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Spontaneous discrimination of small quantities: Shoaling preferences in angelfish (Pterophyllum scalare)
Abstract
The ability to quantify, i.e. to estimate quantity, may provide evolutionary advantages in some contexts and has been demonstrated in a variety of animal species. In a prior study, we showed that angelfish (Pterophyllum scalare) were able to discriminate between groups (shoals) in which a large number of conspecifics swam preferring to join the larger of the two. Our results implied that angelfish can compare relative shoal sizes likely on the basis of some quantitative attributes of the shoal. Here, also using a binary preference test, we examined whether angelfish are able to discriminate between shoals of small numbers of conspecifics, and if so whether their performance reveals a comparable underlying mechanism to that proposed for discrimination of small quantities in human and non-human animals, namely the possible precursor of the ability to count. Our results demonstrate that fish reliably chose 4 versus 1, 3 versus 1, 2 versus 1 and 3 versus 2 individuals, but were at chance performance level when having to choose between 4 versus 3, 5 versus 4 and 6 versus 5. Findings also reveal that the density of the fish in the stimulus shoals did not significantly affect the performance of experimental angelfish. These results are compatible with the hypothesis of the existence of an object-file mechanism to discriminate small quantities in vertebrates and provide evidence for spontaneous discrimination of up to three elements in angelfish, a similar limit to that found in human and non-human animals. The findings add to the growing body of data, suggesting that the mechanisms underlying discrimination between different quantities of items may be shared across different taxa and have an evolutionary ancient origin.
... Revealing the cognitive mechanism underlying quantity discrimination, however, is an important challenge for those interested in understanding the ontogeny and evolution of cognition (Dehaene 1992;Beran 2017;Agrillo and Bisazza 2018;Butterworth et al. 2018;Nieder 2020). The ability to reliably discriminate quantities provides clear evolutionary advantages, by for instance allowing individuals to select more abundant food sources (Perdue et al. 2012), to assess the quantity of potential mates (Lemaître et al. 2011), or to select the larger social subgroup, which can offer better protection against predators when hunting pressure is high (Gómez-Laplaza and Gerlai 2011). In giraffes, in particular, quantity discrimination could allow individuals to select for the location with more trees, for the tree with more leaves or flowers (Berry and Bercovitch 2017), or perhaps even for the place with less predators. ...
... In particular, while small quantities would be encoded by default as object files and larger quantities as magnitudes (as the brain would not manage to encode them as object files), small quantities could also be encoded as magnitudes (e.g., when there are limits in the attentional or working memory resources available; see Hyde 2011). So far, however, experimental evidence has shown that while most animal species rely on the AMS (see, e.g., Cantlon et al. 2010;Beran and Parrish 2016;Beran 2017;Nieder 2020), very few species appear to use the OFS and only in spontaneous-choice tasks (Hauser et al. 2000;Hunt et al. 2008;Gómez-Laplaza and Gerlai 2011;Gómez-Laplaza et al. 2017), perhaps because of the cognitive and perceptual limits in which the OFS should operate (see Hyde 2011) or because only the representation of large numbers may exhibit sufficient variability to result in a ratio effect (Nieder 2020). ...
... In Task 1, the probability to make correct choices was affected by the ratio between the two food quantities in the tray, and this was true for both quantities below and above four. These findings are in line with most research on other non-human animals (Boysen and Berntson 1995;Beran 2017;Rivas-Blanco et al. 2020), including great apes (Cacchione et al. 2014), showing that most species rely on the AMS to discriminate quantities (but see, e.g., Gómez-Laplaza and Gerlai 2011;Agrillo et al. 2014 for evidence of the Object-file System, OFS, in non-human animals). Whether giraffes lack the OFS, or the high cognitive and perceptual demands of the task prevented its use in our study, is a topic for further investigations, although our experimental approach was kept as simple as possible (e.g., simultaneous presentation of two visible food quantities in absence of training). ...
Many species, including humans, rely on an ability to differentiate between quantities to make decisions about social relationships, territories, and food. This study is the first to investigate whether giraffes (Giraffa camelopardalis) are able to select the larger of two sets of quantities in different conditions, and how size and density affect these decisions. In Task 1, we presented captive giraffes with two sets containing a different quantity of identical foods items. In Tasks 2 and 3, we also modified the size and density of the food reward distribution. The results showed that giraffes (i) can successfully make quantity judgments following Weber’s law, (ii) can reliably rely on size to maximize their food income, and (iii) are more successful when comparing sparser than denser distributions. More studies on different taxa are needed to understand whether specific selective pressures have favored the evolution of these skills in certain taxa.
... The first case would show clear adaptive value, as obtaining more food versus less food is critical to survival and reproductive success, and perhaps no other test has been as widely used in this research area as has food quantity judgment. The second case of the guppy also shows a clear instance with immediate survival valuebeing in larger shoals offers more protection, and so finding a way to put oneself into those shoals is important and relies on the ability to discriminate between the choices on the basis of the relative number of fish in them (eg, Gomez-Laplaza & Gerlai, 2011;Piffer, Agrillo, & Hyde, 2012). The third case also is a relational judgment, as those two chimpanzees must assess whether their group size exceeds that of the intruders they hear, and then they must adjust their behavior in terms of approaching or avoiding those intruders. ...
... Variations of these relative quantity judgments of food as well as nonfood items have been given to many species, including fish (eg, Agrillo, Dadda, Serena, & Bisazza, 2008;Agrillo, Piffer, & Bisazza, 2011;Dadda, Piffer, Agrillo, & Bisazza, 2009;Gomez-Laplaza & Gerlai, 2011Piffer et al. 2012), amphibians (Krusche, Uller, & Dicke, 2010;Uller, Jaeger, Guidry, & Martin, 2003), birds (eg, Al Aïn, Giret, Grand, Kreutzer, & Bovet, 2009;Emmerton 1998;Garland, Low, & Burns, 2012;Pepperberg, 2006;Rugani, Regolin, & Vallortigara, 2007, 2008, and a large variety of nonprimate mammals including dogs (Ward & Smuts, 2007), cats (Pisa & Agrillo, 2009), voles (eg, Ferkin, Pierce, Sealand, & delBarco-Trillo, 2005, sea lions (eg, Abramson, Hernández-Lloreda, Call, & Colmenares, 2011), beluga whales (Abramson, Hernández-Lloreda, Call, & Colmenares, 2013), dolphins (eg, Jaakkola, Fellner, Erb, Rodriguez, & Guarino, 2005;Kilian, Yaman, von Fersen, & Gunturkun, 2003), hyenas (Benson-Amram et al., 2010), coyotes (Baker, Shivik, & Jordan, 2011), horses (Uller & Lewis, 2009), bears (Vonk & Beran, 2012), and elephants (Perdue, Talbot, Stone, & Beran, 2012), as well as many nonhuman primates (eg, Addessi, Crecimbene, & Visalberghi, 2008;Anderson et al., 2005;Anderson, Stoinski, Bloomsmith, & Maple, 2007;Barnard et al., 2013;Beran, 2001Beran, , 2004Beran, , 2012Call, 2000;Evans, Beran, Harris, & Rice, 2009;Hanus & Call, 2007;Lewis, Jaffe, & Brannon, 2005). Animals also have shown good discrimination abilities for continuous quantities. ...
... And, ratio effects tend to be evident without any set-size limit effects in many other species ranging from sea lions (Abramson et al., 2011) to parrots (Al Aïn et al., 2009) to pigeons (Roberts, 2010. At present, there is some evidence for set-size limits in some species of fish (eg, Agrillo et al., 2012;Gomez-Laplaza & Gerlai, 2011;Piffer et al., 2012), birds (eg, Garland et al., 2012) and one report with Asian elephants (Irie-Sugimoto, Kobayashi, Sato, & Hasegawa, 2009), but the latter outcome was not replicated in a subsequent study with African elephants (Perdue et al., 2012). Thus, nearly all studies with mammalian species (and particularly primates) show the signature effects of the ANS, but there is much more limited evidence of a second core system for number representation in animals that is precise but limited. ...
The ability to discriminate between quantities of biologically relevant stimuli such as discrete food items or nonfood stimuli such as predators, prey, or conspecifics would serve a clear adaptive purpose in many species, and many species discriminate between quantities. This chapter provides an overview of the kinds of tests given to nonhuman animals to assess their quantity judgment abilities, and the manner in which continuous and discrete quantities are represented by nonhuman animals. This includes a discussion of the potential mechanisms that support quantity representation, and also some of the perceptual and decisional biases that occur across species.
... In all these studies, the elements to be discriminated (conspecifics) were fully visible to the test animals, making the role of continuous physical variables that covary with number (area, contour length, amount of motion, etc.) very prominent. Stancher, Sovrano, Potrich, and Vallortigara (2013) developed a novel procedure that made use of social stimuli as attractors, as in studies of Piffer et al. (2012) and Gómez-Laplaza and Gerlai (2011), that were, however, not visible to the fish during the test, similar to experiments with infants. In this task, fish should compare what they see at the moment of choice (an equal number of conspecifics in two different locations) with memory of the location previously occupied by the larger in number between two groups of conspecifics. ...
... The results apparently confirmed those obtained with redtail splitfin (Stancher et al., 2013) and angel fish (Gómez-Laplaza & Gerlai, 2011), with a drop-off of discrimination with a ratio of 3:4, which seemed to occur irrespective of the largeness of numerical magnitudes, because significant discrimination was apparent with two versus four items (i.e., with a 1:2 ratio, and one number set exceeding any alleged small number system). One problem with this interpretation, however, is that zebrafish failed the fourversus-six discrimination, which has the same ratio (2:3) as the two-versus-three discrimination that zebrafish succeeded to perform. ...
... It should be noted, however, that although most of the evidence available suggest continuity in the processing of small and large numerosity (e.g., nonhuman primates, see Brannon & Terrace, 1998, Cantlon & Brannon, 2007, Judge, Evans, & Vyas, 2005, and Smith, Piel, & Candland, 2003birds, see Rugani, Fontanari, Simoni, Regolin, & Vallortigara (2009) ;Rugani, Regolin & Vallortigara, (2011);Rugani, Cavazzana, Vallortigara, & Regolin (2013); fish, see Gómez-Laplaza & Gerlai, 2011), there are also reports suggesting that the existence of some equivalent of an OFS with a set-size limit at around three to four items (e.g., chimpanzees, see Tomonaga & Matsuzawa, 2002;birds, see Rugani, Regolin, &Vallortigara, 2008, andVallortigara 2010; honeybees, see Dacke & Srinivasan, 2008). It is also important to emphasize that in at least one case, a nonhuman species, an African grey parrot (Psittacus erithacus) named Alex, has shown an ability to use exact, and not approximate, number representations (Pepperberg, 2012;Pepperberg & Carey, 2012). ...
Discrimination of quantity (magnitude) was investigated in zebrafish (Danio rerio). Male zebrafish chose to approach the location previously occupied by the larger in number between 2 groups of female conspecifics (no longer visible at test) in sets of 1 versus 2 items, and 2 versus 3 items, but failed at 3 versus 4 items; similarly, when tested with larger numbers, zebrafish succeeded with 2 versus 4, 4 versus 6, and 4 versus 8 items, but failed with 6 versus 8 items. The results suggest that zebrafish rely on an approximate number system to discriminate memorized sets of conspecifics of different magnitudes, the degree of precision in recall being mainly dependent on the ratio between the sets to be discriminated. (PsycINFO Database Record
... Recently, the work of cognitive ethologists has expanded to encompass fish species. Spontaneous choice tests Piffer et al., 2012;Gómez-Laplaza and Gerlai, 2011b) and training procedures (Agrillo et al., 2010a;Agrillo et al., 2011) showed that fish are capable of processing both small and large numbers with a performance similar to that described in mammals and birds tested with similar paradigms (Barnard et al., 2013;Hunt et al., 2008;Revkin et al., 2008). In particular, guppies (Poecilia reticulata) and mosquitofish (Gambusia holbrooki) show the ability to discriminate between shoals differing by one up to four items (one versus two, two versus three, and three versus four); larger quantities can also be discriminated provided that the numerical ratio between the smaller and the larger quantity is at least 0.50 [i.e. ...
... It would currently be challenging to understand why cavefish differ in numerical acuity from other fish species (Gómez-Laplaza and Gerlai, 2011b;Agrillo et al., 2009;Agrillo et al., 2010a;Agrillo et al., 2011). One may argue that the object representation using non-visual modalities might be less precise. ...
... This species evolved for millions of years in a homogeneous environment in the absence of natural predators and with a scarcity of food resources. Selective pressures might have acted differently from other fish species, on the one hand reducing the cerebral mass in order to optimize the metabolic consumption of the brain, and on the other hand losing the neural circuits supporting cognitive functions not useful in the cave, such as those necessary to discriminate the larger shoal (Buckingham et al., 2007;Gómez-Laplaza and Gerlai, 2011b;Hager and Helfman, 1991). Phreatichthys andruzzii shows an extreme troglomorphic phenotype, and the complete anophthalmy is accompanied by the complete absence of optic nerves and chiasm and by a strong reduction in the size of the entire brain (Berti et al., 2001;Ercolini and Berti, 1975). ...
Over a decade of comparative studies, researchers have found that
rudimentary numerical abilities are widespread among vertebrates.
While experiments in mammals and birds have employed a variety
of stimuli (visual, auditory and tactile), all fish studies involved visual
stimuli and it is unknown whether fish can process numbers in other
sensory modalities. To fill this gap, we studied numerical abilities in
Phreatichthys andruzzii, a blind cave-dwelling species that evolved
in the phreatic layer of the Somalia desert. Fish were trained to
receive a food reward to discriminate between two groups of objects
placed in opposite positions of their home tank. In Experiment 1,
subjects learned to discriminate between two and six objects, with
stimuli not controlled for non-numerical continuous variables that covary
with numbers, such as total area occupied by stimuli or density.
In Experiment 2, the discrimination was two versus four, with half of
the stimuli controlled for continuous quantities and half not controlled
for continuous quantities. The subjects discriminated only the latter
condition, indicating that they spontaneously used non-numerical
information, as other vertebrates tested in similar experiments. In
Experiments 3 and 4, cavefish trained from the beginning only with
stimuli controlled for continuous quantities proved able to learn the
discrimination of quantities based on the sole numerical information.
However, their numerical acuity was lower than that reported in other
teleost fish tested with visual stimuli.
... Furthermore, laboratory studies have found juvenile angelfish to possess complex learning abilities [46][47][48] . Angelfish, as a highly social species, has also been shown to be able to discriminate shoals of conspecifics differing in number [49][50][51][52] , and also to remember the location of the larger vs the smaller shoals [53][54][55][56] . All these behavioural and cognitive traits make the angelfish an ideal species for studying food quantity discrimination. ...
... We found the limit of size discrimination ratio to be 1.5:1. Interestingly a similar ratio limit has been previously found in angelfish in a foraging context 6 , as well as in a social context 50 . A food-size discrimination ratio was also demonstrated in guppies with a performance following Weber's law 4,5 . ...
Comparative studies on quantity discrimination in animals are important for understanding potential evolutionary roots of numerical competence. A previous study with angelfish has shown that they discriminate numerically different sets of same-sized food items and prefer the larger set. However, variables that covary with number were not controlled and choice could have been influenced by variables such as size or density of the food items rather than numerical attributes. Here using a recently developed approach, we examined whether contour length of the food items affects choice in a spontaneous binary choice task. In Experiment 1, a contrast of 1 vs. 1 food item was presented, but the ratio between the size (diameter) of the food items was varied. In Experiment 2, numerically different food sets were equated in overall size by increasing the size (diameter) of the items in the numerically small sets. In both Experiments, subjects showed a preference for the larger sized food items with a discrimination limit. These results show that item size plays a prominent role in foraging decisions in angelfish. Experiment 3 placed numerical and size attributes of the sets in conflict by presenting one larger-sized food item in the numerically smaller set that also had smaller overall size (diameter) of food items. Angelfish showed no preference in any of the contrasts, suggesting that they could not make optimal foraging decisions when these attributes were in conflict. Maximization of energy return is central to optimal foraging. Accordingly, here item size was also found to be a key feature of the sets, although the numerical attributes of the sets also influenced the choice.
... In previous studies, we have demonstrated that angelfish (Pterophyllum scalare), apparently seeking protection from potential threats in a novel environment, exhibited a preference for the larger of two shoals when the contrasting shoals were fully visible and composed of a small number of members, i.e., less than 4 [35]. As subjects were able to discriminate 3 versus 2, 3 versus 1, and 2 versus 1 with similar accuracy, but failed to discriminate 4 versus 3 individuals, the results suggested that angelfish accomplished the task employing an OFS with an upper limit of three individuals. ...
... Yet other studies in which angelfish could view the contrasted shoals during the choice test, found that the experimental fish could cross the large-small boundary (e.g., 4 vs. 1, 4 vs. 2) [35,36], and also when a retention interval of 2 sec was imposed [44]. In such studies, successful quantity discrimination across the boundary occurred when the number of members in the contrasted shoals differed by at least twofold. ...
Rudimentary quantification abilities are found in numerous animal species and in human infants all demonstrating the ability to discriminate between quantities differing in numerical size. An open question is whether individuals rely on different underlying systems to discriminate between large (analogue magnitude system (AMS) for number of items exceeding 3) and small quantities (object-file system (OFS) for number of items below 4), or they use only one system (AMS) for the entire number range. The two-system hypothesis has been supported by finding reduced ability to discriminate between quantities that cross the large-small boundary in several species. Recently, the role of cognitive representation, i.e., memory, in quantity discrimination has also been recognized. Here, we investigated whether angelfish can discriminate quantities across the boundary under two memory conditions. In a binary choice test, single angelfish were allowed to see groups (shoals) of conspecifics of different numerical size on the two sides of their test tank. In Experiment 1, their choice was recorded after a 2-sec retention interval during which shoal size information was unavailable. Angelfish were able to discriminate the larger shoal across the boundary when the shoals differed by a 2:1 or higher ratio, but not when the ratio was lower. In Experiment 2, however, with a 15-sec retention interval, angelfish could only detect a four-fold difference in ratio but failed to detect a three- or a two-fold difference across the boundary. These results suggest that angelfish can remember smaller differences for a short (2 sec) but not for a longer (15 sec) period. Together with previous findings, the current results support the idea that angelfish use two distinct systems for representing quantity, but they may recruit the AMS even for the small number range under some circumstances, e.g., when higher memory demand is imposed by a greater retention interval.
... Fish, however, also proved to be able to compare sets with large numerosity (>4 elements), showing a ratio-dependent accuracy. Angelfish can discriminate up to a 0.56 ratio (e.g., 5 vs. 9; Gómez-Laplaza and Gerlai, 2011b), while other species such as guppies (Agrillo et al., 2012), mosquitofish (Agrillo et al., 2008), and swordtail (Buckingham et al., 2007) show a limit set at 0.5 (one group is two times the other). Fewer evidence indicates that fish can go higher than 0.67: three-spined sticklebacks discriminate up to 0.87, showing, however, a progressive accuracy decrease as the ratio increases (Mehlis et al., 2015). ...
An ability to estimate quantities, such as the number of conspecifics or the size of a predator, has been reported in vertebrates. Fish, in particular zebrafish, may be instrumental in advancing the understanding of magnitude cognition. We review here the behavioral studies that have described the ecological relevance of quantity estimation in fish and the current status of the research aimed at investigating the neurobiological bases of these abilities. By combining behavioral methods with molecular genetics and calcium imaging, the involvement of the retina and the optic tectum has been documented for the estimation of continuous quantities in the larval and adult zebrafish brain, and the contributions of the thalamus and the dorsal-central pallium for discrete magnitude estimation in the adult zebrafish brain. Evidence for basic circuitry can now be complemented and extended to research that make use of transgenic lines to deepen our understanding of quantity cognition at genetic and molecular levels.
... The apparent intelligence of some fish species [e.g., tool use by tuskfish (200), cooperative hunting by moray eels and groupers (201), numerical competency in angelfish (202), and the mirror self-recognition and episodic memory tasks mentioned above] has sometimes been taken as already sufficient evidence for sentience [e.g., (18)]. For two reasons, we believe this is premature. ...
Debates around fishes' ability to feel pain concern sentience : do reactions to tissue damage indicate evaluative consciousness (conscious affect), or mere nociception? Thanks to Braithwaite's discovery of trout nociceptors, and concerns that current practices could compromise welfare in countless fish, this issue's importance is beyond dispute. However, nociceptors are merely necessary, not sufficient, for true pain, and many measures held to indicate sentience have the same problem. The question of whether fish feel pain – or indeed anything at all – therefore stimulates sometimes polarized debate. Here, we try to bridge the divide. After reviewing key consciousness concepts, we identify “red herring” measures that should not be used to infer sentience because also present in non-sentient organisms, notably those lacking nervous systems, like plants and protozoa (P); spines disconnected from brains (S); decerebrate mammals and birds (D); and humans in unaware states (U). These “S.P.U.D. subjects” can show approach/withdrawal; react with apparent emotion; change their reactivity with food deprivation or analgesia; discriminate between stimuli; display Pavlovian learning, including some forms of trace conditioning; and even learn simple instrumental responses. Consequently, none of these responses are good indicators of sentience. Potentially more valid are aspects of working memory, operant conditioning, the self-report of state, and forms of higher order cognition. We suggest new experiments on humans to test these hypotheses, as well as modifications to tests for “mental time travel” and self-awareness (e.g., mirror self-recognition) that could allow these to now probe sentience (since currently they reflect perceptual rather than evaluative, affective aspects of consciousness). Because “bullet-proof” neurological and behavioral indicators of sentience are thus still lacking, agnosticism about fish sentience remains widespread. To end, we address how to balance such doubts with welfare protection, discussing concerns raised by key skeptics in this debate. Overall, we celebrate the rigorous evidential standards required by those unconvinced that fish are sentient; laud the compassion and ethical rigor shown by those advocating for welfare protections; and seek to show how precautionary principles still support protecting fish from physical harm.
... The choice is regarded "spontaneous" as no training is involved, and thus the test relies upon inherent behavioral tendencies of the studied fish species. Such spontaneous choice tasks have been employed with a cichlid, the angelfish (Pterophyllum scalare), and the results have demonstrated excellent discrimination abilities (Gómez-Laplaza, 2006;Gómez-Laplaza, 2009;Gómez-Laplaza & Fuente, 2007), including the capability to discriminate quantities based on visual cues as well as the ability to discriminate based upon short-term memory of where the shoals were (Gómez-Laplaza et al., 2017;Gómez-Laplaza & Gerlai, 2011a, 2011bGómez-Laplaza & Gerlai, 2015;Gómez-Laplaza & Gerlai, 2016a, 2016b. Briefly, when conspecific shoals differing in numerical size (number of shoal members) were contrasted, angelfish chose the numerically larger one. ...
Video playback is a widely used technique for presentation of visual stimuli in animal behavior research. In the analysis of behavioral responses to social cues, presentation of video recordings of live conspecifics represents a consistently reproducible stimulus. However, video-recordings do not interact with the experimental subject, and thus this stimulus may be inferior in the social context. Here, we evaluated how angelfish ( Pterophyllum scalare ) respond to a video playback of conspecifics versus a live shoal of conspecifics. Using binary choice tests, subjects were presented different stimuli. Time spent close to one versus the other stimulus was considered an index of preference. We found angelfish to prefer a live shoal of conspecifics to an empty tank, and also the video playback of a shoal of conspecifics to a blank screen, although the level of preference in the latter was lower than in the former. These results indicate that video-playback of live conspecifics may be appropriate in angelfish, thus allowing manipulation of specific cues that angelfish may use in quantity discrimination. However, when we directly contrasted a live and a video recorded shoal, both having the same number of members, experimental fish preferred the live shoal. When the choice consisted of a live shoal of four conspecifics versus a video playback of a shoal of nine conspecifics no clear preference emerged. These results imply that video-playback has disadvantages in quantity discrimination studies with angelfish. Exploring procedural and/or technological parameters will verify the suitability of video-recording-based stimulus presentation for future use in angelfish.
... The results match the findings of other studies in fish, mammals, and amphibians [35,63,64]. OFS limits have shown to be at approximately 3 for the angelfish, goldbelly topminnow and redtail splitfin [65][66][67], 4 in primates (Macaca mulatta) and possibly 5 in guppies [1,68,69]. The present data could indicate that the OFS in C. griseum is limited to numbers of 4 or below. ...
Over the last decade, studies examining the cognitive abilities of fish have increased, using a broad range of approaches. One of the foci has been to test the ability of fish to discriminate quantities of items and to determine whether fish can solve tasks solely on the basis of numerical information. This study is the first to investigate this ability in two elasmobranch species. All animals were trained in two-alternative forced-choice visual experiments and then examined in transfer tests, to determine if previously gained knowledge could be applied to new tasks. Results show that the grey bamboo shark (Chiloscyllium griseum) and the ocellate river stingray (Potamotrygon motoro) can discriminate quantities based on numerical information alone, while continuous variables were controlled for. Furthermore, the data indicates that similar magnitudes and limits for quantity discrimination exist as in other animals. However, the high degree of intraspecific variation that was observed as well as the low rate of animals proving to be successful suggest that the ability to discriminate quantities may not be as important to these species as to some other vertebrate and invertebrate species tested so far.
... P. scalare can spontaneously discriminate the quantity of conspecifics (Agrillo et al., 2017;G omez-Laplaza & Gerlai, 2011a, 2013a, 2016 even without previous training (see Agrillo et al., 2017;G omez-Laplaza & Gerlai, 2011b). ...
Some common practices in aquaculture, ornamental trade, and fish facilities may disturb the behavioural repertoire of fish and its natural adaptive value, reducing welfare and unsettling fish production. Hence, it is necessary to understand individuals’ behaviour, as well as the factors affecting it, in order to improve their quality of life. Here, we reviewed the behaviour of the angelfish Pterophyllum scalare, an Amazonian cichlid used worldwide both as an ornamental fish and as a fish model in scientific research. We characterized social, reproductive, feeding behaviour, as wells as the amazing cognitive ability of the angelfish. In addition, we reviewed the effects of environmental enrichment and suggested some important variables that need to be considered for rearing P. scalare. In this review, we showed for the first time a synthesis on behaviour and a best practice overview to improve the welfare of angelfish. Nonetheless, most topics reviewed fit a broader set of fish species, particularly ornamental ones. This synthesis can, therefore, open a path for further behavioural research applied to the welfare of angelfish and bring insights to other fish species.
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... Quick estimation of stimulus magnitude, such as number discrimination, is often essential to performing adaptive behavior in the shortest time. For instance, group-living animals, such as some fish or anuran tadpole, in the presence of a predator need to rapidly estimate the largest group of conspecifics and to aggregate with it, thus maximizing their survival rate (Hoare et al., 2004;Agrillo et al., 2008;Gómez-Laplaza and Gerlai, 2011;Balestrieri et al., 2019). The tectum could be an ideal candidate region to perform fast and coarse number discrimination classifying sensory information, in this case larger and smaller number of conspecifics, and give rise directly to motor output toward the larger group. ...
The ability to represent, discriminate, and perform arithmetic operations on discrete quantities (numerosities) has been documented in a variety of species of different taxonomic groups, both vertebrates and invertebrates. We do not know, however, to what extent similarity in behavioral data corresponds to basic similarity in underlying neural mechanisms. Here, we review evidence for magnitude representation, both discrete (countable) and continuous, following the sensory input path from primary sensory systems to associative pallial territories in the vertebrate brains. We also speculate on possible underlying mechanisms in invertebrate brains and on the role played by modeling with artificial neural networks. This may provide a general overview on the nervous system involvement in approximating quantity in different animal species, and a general theoretical framework to future comparative studies on the neurobiology of number cognition.
... In the large number range, the presented contrast was discriminated against, but in the small number range, it was not. Since in angelfish four items exceed the upper limit of OFS-based discrimination (Gómez-Laplaza and Gerlai, 2011b;Gómez-Laplaza et al., 2018), the contrast 4 vs. 2 could be considered as crossing the boundary of the large/small set. Thus, the failure in the discrimination may be the result of having to use two distinct representational systems for the same contrast, as argued by Cordes and Brannon (2009), a conflict between representational systems that impedes comparison. ...
Quantity discrimination, the ability to identify, process, and respond to differences in number, has been shown in a variety of animal species and may have fitness value. In fish, the ability to distinguish between numerically different shoals has been well studied. However, little work has been devoted to the investigation of such ability in a foraging context. Nevertheless, angelfish (Pterophyllum scalare) have been previously shown to be able to discriminate numerically different sets of food items, with variables such as size and density of the food items playing important roles in making the choice. Here, we examine the possible role of other numerical and non-numerical variables. Using a spontaneous binary choice task, we contrasted sets of food items differing in specifically controlled ways: (1) different numerical size but equal inter-item distance; (2) different numerical size and different inter-item distance; and (3) identical total contour length and area occupied but different individual food size and inter-food distance between the contrasted food sets. In Experiment 1, angelfish were found to prefer the sets with a large number of food items. In Experiment 2, they preferred the numerically smaller sets with clustered items to the numerically larger sets with scattered items, but only when the sets were in the large number range (10 vs. 5 food items). Finally, in Experiment 3 fish preferred numerically smaller sets with large-sized and scattered food items in the large number range sets. We conclude that food item number, density, and size may not be considered individually by angelfish, but instead, the fish respond to all these factors attempting to maximize energy gained from eating the food while minimizing energy expenditure collecting and/or protecting the food.
... Such ability is supposed to be highly useful in nature to reduce the risks of being predated. It has been shown that the capacity to discriminate between a large and a small shoal varies as a function of the species: when the two shoals differ by one unit, angelfish seem to be able to find the larger shoal up to 3 units (2 vs. 3, Gómez-Laplaza and Gerlai, 2011), mosquitofish up to 4 (3 vs. 4, Agrillo et al., 2008), guppies up to 5 (4 vs. 5, Lucon-Xiccato et al., 2017), while stickleback seem to be able to discriminate even 6 from 7 conspecifics ( Mehlis et al., 2015). As these species are highly social, it is unlikely that the variability here observed could be explained by the different degree of motivation in reaching social companions. ...
In 1985, Macphail argued that there are no differences among the intellects of non-human vertebrates and that humans display unique cognitive skills because of language. Mathematical abilities represent one of the most sophisticated cognitive skills. While it is unquestionable that humans exhibit impressive mathematical skills associated with language, a large body of experimental evidence suggests that Macphail hypothesis must be refined in this field. In particular, the evidence that also small-brained organisms, such as fish, are capable of processing numerical information challenges the idea that humans display unique cognitive skills. Like humans, fish may take advantage of using continuous quantities (such as the area occupied by the objects) as proxy of number to select the larger/smaller group. Fish and humans also showed interesting similarities in the strategy adopted to learn a numerical rule. Collective intelligence in numerical estimation has been also observed in humans and guppies. However, numerical acuity in humans is considerably higher than that reported in any fish species investigated, suggesting that quantitative but not qualitative differences do exist between humans and fish. Lastly, while it is clear that contextual factors play an important role in the performance of numerical tasks, inter-species variability can be found also when different fish species were tested in comparable conditions, a fact that does not align with the null hypothesis of vertebrate intelligence. Taken together, we believe that the recent evidence of numerical abilities in fish call for a deeper reflection of Macphail's hypothesis.
... Social species such as hyenas and dogs adopt different strategies in competitive situations in relation to the number of potential competitors (Benson-Amram, Heinen, Dryer, & Holekamp, 2011;Bonanni, Natoli, Cafazzo, & Valsecchi, 2011). In an unfamiliar and dangerous environment, prey species may minimize predatory risks by joining a larger group of conspecifics ( Gomez-Laplaza & Gerlai, 2011;Hager & Helfman, 1991;. ...
Recent studies have reported that the ability to discriminate among quantities is not a prerogative of vertebrates. Ants, bees, and spiders can solve tasks in which they are required to discriminate between groups of objects. Although many studies regarding numerical cognition on invertebrates proposed a proto‐counting system, more control experiments for non‐numerical variables are necessary to confirm this hypothesis. Here, we developed a new method to investigate quantity discrimination abilities in invertebrates. We investigated the spontaneous choice of a cricket, Acheta domesticus. We exploited its natural shelter‐seeking behavior by presenting sets of geometrical shapes that simulated potential shelters. In a dichotomous choice between sets of geometrical black shapes differing in number of items, the majority of crickets chose the set containing the larger numerosity up to 2 versus 3 items. Control experiments suggested that crickets discriminated between sets consisting of different numbers of items by attending to continuous variables (i.e., convex hull and cumulative surface area) rather than by attending to numerosity. Secondly, when discriminating between single geometrical shapes, crickets attend to the width but not to the height of the stimuli.
... Numbers play an essential role in everyday life for most, if not all, animals. The ability to assess quantitative differences enables individuals from prey species, such as guppies, to reduce predation risk by preferentially joining larger social groups [1][2][3][4][5][6]. Likewise, predators benefit from the ability to discriminate quantities when selecting foraging patches and when choosing which size of prey to hunt, which can change depending on the number of partners in a hunting party [7][8][9][10]. ...
Playback experiments have proved to be a useful tool to investigate the extent to which wild animals understand numerical concepts and the factors that play into their decisions to respond to different numbers of vocalizing conspecifics. In particular, playback experiments have broadened our understanding of the cognitive abilities of historically understudied species that are challenging to test in the traditional laboratory, such as members of the Order Carnivora. Additionally, playback experiments allow us to assess the importance of numerical information versus other ecologically important variables when animals are making adaptive decisions in their natural habitats. Here, we begin by reviewing what we know about quantity discrimination in carnivores from studies conducted in captivity. We then review a series of playback experiments conducted with wild social carnivores, including African lions, spotted hyenas and wolves, which demonstrate that these animals can assess the number of conspecifics calling and respond based on numerical advantage. We discuss how the wild studies complement those conducted in captivity and allow us to gain insights into why wild animals may not always respond based solely on differences in quantity. We then consider the key roles that individual discrimination and cross-modal recognition play in the ability of animals to assess the number of conspecifics vocalizing nearby. Finally, we explore new directions for future research in this area, highlighting in particular the need for further work on the cognitive basis of numerical assessment skills and experimental paradigms that can be effective in both captive and wild settings.
This article is part of a discussion meeting issue ‘The origins of numerical abilities’.
... 28 Previous studies with zebrafish and other fish species consistently showed that larger shoals tend to decrease the risk of being caught for individual shoal members, 29 and, thus, most fish species exhibit preference for the numerically larger to the smaller shoal. 30,31 However, during spawning, a large number of individuals may increase the risk of predation of fertilized eggs. 28 Thus, there may be an optimal compromise for shoal size that minimizes predation risk for adults but maximizes survival of fertilized eggs, and this optimum may change depending on the breeding season. ...
In recent years, a rapidly increasing number of scientific papers have been published that utilize zebrafish (Danio rerio) as an alternative model organism in the study of a wide range of biological phenomena from cancer to behavior. This is, in large part, due to the prolific nature, relative ease of maintenance, and sufficiently high genetic homology of zebrafish to humans. With the surge of zebrafish use in animal research, the variations in methodologies of breeding and husbandry of this species have also increased. Investigators usually focus on the development and implementation of rigorous laboratory control that is specific to their studies. We suggest that the same scrutiny and attention may be required for the methods of breeding and housing of zebrafish. This article reviews a variety of zebrafish husbandry and breeding techniques and conditions employed around the world. It discusses factors ranging from numerous aspects of rearing/housing conditions through the sex ratio of the breeding group to the composition of the diet of zebrafish that may vary across laboratories. It provides some feedback on the potential pros and cons of the different methods. It argues that there is a substantial need for systematic analysis of these methods, that is, the effects of environmental factors on zebrafish health and breeding. It also discusses the question as to whether some degree of standardization of these methods is needed to enhance cross-laboratory comparability of results.
... We found that with a retention interval of 30 sec, angelfish failed to discriminate in the comparisons involving shoals whose numerical size was in the small number range (maximum 4), but not when the two contrasted shoals had at least 4 members each and the ratio of these shoals reached or exceeded 2:1. The failure to distinguish shoals of different numerical size in the small number range contrasts with results previously obtained with angelfish (Gómez-Laplaza & Gerlai, 2011b;Gómez-Laplaza & Gerlai, 2015). Notably in these latter studies either no memory demand was placed on performance (Gómez-Laplaza & Gerlai, 2011b) or the retention interval was not longer than 2 sec (Gómez-Laplaza & Gerlai, 2015). ...
The ability to discriminate between sets that differ in the number of elements can be useful in different contexts and may have survival and fitness consequences. As such, numerical/quantity discrimination has been demonstrated in a diversity of animal species. In the laboratory, this ability has been analyzed, for example, using binary choice tests. Furthermore, when the different number of items first presented to the subjects are subsequently obscured, i.e., are not visible at the moment of making a choice, the task requires memory for the size of the sets. In previous work, angelfish (Pterophyllum scalare) have been found to be able to discriminate shoals differing in the number of shoal members both in the small (less than 4) and the large (4 or more) number range, and they were able to perform well even when a short memory retention interval (2-15 s) was imposed. In the current study, we increased the retention interval to 30 s during which the shoals to choose between were obscured, and investigated whether angelfish could show preference for the larger shoal they saw before this interval. Subjects were faced with a discrimination between numerically small shoals (≤4 fish) and also between numerically large (≥4 fish) shoals of conspecifics. We found angelfish not to be able to remember the location of larger versus smaller shoals in the small number range, but to exhibit significant memory for the larger shoal in the large number range as long as the ratio between these shoals was at least 2:1. These results, together with prior findings, suggest the existence of two separate quantity estimation systems, the object file system for small number of items that does not work with the longer retention interval and the analogue magnitude system for larger number of items that does.
... The proportion of choices for the larger food quantity is recorded as measure of accuracy. (Agrillo et al., 2012a) while angelfish (Pterophillum scalare) discriminated 1 vs. 2 and 2 vs. 3, but not 3 vs. 4 fish (Gómez-Laplaza and Gerlai, 2011b). Larger quantities can also be discriminated by increasing the ratio between the smaller and the larger quantity: guppies (Agrillo et al., 2012a), mosquitofish (Agrillo et al., 2008a) and swordtails (Xiphophorus elleri, Buckingham et al., 2007) could discriminate up to a 0.50 ratio (e.g., 8 vs. 16) but not 0.67 ratio (e.g., 8 vs. 12). ...
The ability to utilize numerical information can be adaptive in a number of ecological contexts including foraging, mating, parental care, and anti-predator strategies. Numerical abilities of mammals and birds have been studied both in natural conditions and in controlled laboratory conditions using a variety of approaches. During the last decade this ability was also investigated in some fish species. Here we reviewed the main methods used to study this group, highlighting the strengths and weaknesses of each of the methods used. Fish have only been studied under laboratory conditions and among the methods used with other species, only two have been systematically used in fish-spontaneous choice tests and discrimination learning procedures. In the former case, the choice between two options is observed in a biologically relevant situation and the degree of preference for the larger/smaller group is taken as a measure of the capacity to discriminate the two quantities (e.g., two shoals differing in number). In discrimination learning tasks, fish are trained to select the larger or the smaller of two sets of abstract objects, typically two-dimensional geometric figures, using food or social companions as reward. Beyond methodological differences, what emerges from the literature is a substantial similarity of the numerical abilities of fish with those of other vertebrates studied.
... A model of this system, often called the object file model, has been described in detail elsewhere (e.g., Feigenson & Carey 2005;Feigenson, Carey, & Hauser, 2002;Uller, Carey, Huntley-Fennner, & Klatt, 1999), but the key point here is that a two core-systems hypothesis for numerical cognition requires evidence that performance with small sets of items looks better than performance with large sets (Feigenson, Dehaene, & Spelke, 2004;Xu, 2003; also see Hyde, 2011;Hyde & Wood, 2011, for a discussion about other stimulus features that impact which of these two systems might be activated). Although there has been less evidence for this system in the comparative literature than in the developmental literature, some reports have suggested that fish may show evidence of these two systems (Agrillo, Miletto Petrazzini, & Bisazza, 2014;Agrillo, Piffer, Bisazza, & Butterworth, 2012;Gómez-Laplaza & Gerlai, 2011;Piffer, Agrillo, & Hyde, 2012; but see Gomez-Laplaza & Gerlai, 2011;Potrich, Sovrano, Stancher, & Vallortigara, 2015), as might birds (e.g., Garland, Low, & Burns, 2012;Hunt, Low, & Burns, 2008), salamanders (Krusche, Uller, & Dicke, 2010;Uller et al. 2003), and beluga whales (Abramson, Hernandez-Lloreda, Call, & Colmenares, 2013). Among primate studies, only one paper reported that semi-free ranging rhesus monkeys also showed the hallmark behavioral features of two core systems (Hauser et al. 2000a, b), whereas the vast majority of the studies with primates show ratio effects indicative of the approximate number system only (e.g., Barnard et al., 2013;Beran, 2004Beran, , 2008Evans, Beran, Harris, & Rice, 2009;Cantlon & Brannon, 2006a, b;Hanus & Call, 2007;Merritt, MacLean, Crawford, & Brannon, 2011;Nieder & Miller, 2004). ...
A key issue in understanding the evolutionary and developmental emergence of numerical cognition is to learn what mechanism(s) support perception and representation of quantitative information. Two such systems have been proposed, one for dealing with approximate representation of sets of items across an extended numerical range and another for highly precise representation of only small numbers of items. Evidence for the first system is abundant across species and in many tests with human adults and children, whereas the second system is primarily evident in research with children and in some tests with non-human animals. A recent paper (Choo & Franconeri, Psychonomic Bulletin & Review, 21, 93-99, 2014) with adult humans also reported "superprecise" representation of small sets of items in comparison to large sets of items, which would provide more support for the presence of a second system in human adults. We first presented capuchin monkeys with a test similar to that of Choo and Franconeri in which small or large sets with the same ratios had to be discriminated. We then presented the same monkeys with an expanded range of comparisons in the small number range (all comparisons of 1-9 items) and the large number range (all comparisons of 10-90 items in 10-item increments). Capuchin monkeys showed no increased precision for small over large sets in making these discriminations in either experiment. These data indicate a difference in the performance of monkeys to that of adult humans, and specifically that monkeys do not show improved discrimination performance for small sets relative to large sets when the relative numerical differences are held constant.
... This debate has been recently enlarged to encompass non-human species. Some authors found evidence of a different ratio dependence of the performance within and outside the subitizing range in mammals, birds and fish (Hauser et al., 2000;Hunt et al., 2008;Bonanni et al., 2011;Gòmez-Laplaza and Gerlai, 2011;, while other studies report a similar ratio sensitivity in the two numerical ranges, supporting the idea of a single ANS (Cantlon and Brannon, 2007;Ward and Smuts, 2007;Tomonaga, 2008;Al Aïn et al., 2009). ...
Adults, infants and some non-human animals share an approximate number system (ANS) to estimate numerical quantities, and are supposed to share a second, ‘object-tracking’, system (OTS) that supports the precise representation of a small number of items (up to 3 or 4). In relative numerosity judgments, accuracy depends on the ratio of the two numerosities (Weber’s Law), for numerosities > 4 (the typical ANS range), while for numerosities ≤ 4 (OTS range) there is usually no ratio effect. However, recent studies have found evidence for ratio effects for small numerosities, challenging the idea that the OTS might be involved for small number discrimination. Here we tested the hypothesis that the lack of ratio effect in the numbers 1-4 is largely dependent on the type of stimulus presentation.
We investigated relative numerosity judgments in college students using three different procedures: a simultaneous presentation of intermingled and separate groups of dots in separate experiments, and a further experiment with sequential presentation. As predicted, in the large number range, ratio dependence was observed in all tasks. By contrast, in the small number range, ratio insensitivity was found in one task (sequential presentation). In a fourth experiment, we showed that the presence of intermingled distractors elicited a ratio effect, while easily distinguishable distractors did not. As the different ratio sensitivity for small and large numbers has been often interpreted in terms of the activation of the OTS and ANS, our results suggest that numbers 1-4 may be represented by both numerical systems and that the experimental context, such as the presence/absence of task-irrelevant items in the visual field, would determine which system is activated.
... New Zealand robins were found to discriminate 1 vs 2, 2 vs 3 and 3 vs 4 with the same accuracy, while they can discriminate larger quantities only with a 0.50 ratio (4 vs 8, HUNT et al. 2008). Similarly, angelfish could discriminate 1 vs 2 and 2 vs 3 with the same accuracy, while in the ANS range they needed at most a 0.56 ratio (5 vs 9) to discriminate between the presented quantities (GÓMEZ-LAPLAZA & GERLAI 2011a, 2011b. ...
Quantitative ability in non-human animals represents one of the topics most investigated in cognitive ethology during the last decade. Vertebrates as diverse as mammals, birds, and fish proved able to discriminate between two quantities in several ecological contexts. Recently, there has been a wide debate as to whether non-human animals share a single mechanism of numerical representation (commonly referred to as the “approximate number system”, ANS) or instead have also a distinct mechanism for enumerating small numbers (≤ 4), referred to as “subitizing”. To date, little attention has been devoted to assess whether individual differences exist in quantity abilities within the boundaries of the two supposed mechanisms.
In the present study, we compared the performance of guppies (Poecilia reticulata) in small- and large-quantity discrimination. Subjects were inserted in an unfamiliar tank where two groups of conspecifics differing in numerosity were visible, and their spontaneous preference of joining the larger shoal was taken as a measure of their numerical acuity. Each subject was tested in two numerical contrasts: 2 vs 3 and 6 vs 10. A positive correlation in the performance in the two numerical contrasts was found: subjects showing a better performance in the subitizing range also showed a better performance in the ANS range. Our data do not contradict the hypothesis of two distinct mechanisms of numerical representation but may be more parsimoniously explained by the existence of a single ANS mechanism across the whole numerical range.
... First we have manipulated the size of the conspecific images relative to the size of the experimental subjects. Intuitively, and based upon indirect evidence obtained with other fish species (Laplaza and Gerlai, 2011b; Agrillo et al., 2012 and references therein), investigators tended to use zebrafish images of a size identical to that of their experimental fish. However, according to our knowledge, our study is the first that actually demonstrated that similarity between the size of the stimulus and the experimental fish the strongest reduction of distance (shoaling response) towards animated images of conspecifics whose size matched or was bigger than their own. ...
... But, we also know that animals do not count, at least not without massive efforts to instill such counting routines (Beran & Rumbaugh, 20001;Boysen & Berntson, 1989;Matsuzawa, 1985;Pepperberg, 1994Pepperberg, , 2012Pepperberg & Carey, 2012;Pepperberg & Gordon, 2005), and even then performance is underwhelming compared to what a 4-or 5-year-old child can do (Gelman & Gallistel, 1978). The more recent controversy has been about the nature of nonverbal representation of number and quantity, and whether animals require and access one or two "core systems of number," and a large amount of research has been conducted as part of this debate (Beran, 2004(Beran, , 2007(Beran, , 2008(Beran, , 2012Beran & Beran, 2004;Feigenson, Dehaene, & Spelke, 2004;Gomez-Laplaza, & Gerlai, 2011;Hanus & Call, 2007;Hauser, Carey, & Hauser, 2000;Jordan & Brannon, 2006;Nieder, 2005;Piffer, Agrillo, & Hyde, 2012;Perdue, Talbot, Stone, & Beran, 2012;Rugani, Cavazzana, Vallortigara, & Regolin, 2013;Tomonaga, 2007;Ujfalussy, Miklósi, Bugnyar, & Kotrschal, in press;Vonk & Beran, 2012). ...
Comparative cognition is the field of inquiry concerned with understanding the cognitive abilities and mechanisms that are evident in nonhuman species. Assessments of animal cognition have a long history, but in recent years there has been an explosion of new research topics, and a general broadening of the phylogenetic map of animal cognition. To review the past of comparative cognition, we describe the historical trends. In regards to the present state, we examine current "hot topics" in comparative cognition. Finally, we offer our unique and combined thoughts on the future of the field.
This paper argues that the core phi-features behind grammatical person, number, and gender are widely used in animal cognition and are in no way limited to humans or to communication. Based on this, it is hypothesized (i) that the semantics behind phi-features were fixed long before primates evolved, (ii) that most go back as far as far as vertebrates, and (iii) that some are shared with insects and plants.
Zebrafish ( Danio rerio ) constitute an excellent model system to investigate the neural and genetic basis of quantitative cognition because of the single neuron resolution of calcium imaging of awake, behaving fish. While nonsymbolic numerical cognition has been investigated across many taxa, symbolic numerical cognition has not been investigated among fish. We developed a novel quantitative symbolic test for zebrafish using an operant conditioning paradigm in which the number of horizontal lines zebrafish approached in a 2-alternative forced choice task predicted the number of food reward pellets they would receive. Zebrafish did not at the population level learn a preference for the 2-line stimulus predictive of receiving 2 food pellets. However, they performed significantly above chance in a nonsymbolic discrimination task with the same apparatus, in which the 2-line stimulus was associated with the same reward but the choice of the 1-line stimulus was not rewarded. We also explored the explanatory value of alternative spatial learning hypotheses such as a Win-Stay, Lose-Shift (WSLS) strategy at the individual level for fish in navigating these spatially randomised tasks. The implications of this for symbolic versus nonsymbolic quantitative cognition in this model system are discussed relative to reward type and stimulus modality.
Non-symbolic number cognition based on an approximate sense of magnitude has been documented in adult zebrafish. Here we investigated the ontogeny of this ability using a group size preference task in juvenile zebrafish. Fish showed group size preference from 26 days post fertilization (dpf) and from 27 dpf fish reliably chose the larger group when presented with discrimination ratios from 1:8 to 2:3. When the ratio between the number of conspecifics in each group was maintained at 1:2, fish could discriminate between 1 vs. 2 individuals and 3 vs. 6, but not when given a choice between 2 vs. 4 individuals. These findings suggest that the systems involved in numerosity representation in fish do not operate separately from other cognitive mechanisms. Rather they suggest numerosity processing is the result of an interplay between attentional, cognitive and memory-related mechanisms that orchestrate numerical competence both in humans and animals. Our results pave the way for the use of zebrafish to explore the genetic and neural processes underlying the ontogeny of number cognition.
Many animals need to process numerical and quantity information in order to survive. Spontaneous quantity discrimination allows differentiation between two or more quantities without reinforcement or prior training on any numerical task. It is useful for assessing food resources, aggressive interactions, predator avoidance and prey choice. Honeybees have previously demonstrated landmark counting, quantity matching, use of numerical rules, quantity discrimination and arithmetic, but have not been tested for spontaneous quantity discrimination. In bees, spontaneous quantity discrimination could be useful when assessing the quantity of flowers available in a patch and thus maximizing foraging efficiency. In the current study, we assessed the spontaneous quantity discrimination behaviour of honeybees. Bees were trained to associate a single yellow artificial flower with sucrose. Bees were then tested for their ability to discriminate between 13 different quantity comparisons of artificial flowers (numeric ratio range: 0.08-0.8). Bees significantly preferred the higher quantity only in comparisons where '1' was the lower quantity and where there was a sufficient magnitudinal distance between quantities (e.g. 1 versus 12, 1 versus 4, and 1 versus 3 but not 1 versus 2). Our results suggest a possible evolutionary benefit to choosing a foraging patch with a higher quantity of flowers when resources are scarce.
Many animal species share the ability to discriminate between sets with different quantity of food items. In fish, this ability has rarely been investigated, although findings have been obtained do indicate a preference, as in other animals, for sets with large over small quantities. The role played by food item size has also been found to be important in the discrimination. However, another potentially important non-numerical variable, food density, has not been investigated. In this study, we examined the influence of density (inter-item distance) in the decision-making process of food discrimination in angelfish (Pterophyllum scalare). In a binary choice task, we kept the number and size of food items constant, but contrasted a set containing food items spaced further apart (sparse set) to another set with food items spaced more closely (dense set). We conducted this analysis with sets in the small (3 vs 3 food items) and in the large number range (5 vs 5 food items) and also varied the specific spatial arrangements of the food items in the sets. Contrary to expectations, angelfish showed a preference for the sparse sets over the dense sets in the five vs five contrasts irrespective of the specific spatial arrangement, but exhibited no preference in case of the three vs three contrasts. Subsequently, we slightly lengthened the inter-item distance in the dense sets, and found preference for the dense over the sparse sets. Last, we further examined the potential effect of spatial configuration of the items in the sets, but found no effect of this latter factor. Overall, these results indicate that higher density of the contrasted food item sets significantly influences choice in angelfish, which prefer denser sets if a clear discriminability of each individual item within the sets is provided.
Animals including humans, fish and honeybees have demonstrated a quantity discrimination threshold at four objects, often known as subitizing elements. Discrimination between numerosities at or above the subitizing range is considered a complex capacity. In the current study, we trained and tested two groups of bees on their ability to differentiate between quantities (4 versus 5 through to 4 versus 8) when trained with different conditioning procedures. Bees trained with appetitive (reward) differential conditioning demonstrated no significant learning of this task, and limited discrimination above the subitizing range. In contrast, bees trained using appetitive-aversive (reward-aversion) differential conditioning demonstrated significant learning and subsequent discrimination of all tested comparisons from 4 versus 5 to 4 versus 8. Our results show conditioning procedure is vital to performance on numerically challenging tasks, and may inform future research on numerical abilities in other animals.
For social animals, group size discrimination may play a major role in setting the trade-off between the costs and benefits of membership. Several anuran tadpoles show different degrees of social aggregation when exposed to the risk of predation. Despite the importance of aggregative behaviour as an anti-predatory response, the mechanism underlying tadpole choice of the group to join to has not been sufficiently investigated. To establish whether visual cues provide sufficient information to enable tadpoles to choose between aggregations differing in size, we explored the abilities of the larvae of two anuran species (green toad Bufotes balearicus and edible frog Pelophylax esculentus) to discriminate among four numerical combinations of conspecific tadpoles (1 vs. 4, 3 vs. 4, 4 vs. 6 and 4 vs. 8), either in the presence or absence of predatory cues. Our results suggest that in anuran larvae the capacity to discriminate between quantities is limited to small numbers (1 vs. 4 for B. balearicus and both 1 vs. 4 and 3 vs. 4 for P. esculentus). Predator-exposed toad tadpoles stayed longer close to the larger group, supporting aggregation as a major anti-predator behaviour in bufonids, while frog tadpoles showed a preference for the smaller groups, though in predator-free trials only, probably associated with lower intra-specific competition.
The ability to distinguish between different quantities of items is fundamental in many ecological contexts, and it has been shown in different animal species. This ability may also be context specific. Quantity estimation in fish has mainly been analysed in the context of social behaviour, whereas a majority of studies conducted with species other than fish tested it in the context of foraging. Surprisingly, little is known about the capacity of fish to discriminate between food quantities, possibly because of difficulties in testing individual fish in a novel, and thus aversive, test environment. Here, we present a novel approach that allowed us to test single angelfish, Pterophyllum scalare, while minimizing isolation-related stress. In binary choice tests, sets composed of similarly sized discrete food items differing in numerical size were presented and the spontaneous (untrained) choice of angelfish was investigated. In all contrasts tested in three experiments, angelfish preferred the numerically larger to the smaller food set. The performance of the fish was ratio dependent in the small but not in the large number range (more than four food items, contrasts that were investigated for the first time in fishes), and there was no significant difference in the magnitude of preference for the small versus the large values. However, overall results indicated that the response was ratio dependent, with an increase in accuracy as the numerical ratio between the contrasts increased. Furthermore, the same numerical ratios that were successfully discriminated with small quantities were also similarly discriminated with large quantities. Altogether, our results thus imply that angelfish utilize the approximate number system of quantity representation for the entire numerical range tested, and that their response is an attempt to maximize foraging success.
The ability to use quantitative information is thought to be adaptive in a wide range of ecological contexts. For nearly a century, the numerical abilities of mammals and birds have been extensively studied using a variety of approaches. However, in the last two decades, there has been increasing interest in investigating the numerical abilities of teleosts (i.e. a large group of ray-finned fish), mainly due to the practical advantages of using fish species as models in laboratory research. Here, we review the current state of the art in this field. In the first part, we highlight some potential ecological functions of numerical abilities in fish and summarize the existing literature that demonstrates numerical abilities in different fish species. In many cases, surprising similarities have been reported among the numerical performance of mammals, birds and fish, raising the question as to whether vertebrates' numerical systems have been inherited from a common ancestor. In the second part, we will focus on what we still need to investigate, specifically the research fields in which the use of fish would be particularly beneficial, such as the genetic bases of numerical abilities, the development of these abilities and the evolutionary foundation of vertebrate number sense.
This article is part of a discussion meeting issue ‘The origins of numerical abilities’.
The zebrafish represents an excellent compromise between system complexity and practical simplicity, features that make it useful for modeling and mechanistic analysis of complex brain disorders. Also promising are screens for psychoactive drugs with effects on larval and adult zebrafish behavior. This review, based upon a recent symposium held at the 2016 IBNS Congress, provides different perspectives on how the zebrafish may be utilized to advance research into human central nervous system disorders. It starts with a discussion on an important bottleneck in zebrafish research, measuring the behavior of this species (specifically shoaling), and continues with examples on research on autism spectrum disorder in larval zebrafish, on screening natural products for compounds with psychoactive properties in adult zebrafish, and on the development of a zebrafish model of fetal alcohol spectrum disorders. By providing information on a broad spectrum of brain disorders, experimental methods, and scientific approaches using both larval and adult zebrafish, the review is intended to showcase this underutilized laboratory species for behavioral neuroscience and psychopharmacology research.
In a social environment, individual behavior is modulated by surrounding observers (a phenomenon known as the audience effect). Here, we used mirrors to test the effect of two audience sizes (one virtual bystander vs. three virtual bystanders) on the aggressive behavior of a focal fish when bystander’s fighting ability was not clear (i.e., information about the ability of virtual conspecifics limited by their mirror images). We found that the Nile tilapia, a cichlid fish, responds to its image as an audience by reducing overt aggression in the presence of larger audience.
Numerical abilities have been demonstrated in a variety of non-human vertebrates. However, underlying biological mechanisms have been difficult to study due to a paucity of experimental tools. Powerful genetic and neurobiological tools already exist for the zebrafish, but numerical abilities remain scarcely explored with this species. Here, we investigate the choice made by single experimental zebrafish between numerically different shoals of conspecifics presented concurrently on opposite sides of the experimental tank. We examined this choice using the AB strain and pet store zebrafish. We found zebrafish of both populations to generally prefer the numerically larger shoal to the smaller one. This preference was significant for contrasted ratios above or equalling 2:1 (i.e. 4 vs. 0, 4 vs. 1, 8 vs. 2, 6 vs. 2 and 6 vs. 3). Interestingly, zebrafish showed no significant preference when each of the two contrasted shoals had at least 4 members, e.g. in a contrast 8 versus 4. These results confirm that zebrafish possess the ability to distinguish larger numbers of items from smaller number of items, in a shoaling context, with a potential limit above 4. Our findings confirm the utility of the zebrafish for the exploration of both the behavioural and the biological mechanisms underlying numerical abilities in vertebrates.
Fathead minnow (FM, Pimephales promelas) are a species of small fish native to North America. Their small size, fast development, and ability to breed in the lab make them an ideal species to use in research, especially in toxicology. Behaviour in general is poorly studied in FM. The aim of this study was to characterize the normal behaviour of fathead minnow at 3 different stages of development in a light-dark box and in a social behaviour test. Fish larvae showed a preference for the light area, and then an increase in dark preference was seen as the fish aged. FM preferred to be with conspecifics at each age, but this preference was much stronger at the adult stage. The time of first entry into the conspecific area was reduced with increasing age of the fish. The time spent in the conspecific area increased between the juvenile and adult stage, and adults stayed more in this area when they entered it. Maturation of behavior in FM was demonstrated in our study. The FM is another good model fish to assess behavioral effects of chemicals, and this study helps to define the appropriate ages for behavioral studies with FM.
Previous studies on relative quantity discrimination in birds and mammals with training procedures have employed hundreds or thousands of trials whereas studies with fish typically use dozens of trials. The goal of this study was to examine whether more extensive training improves the performance of fish tested on stimuli in the small (<4) and large (>4) number range. Goldfish were trained with dot array stimuli using the ratio 0.5 (2 vs. 4, 6 vs.12) across two blocks of training sessions with a total of approximately 1200 trials. They were tested after each block of training sessions with the ratios 0.33 (1 vs. 3, 5 vs. 15), 0.5 (2 vs. 4, 6 vs. 12), and 0.67 (2 vs. 3, 10 vs.15). Performance exceeded 90% correct on both test blocks. Accuracy was not affected by manipulating the surface area, density, or space of stimuli. Performance was best on the ratio 0.5 in test block 1, but ratio-independent in test block 2. There was no difference in performance in the small vs. large number range across the study. These results suggest that fish given extensive training can achieve accuracy on a numerical task comparable to well-trained birds, humans, or non-human primates.
Numerous studies have shown that many animal species can be trained to discriminate between stimuli differing in numerosity. However, in the absence of generalization tests with untrained numerosities, what decision criterion was used by subjects remains unclear: the subjects may succeed by selecting a specific number of items (a criterion over absolute numerosities), or by applying a more general relative numerosity rule, for example, selecting the larger/smaller quantity of items. The latter case may require more powerful representations, supporting judgments of order (“more/less”) beyond simple “same/different” judgments, but a relative numerosity rule may also be more adaptive. In previous research, we showed that guppies (Poecilia reticulata) spontaneously prefer relative numerosity rules. To date it is unclear whether this preference is shared by other fish and, more broadly, other species. Here we compared the performance of angelfish (Pterophyllum scalare) with that of human adults (Homo sapiens) in a task in which subjects were initially trained to select arrays containing 10 dots (either in 5 vs. 10 or 10 vs. 20 comparisons). Subsequently they were tested with the previously trained numerosity and a novel numerosity (respectively, 20 or 5). In the absence of explicit instructions, both species spontaneously favored a relative rule, selecting the novel numerosity. These similarities demonstrate that, beyond shared representations for numerical quantities, vertebrate species may also share a system for taking decisions about quantities.
There is a well-established tradition of studying numerical abilities in mammals and birds; however, only recently has it been proposed that some species of fish possess similar capacities. Here, we review the evidence for the presence of rudimentary numerical abilities in fish. There is substantial evidence that fish can use purely numerical information both when they are trained to discriminate among different quantities and when they are required to choose which group of social companions is less/more numerous. In both contexts, however, they tend to use primarily continuous quantities (e.g., area) instead of discrete numbers when both types of information are available. Similarities among species appear greater than differences and, in general, fish numerical capacities closely match those reported in mammals and birds. The study of developmental trajectories suggests that fish have multiple core number systems that are domain-specific and serve to solve a limited set of problems.
A longstanding question in comparative psychology is whether animals can use quantitative information to guide behavior in ways similar to how humans use that information. The question has been the focus of significant research across many species. In this chapter, we concentrate on some of the broader questions that have been asked, beginning with whether animals can count the way that humans do in the absence of language. We then review some of the research conducted with nonhuman primates demonstrating their abilities to judge quantities and to learn ordinal relations between quantities or symbols representing numbers of items, and that explores the extent to which they fall prey to some of the same visual numerical illusions as humans do. We compare their performances to those of humans in certain tasks, and we conclude by assessing the different mechanisms that have been proposed to support quantitative processes in nonhuman animals.
We examined spontaneous quantity discrimination in untrained domestic cats in three food choice experiments. In Experiment 1, we presented the cats with two different quantities of food in eight numerical combinations. Overall, the subjects chose the larger quantity more often than the smaller one, and significantly so when the ratio between the quantities was less than 0.5. In Experiment 2, we presented the cats with two pieces of food in four different size combinations. Again, subjects chose the larger piece above chance, although not in the combination where the largest item was presented. In Experiment 3, a subset of the cats was presented multiple times with two different quantities of food, which were hidden from view. In this case, the cats did not choose the larger quantity more often than the smaller one, suggesting that in the present experiments they mainly used visual cues when comparing quantities. We conclude that domestic cats are capable of spontaneously discriminating quantities when faced with different numbers or sizes of food items, and we suggest why they may not always be motivated to choose the larger quantity. In doing so, we highlight the advantages of testing spontaneous choice behavior, which is more likely to reflect animals' everyday manner of responding than is the case when training them in order to test their absolute limits of performance which may not always coincide with their daily needs.
The ability to discriminate between different quantities has important ecological relevance for animals when engaging in behaviours such as forming groups, foraging or trying to avoid predators. Quantity discrimination has been shown in a diversity of human and nonhuman animal species. In angelfish this discrimination ability has been investigated using dichotomous choice tests when the numerically different stimulus groups (shoals) of conspecifics were fully visible to the test fish. Here, using a new procedure we investigated whether test fish were able to discriminate between the contrasting shoals using their memory. After a period of full visual access to the contrasted shoals on the two sides of their test tank, the test fish was required to make a choice while being able to see only a single member of the stimulus shoals on each side. With this cognitively more demanding procedure we tested discrimination between numerically large shoals (≥ four fish per stimulus shoal). As in our previous studies, we found that angelfish consistently chose the larger of the two shoals when the shoals differed by a 2:1 or higher ratio, but not those that differed by a 3:2 or 4:3 ratio. The results followed Weber's law in that performance became poorer as the ratio between the two stimulus shoals approached one. In addition, when we kept the absolute difference between the contrasted shoals constant, discrimination was less accurate as the shoal sizes increased. This pattern of results lends support for the analogue magnitude representational system in the angelfish, a nonverbal approximation system believed to be employed by a diversity of human and nonhuman animal species. Furthermore, our results also demonstrate that angelfish remember the different shoals presented to them, i.e. they make their choice based upon mental representation of the different quantities.
It has been hypothesized that cerebral lateralization can significantly enhance cognition and that this was one of the primary selective forces shaping its wide-spread evolution amongst vertebrate taxa. Here we tested this hypothesis by examining the link between cerebral lateralization and numerical discrimination. Guppies, Poecilia reticulata, were sorted into left, right and non-lateralized groups using a standard mirror test and their numerical discrimination abilities tested in both natural shoal choice and abstract contexts. Our results show that strongly lateralized guppies have enhanced numerical abilities compared to non-lateralized guppies irrespective of context. These data provide further credence to the notion that cerebral lateralization can enhance cognitive efficiency.
Previous research has shown that the ratio between competing quantities of food significantly mediates coyotes‘ (Canis latrans) ability to choose the larger of two food options. These previous findings are consistent with predictions made by Weber‘s Law and indicate that coyotes possess quantity discrimination abilities that are similar to other species. Importantly, coyotes‘ discrimination abilities are similar to domestic dogs (Canis lupus familiaris), indicating that quantitative discrimination may remain stable throughout certain species‘ evolution. However, while previously shown in two domestic dogs, it is unknown whether coyotes possess the ability to discriminate visual quantities from memory. Here, we address this question by displaying different ratios of food quantities to 14 coyotes before placing the choices out of sight. The coyotes were then allowed to select one of either non-visible food quantities. Coyotes‘ discrimination of quantity from memory does not follow Weber‘s Law in this particular task. These results suggest that working memory in coyotes may not be adapted to maintain information regarding quantity as well as in domestic dogs. The likelihood of a coyote‘s choosing the large option increased when it was presented with difficult ratios of food options first, before it was later presented with trials using more easily discriminable ratios, and when the large option was placed on one particular side. This suggests that learning or motivation increased across trials when coyotes experienced difficult ratios first, and that location of food may have been more salient in working memory than quantity of food.
The ability to discriminate between sets of items differing in quantity has shown a growing interest in comparative studies as a diversity of animal species exhibit such quantitative competence. Previous studies with angelfish (Pterophyllum scalare) have demonstrated that this species is capable of spontaneously discriminating between fully visible groups (shoals) of conspecifics of different numerical size. In the present study, we investigated quantity discrimination in angelfish adopting a new procedure that we expected to make the task more difficult for the fish. During a pretest period, angelfish were allowed to fully see shoals of conspecifics of different numerical size, subsequently all fish but 1 in each stimulus shoal were hidden behind opaque barriers. Thus, during testing, experimental fish had to rely on their working memory, which implies a certain level of mental representation of the quantities or numbers discriminated. Angelfish chose the larger shoal with similar accuracy when 1 versus 2, 1 versus 3, 1 versus 4, 2 versus 3, and 2 versus 4 stimulus fish were contrasted, but failed to distinguish shoals when 3 versus 4, 4 versus 5, and 4 versus 6 fish were contrasted. Strong similarities were observed in relation with our previous procedure indicating the robustness of the quantity discrimination abilities of this species. Our results imply that angelfish form internal representations and demonstrate that these fish can make comparisons between small quantities of items while relying on their working memory alone. (PsycINFO Database Record (c) 2014 APA, all rights reserved).
Despite considerable research on numeric judgements in animals, uncertainty remains about both the underlying mechanisms and the role of training. To address these issues, we study quantity discrimination in jungle crows that have previously been shown to select the larger of two quantities following training. In the current study, we examined whether jungle crows are able to discriminate between different quantities of food items without prior training. Using a simultaneous two-alternative test, we studied whether their performance reveals the underlying mechanism being used for discrimination of small and large quantities in other animals. First, jungle crows were tested with a choice between two discrete homogeneous quantities; one smaller and one larger, with ratios of 0.5 (1 versus 2, 2 versus 4 and 4 versus 8), 0.67 (2 versus 3, 4 versus 6 and 8 versus 12) and 0.75 (3 versus 4, 6 versus 8 and 12 versus 16). Then, we conducted a test using a non-numerical cue where the volume of comparison stimuli was equalized. Jungle crows selected the larger of the two quantities in all comparisons, except those when both quantities were large (6 versus 8, 8 versus 12 and 12 versus 16). Furthermore, accuracy of selection of the larger quantity declined with increasing numerical magnitude. These results suggest that in a spontaneous discrimination task, jungle crows use an object-file mechanism to compare quantities, even when the number of items in one of the arrays exceeds four. The crows showed no preference for the smaller or the larger quantity when the volume cue was removed. This lack of bias may suggest an ecological role of selection for a patch with higher amounts of food and not necessarily the exact number of food items for optimal foraging in jungle crows.
Fish are one of the most highly utilised vertebrate taxa by humans; they are harvested from wild stocks as part of global fishing industries, grown under intensive aquaculture conditions, are the most common pet and are widely used for scientific research. But fish are seldom afforded the same level of compassion or welfare as warm-blooded vertebrates. Part of the problem is the large gap between people's perception of fish intelligence and the scientific reality. This is an important issue because public perception guides government policy. The perception of an animal's intelligence often drives our decision whether or not to include them in our moral circle. From a welfare perspective, most researchers would suggest that if an animal is sentient, then it can most likely suffer and should therefore be offered some form of formal protection. There has been a debate about fish welfare for decades which centres on the question of whether they are sentient or conscious. The implications for affording the same level of protection to fish as other vertebrates are great, not least because of fishing-related industries. Here, I review the current state of knowledge of fish cognition starting with their sensory perception and moving on to cognition. The review reveals that fish perception and cognitive abilities often match or exceed other vertebrates. A review of the evidence for pain perception strongly suggests that fish experience pain in a manner similar to the rest of the vertebrates. Although scientists cannot provide a definitive answer on the level of consciousness for any non-human vertebrate, the extensive evidence of fish behavioural and cognitive sophistication and pain perception suggests that best practice would be to lend fish the same level of protection as any other vertebrate.
This study examined infants' use of contour length in num- ber discrimination tasks. We systematically varied number and con- tour length in a visual habituation experiment in order to separate these two variables. Sixteen 6- to 8-month-old infants were habituated to displays of either two or three black squares on a page. They were then tested with alternating displays of either a familiar number of squares with a novel contour length or a novel number of squares with a familiar contour length. Infants dishabituated to the display that changed in contour length, but not to the display that changed in num- ber. We conclude that infants base their discriminations on contour length or some other continuous variable that correlates with it, rather than on number.
Single three-spined sticklebacks, Gasterosteus aculeatus, were frightened with a light stimulus simulating an aerial predator while facing a choice between two conspecific display shoals of different membership sizes. We observed which shoal the test fish approached. Initially, both display shoals were equidistant from the test fish. The smaller shoal was then moved gradually closer whereas the larger shoal stayed at a constant distance. This experiment modelled an early stage of the aggregation behaviour of sticklebacks in response to perceived imminent predation risk. When the two display shoals were equidistant from the test fish, we found that the test animal preferred approaching the larger display shoal, and the magnitude of this preference increased with increasing display shoal size difference. This demonstrates that the aggregation behaviour of frightened sticklebacks is density dependent. Further, we found that sticklebacks made a trade-off between the distance to a display shoal and its membership size. In particular, for a given ratio of display shoal sizes, there was a critical distance at which half of all tested animals turned to one and the other half to the other display shoal. This demonstrates that the observed aggregation behaviour is also distance dependent. We introduce several elementary models which formalize individual predation risk and explore how distance and display shoal size contribute to total risk. In particular, we distinguish between total risk as a product or as a sum of the risk components associated with swimming distance and display shoal size, respectively. All models follow the `partial preferences' paradigm of McNamara & Houston (Anim. Behav. 35, 1084-1099 (1987)). We compare how closely these models match the observed data and how well they predict the empirical critical distances. We find a consistent discrepancy between theory and data, which we resolve by invoking a fundamental perceptual limit (numerosity) for shoal size discrimination.
Two decades of comparative studies have shown that numerical abilities are widespread in the animal kingdom. In particular, the ability to select the larger of two quantities (known as the ability to go for “more”) has been found in apes, dolphins, birds and amphibians. This primary quantity discrimination has not been investigated in fish in a systematic way although several studies suggest that fish have the ability to distinguish among different-sized groups of conspecifics. We investigated the spontaneous choice for the larger of two shoals in female goldbelly topminnows (Girardinus falcatus) and compared two different methods to determine an efficient procedure for studying the numerical competence of fish.Female topminnows that were placed in a novel and potentially dangerous environment were found to choose the larger shoal suggesting that the capacity to go for “more” was similar to that observed in other non-verbal animals. The two methods proved not to be sensitive in revealing differences between the numerical contrasts, however, combining the results of the two experiments, we found that when groups differed in size by one individual, choice of the larger shoal was significantly above that predicted by chance up to a maximum of three fish in the larger shoal (i.e. 2 vs 1 and 3 vs 2 but not 4 vs 3 or larger). When presented with two shoals with a 1:2 numerical ratio, choice was significantly above chance in 4 vs 2 but not in 8 vs 4 comparisons.
Breeding birds use a variety of cues to choose a nest site. For conspecific brood parasites, the number of eggs in a host nest may be an important indicator of nest stage (laying or incubating) and the resulting prospective value of the nest. In precocial birds, such as wood ducks (Aix sponsa), a parasitic female should lay her eggs during the host's laying stage to ensure hatch synchrony with the host. Incubation and hatching success may also be compromised in large clutches. Accordingly, parasitic females should respond to the number of eggs already present in a potential hosts' nest and should preferentially lay eggs in nests with smaller clutches. We conducted a field experiment using simulated nests containing different numbers of "host" eggs to test this hypothesis. When offered a choice of nests containing clutches of 5, 10, 15, or 20 eggs, females were significantly more likely to lay eggs in the 5- and 10-egg treatments, laid more eggs in total in the smaller clutch treatments, and were more likely to incubate the nests in the 5- and 10-egg treatments. These results indicate that wood ducks are responsive to quantitative cues, such as the number of eggs in a nest, although we do not yet know if they are able to do so directly by numerical discrimination (i.e., counting). Copyright 2010, Oxford University Press.
A large body of studies has investigated the capacity of non-human primates, dogs, birds and lower vertebrates to estimate
different quantities of objects or events. Little attention, however, has been devoted to felines, and no study has specifically
concentrated on cats’ numerical cognition. The present study aims to investigate the capacity of domestic cats to distinguish
between two and three dots in order to obtain food; results showed that cats can be trained to discriminate between the two
quantities. Furthermore our research suggests that cats do not spontaneously use numerical information, but rather seem to
make use of visual cues that co-vary with numerosity in order to solve the task.
Many animals live in groups most of their life. One function of this behaviour is an increased predator protection whereas
larger groups provide better protection than smaller ones. A causal explanation is that due to a higher number of shoal members
the individual risk of being predated will decrease (“dilution effect”). Additionally, shoaling leads to increased predator
confusion. This “confusion effect” can be strengthened by an increased group density, which often correlates with group size.
Many studies found that individuals prefer the larger of two groups. However, whether this preference is due to a larger group
size or because of an increased density of the larger group remained unclear. To disentangle these factors we gave three-spined
sticklebacks (Gasterosteus aculeatus) the choice between shoals of (1) different group size and density, (2) different group size, but equal density and (3) equal
group sizes, but different densities. As expected, test fish preferred the larger and denser shoal over the smaller, less
dense one. This preference was lost when shoal size differed but density was kept constant. When shoal size was equal but
density differed, test fish preferred the less dense shoal. However, this was only the case when test fish chose between two
relatively dense shoals. On the other hand, when overall density was low, test fish did not discriminate between shoals of
different densities. This result may be explained in terms of predator avoidance. The results show that shoaling preferences
might not always be influenced by a higher number of group members but also by the density and cohesiveness of the respective
groups.
The costs and benefits of group membership vary with the size of groups, and individuals are expected to modify their choice of groups in response to ecological factors such as food availability and predation risk. We experimentally examined context-dependent group size choice in a shoaling fish, the banded killifish, Fundulus diaphanus, by using nondirectional odour cues to simulate a food source or a successful attack by a predator (food or alarm treatments) in the laboratory. Group sizes were significantly smaller in the food treatment and larger in the alarm treatment than in control trials. When presented with food and alarm cues together, fish formed groups that were larger than control groups but smaller than those seen with alarm cues alone. These results are consistent with theoretical predictions based on the known benefits and costs of grouping and with previous laboratory work examining the individual shoal choice behaviour of single fish. To examine possible mechanisms of group formation, we developed an individual-based model of shoaling behaviour in which simulated fish were allowed to modify the area over which they interacted with neighbouring individuals. Group size distributions produced by the model were a good approximation of our experimental data. We suggest that local behavioural interaction rules of this type are a potential mechanism by which fish may individually adjust grouping behaviour without requiring extensive information on the position and movement of all possible shoalmates.
Data on numerical processing by verbal (human) and non-verbal (animal and human) subjects are integrated by the hypothesis that a non-verbal counting process represents discrete (countable) quantities by means of magnitudes with scalar variability. These appear to be identical to the magnitudes that represent continuous (uncountable) quantities such as duration. The magnitudes representing countable quantity are generated by a discrete incrementing process, which defines next magnitudes and yields a discrete ordering. In the case of continuous quantities, the continuous accumulation process does not define next magnitudes, so the ordering is also continuous (‘dense’). The magnitudes representing both countable and uncountable quantity are arithmetically combined in, for example, the computation of the income to be expected from a foraging patch. Thus, on the hypothesis presented here, the primitive machinery for arithmetic processing works with real numbers (magnitudes).
It has been hypothesised that human adults, infants, and non-human primates share two non-verbal systems for enumerating objects, one for representing precisely small quantities (up to 3-4 items) and one for representing approximately larger quantities. Recent studies exploiting fish's spontaneous tendency to join the larger group showed that their ability in numerical discrimination closely resembles that of primates but little is known as to whether these capacities are innate or acquired.
We used the spontaneous tendency to join the larger shoal to study the limits of the quantity discrimination of newborn and juvenile guppies. One-day old fish chose the larger shoal when the choice was between numbers in the small quantity range, 2 vs. 3 fish, but not when they had to choose between large numbers, 4 vs. 8 or 4 vs. 12, although the numerical ratio was larger in the latter case. To investigate the relative role of maturation and experience in large number discrimination, fish were raised in pairs (with no numerical experience) or in large social groups and tested at three ages. Forty-day old guppies from both treatments were able to discriminate 4 vs. 8 fish while at 20 days this was only observed in fish grown in groups. Control experiments showed that these capacities were maintained after guppies were prevented from using non numerical perceptual variables that co-vary with numerosity.
Overall, our results suggest the ability of guppies to discriminate small numbers is innate and is displayed immediately at birth while discrimination of large numbers emerges later as a result of both maturation and social experience. This developmental dissociation suggests that fish like primates might have separate systems for small and large number representation.
In conflicts between social groups, the decision of competitors whether to attack/retreat should be based on the assessment of the quantity of individuals in their own and the opposing group. Experimental studies on numerical cognition in animals suggest that they may represent both large and small numbers as noisy mental magnitudes subject to scalar variability, and small numbers (≤4) also as discrete object-files. Consequently, discriminating between large quantities, but not between smaller ones, should become easier as the asymmetry between quantities increases. Here, we tested these hypotheses by recording naturally occurring conflicts in a population of free-ranging dogs, Canis lupus familiaris, living in a suburban environment. The overall probability of at least one pack member approaching opponents aggressively increased with a decreasing ratio of the number of rivals to that of companions. Moreover, the probability that more than half of the pack members withdrew from a conflict increased when this ratio increased. The skill of dogs in correctly assessing relative group size appeared to improve with increasing the asymmetry in size when at least one pack comprised more than four individuals, and appeared affected to a lesser extent by group size asymmetries when dogs had to compare only small numbers. These results provide the first indications that a representation of quantity based on noisy mental magnitudes may be involved in the assessment of opponents in intergroup conflicts and leave open the possibility that an additional, more precise mechanism may operate with small numbers.
Although young infants have repeatedly demonstrated successful numerosity discrimination across large sets when the number of items in the sets changes twofold (E. M. Brannon, S. Abbott, & D. J. Lutz, 2004; J. N. Wood & E. S. Spelke, 2005; F. Xu & E. S. Spelke, 2000), they consistently fail to discriminate a twofold change in number when one set is large and the other is small (<4 items; F. Feigenson, S. Carey, & M. Hauser, 2002; F. Xu, 2003). It has been theorized that this failure reflects an incompatibility in representational systems for small and large sets. The authors investigated the ability of 7-month-old infants to compare small and large sets over a variety of conditions. Results reveal that infants can successfully discriminate small from large sets when given a fourfold change, but not a twofold change, in number. The implications of these results are discussed in light of current theories of number representation.
Newly hatched domestic chicks were reared with five identical objects. On days 3 or 4, chicks underwent free-choice tests in which sets of three and two of the five original objects disappeared (either simultaneously or one by one), each behind one of two opaque identical screens. Chicks spontaneously inspected the screen occluding the larger set (experiment 1). Results were confirmed under conditions controlling for continuous variables (total surface area or contour length; experiment 2). In the third experiment, after the initial disappearance of the two sets (first event, FE), some of the objects were visibly transferred, one by one, from one screen to the other (second event, SE). Thus, computation of a series of subsequent additions or subtractions of elements that appeared and disappeared, one by one, was needed in order to perform the task successfully. Chicks spontaneously chose the screen, hiding the larger number of elements at the end of the SE, irrespective of the directional cues provided by the initial (FE) and final (SE) displacements. Results suggest impressive proto-arithmetic capacities in the young and relatively inexperienced chicks of this precocial species.
Research on human infants, mammals, birds and fish has demonstrated that rudimentary numerical abilities pre-date the evolution of human language. Yet there is controversy as to whether animals represent numbers mentally or rather base their judgments on non-numerical perceptual variables that co-vary with numerosity. To date, mental representation of number has been convincingly documented only for a few mammals.
Here we used a training procedure to investigate whether mosquitofish could learn to discriminate between two and three objects even when denied access to non-numerical information. In the first experiment, fish were trained to discriminate between two sets of geometric figures. These varied in shape, size, brightness and distance, but no control for non-numerical variables was made. Subjects were then re-tested while controlling for one non-numerical variable at a time. Total luminance of the stimuli and the sum of perimeter of figures appeared irrelevant, but performance dropped to chance level when stimuli were matched for the cumulative surface area or for the overall space occupied by the arrays, indicating that these latter cues had been spontaneously used by the fish during the learning process. In a second experiment, where the task consisted of discriminating 2 vs 3 elements with all non-numerical variables simultaneously controlled for, all subjects proved able to learn the discrimination, and interestingly they did not make more errors than the fish in Experiment 1 that could access non-numerical information in order to accomplish the task.
Mosquitofish can learn to discriminate small quantities, even when non-numerical indicators of quantity are unavailable, hence providing the first evidence that fish, like primates, can use numbers. As in humans and non-human primates, genuine counting appears to be a 'last resort' strategy in fish, when no other perceptual mechanism may suggest the quantity of the elements. However, our data suggest that, at least in fish, the priority of perceptual over numerical information is not related to a greater cognitive load imposed by direct numerical computation.
Numerosity discrimination, the ability to distinguish between sets with more and less items, is recognised as the foundation for higher numerical abilities. Understanding numerosity discrimination from a comparative perspective is hence pivotal in tracing the evolution of numerical representation systems. However, numerosity discrimination has been well studied only in vertebrates, where two innate systems of number representation have been described: an 'analog magnitude system' used to discriminate among numerosities by representing them as cardinal magnitudes and a 'parallel individualisation system' that allows precise discrimination among small arrays of items (< or =4) by representing objects individually. We investigated the existence of quantity discrimination in an insect species (Tenebrio molitor) by using a spontaneous two-choice procedure in which males were exposed to substrates bearing odours from different numbers of females (< or =4) in increasing numerosity ratios (1:4, 1:3 and 1:2). We show that males can discriminate sources of odours reflecting 1 versus 4 and 1 versus 3 females, but not 2 versus 4 or 1 versus 2, indicating that T. molitor males exhibit a marked preference for sources reflecting more female donors only when numerosity ratios are below 1:2. We discuss the functional significance of this finding and whether our pattern of results could be best explained by summation of a non-numerical continuous variable or by the existence of a numerosity discrimination mechanism with an operational signature ratio of 1:2.
Human mathematical competence emerges from two representational systems. Competence in some domains of mathematics, such as calculus, relies on symbolic representations that are unique to humans who have undergone explicit teaching. More basic numerical intuitions are supported by an evolutionarily ancient approximate number system that is shared by adults, infants and non-human animals-these groups can all represent the approximate number of items in visual or auditory arrays without verbally counting, and use this capacity to guide everyday behaviour such as foraging. Despite the widespread nature of the approximate number system both across species and across development, it is not known whether some individuals have a more precise non-verbal 'number sense' than others. Furthermore, the extent to which this system interfaces with the formal, symbolic maths abilities that humans acquire by explicit instruction remains unknown. Here we show that there are large individual differences in the non-verbal approximation abilities of 14-year-old children, and that these individual differences in the present correlate with children's past scores on standardized maths achievement tests, extending all the way back to kindergarten. Moreover, this correlation remains significant when controlling for individual differences in other cognitive and performance factors. Our results show that individual differences in achievement in school mathematics are related to individual differences in the acuity of an evolutionarily ancient, unlearned approximate number sense. Further research will determine whether early differences in number sense acuity affect later maths learning, whether maths education enhances number sense acuity, and the extent to which tertiary factors can affect both.
A wealth of research in infants and animals demonstrates discrimination of quantities, in some cases nonverbal numerical perception, and even elementary calculation capacities. We investigated the ability of three African grey parrots (Psittacus erithacus) to select the largest amount of food between two sets, either discrete food items (experiment 1) or as volume of a food substance (experiment 2). The two amounts were presented simultaneously and were visible at the time of choice. Parrots were tested several times for all possible combinations between 1 and 5 seeds or 0.2 and 1 ml of food substance. In both conditions, subjects performed above chance for almost all combinations. Accuracy was negatively correlated with the ratio, that is performance improved with greater differences between amounts. Therefore, these results with both individual items and volume discrimination suggest that parrots use an analogue of magnitude, rather than object-file mechanisms to quantify items and substances.
Chicks were trained to discriminate small sets of identical elements. They were then tested for choices (unrewarded) between sets of similar numerosities, when continuous physical variables such as spatial distribution, contour length, and overall surface were equalized. In all conditions chicks discriminated one versus two and two versus three stimulus sets. Similar results were obtained when elements were presented under conditions of partial occlusion. In contrast, with sets of four versus five, four versus six, and three versus four elements chicks seemed unable to discriminate on the basis of number, although nonnumerical discrimination based on perceptual cues was observed. This adds to increasing evidence for discrimination of small numerosities of up to three elements in human infants and nonhuman animals.
"Subitizing," the process of enumeration when there are fewer than 4 items, is rapid (40-100 ms/item), effortless, and accurate. "Counting," the process of enumeration when there are more than 4 items, is slow (250-350 ms/item), effortful, and error-prone. Why is there a difference in the way the small and large numbers of items are enumerated? A theory of enumeration is proposed that emerges from a general theory of vision, yet explains the numeric abilities of preverbal infants, children, and adults. We argue that subitizing exploits a limited-capacity parallel mechanism for item individuation, the FINST mechanism, associated with the multiple target tracking task (Pylyshyn, 1989; Pylyshyn & Storm, 1988). Two kinds of evidence support the claim that subitizing relies on preattentive information, whereas counting requires spatial attention. First, whenever spatial attention is needed to compute a spatial relation (cf. Ullman, 1984) or to perform feature integration (cf. Treisman & Gelade, 1980), subitizing does not occur (Trick & Pylyshyn, 1993a). Second, the position of the attentional focus, as manipulated by cue validity, has a greater effect on counting than subitizing latencies (Trick & Pylyshyn, 1993b).
Data on numerical processing by verbal (human) and non-verbal (animal and human) subjects are integrated by the hypothesis that a non-verbal counting process represents discrete (countable) quantities by means of magnitudes with scalar variability. These appear to be identical to the magnitudes that represent continuous (uncountable) quantities such as duration. The magnitudes representing countable quantity are generated by a discrete incrementing process, which defines next magnitudes and yields a discrete ordering. In the case of continuous quantities, the continuous accumulation process does not define next magnitudes, so the ordering is also continuous ('dense'). The magnitudes representing both countable and uncountable quantity are arithmetically combined in, for example, the computation of the income to be expected from a foraging patch. Thus, on the hypothesis presented here, the primitive machinery for arithmetic processing works with real numbers (magnitudes).
Seven studies explored the empirical basis for claims that infants represent cardinal values of small sets of objects. Many studies investigating numerical ability did not properly control for continuous stimulus properties such as surface area, volume, contour length, or dimensions that correlate with these properties. Experiment 1 extended the standard habituation/dishabituation paradigm to a 1 vs 2 comparison with three-dimensional objects and confirmed that when number and total front surface area are confounded, infants discriminate the arrays. Experiment 2 revealed that infants dishabituated to a change in front surface area but not to a change in number when the two variables were pitted against each other. Experiments 3 through 5 revealed no sensitivity to number when front surface area was controlled, and Experiments 6 and 7 extended this pattern of findings to the Wynn (1992) transformation task. Infants' lack of a response to number, combined with their demonstrated sensitivity to one or more dimensions of continuous extent, supports the hypothesis that the representations subserving object-based attention, rather than those subserving enumeration, underlie performance in the above tasks.
Techniques traditionally used in developmental research with infants have been widely used with nonhuman primates in the investigation of comparative cognitive abilities. Recently, researchers have shown that human infants and monkeys select the larger of two numerosities in a spontaneous forced-choice discrimination task. Here we adopt the same method to assess in a series of experiments spontaneous choice of the larger of two numerosities in a species of amphibian, red-backed salamanders ( Plethodon cinereus). The findings indicate that salamanders "go for more," just like human babies and monkeys. This rudimentary capacity is a type of numerical discrimination that is spontaneously present in this amphibian.
A bottlenose dolphin was trained to discriminate two simultaneously presented stimuli differing in numerosity (defined by the number of constituent elements). After responding correctly to stimuli consisting of three-dimensional objects, the dolphin transferred to two-dimensional stimuli. Initially, a variety of stimulus parameters covaried with the numerosity feature. By systematically controlling for these stimulus parameters, it was demonstrated that some of these attributes, such as element configuration and overall brightness, affected the animal's discrimination performance. However, after all the confounding parameters were under control, the dolphin was able to discriminate the stimuli exclusively on the basis of the numerosity feature. The animal then achieved a successful transfer to novel numerosities, both intervening numerosities and numerosities outside the former range. These findings provide substantial evidence that the dolphin could base his behavior on the numerosity of a set independently of its other attributes and that he represented ordinal relations among numerosities.
''Subitizing,'' the process of enumeration when there are fewer than 4 items, is rapid (40-100 ms/item), effortless, and accurate. ''Counting' the process of enumeration when there are more than 4 items, is slow (250-350 ms/item), effortful, and error-prone. Why is there a difference in the way the small and large numbers of items are enumerated? A theory of enumeration is proposed that emerges from a general theory of vision, yet explains the numeric abilities of preverbal infants, children, and adults. We argue that subitizing exploits a limited-capacity parallel mechanism for item individuation, the FINST mechanism, associated with the multiple target tracking task (Pylyshyn, 1989; Pylyshyn & Storm, 1988). Two kinds of evidence support the claim that subitizing relies on preattentive information, whereas counting requires spatial attention. First, whenever spatial attention is needed to compute a spatial relation (cf. Ullman, 1984) or to perform feature integration (cf. Treisman & Gelade, 1980), subitizing does not occur (Trick & Pylyshyn, 1993a). Second, the position of the attentional focus, as manipulated by cue validity, has a greater effect on counting than subitizing latencies (Trick & Pylyshyn, 1993b).
Male chimpanzees, Pan troglodytes, engage in cooperative territorial defence and sometimes kill members of neighbouring communities. Observations of intergroup interactions suggest that escalation of aggression depends on numerical assessment, with lethal attacks occurring when numerical advantage reduces the costs of attacking. To gain a better understanding of the factors guiding participation in intergroup conflict, we conducted a series of playback experiments with the Kanyawara chimpanzee community of the Kibale National Park, Uganda. We tested whether the response to the playback of the 'pant-hoot' call of a single extragroup male depended on the number of adult males in the listening party, the location of the speaker relative to the territory edge, and each male's agonistic rank. These playbacks elicited cooperative responses, with the nature of the response depending on the number of adult males in the party. Parties with three or more males consistently joined in a chorus of loud vocalizations and approached the speaker together. Parties with fewer adult males usually stayed silent, approached the speaker less often, and travelled more slowly if they did approach. In contrast to many territorial species, the location of the simulated intruder did not affect the response. Although high-ranking males might be expected to benefit more from repelling outside males, both high-and low-ranking males showed a similar pattern of response. Each male responded as if he benefited from repelling intruders, but only if he had strength in numbers. This pattern of response is consistent with cooperation based on mutualism.
Shoaling fish are expected, in many cases, to gain fitness benefits from being in a larger shoal and previous experiments have shown that fish are indeed capable of choosing between shoals of different sizes. We investigated the influence of shoal activity on shoal size preference in the zebrafish. We gave test fish the choice between shoals of one to four stimulus fish, presented at two different water temperatures, and so differing in their activity levels. Where all stimulus fish were in water of the same temperature, test fish generally preferred the larger shoal. However, this preference could be reduced by presenting the larger shoal in colder water and so reducing its activity. We discuss these findings with reference to the factors that may influence shoal activity, the effect of temperature on shoaling behaviour and the mechanisms that may be used by fish to discriminate shoal size.
What are the brain and cognitive systems that allow humans to play baseball, compute square roots, cook souffl茅s, or navigate the Tokyo subways? It may seem that studies of human infants and of non-human animals will tell us little about these abilities, because only educated, enculturated human adults engage in organized games, formal mathematics, gourmet cooking, or map-reading. In this chapter, we argue against this seemingly sensible conclusion. When human adults exhibit complex, uniquely human, culture-specific skills, they draw on a set of psychological and neural mechanisms with two distinctive properties: they evolved before humanity and thus are shared with other animals, and they emerge early in human development and thus are common to infants, children, and adults. These core knowledge systems form the building blocks for uniquely human skills. Without them we wouldn't be able to learn about different kinds of games, mathematics, cooking, or maps. To understand what is special about human intelligence, therefore, we must study both the core knowledge systems on which it rests and the mechanisms by which these systems are orchestrated to permit new kinds of concepts and cognitive processes. What is core knowledge? A wealth of research on non-human primates and on human infants suggests that a system of core knowledge is characterized by four properties (Hauser, 2000; Spelke, 2000). First, it is domain-specific: each system functions to represent particular kinds of entities such as conspecific agents, manipulable objects, places in the environmental layout, and numerosities. Second, it is task-specific: each system uses its representations to address specific questions about the world, such as "who is this?" [face recognition], "what does this do?" [categorization of artifacts], "where am I?" [spatial orientation], and "how many are here?" [enumeration]. Third, it is relatively encapsulated: each uses only a subset of the information delivered by an animal's input systems and sends information only to a subset of the animal's output systems. Finally, the system is relatively automatic and impervious to explicitly held beliefs and goals,. In this chapter, we use the domain of number to illustrate how core knowledge systems are assembled to permit uniquely human cognitive advances. We consider first two lines of research that elucidate core knowledge systems: comparative evolutionary studies and studies of human development. These studies provide evidence for two core knowledge systems that
Predators and food are the keys to understanding fish shoals; synchronised co-operation defeats predators, and optimal food gathering in shoals reflects a shifting balance between joining, competing in, or leaving the group. In the wild, predators may arrive while shoaling fish are feeding, and so vigilance is a crucial behaviour. Once detected, predator defence takes precedence over feeding, since an animal’s life is worth more than today’s dinner.
Individual white and black mollies Poecilia latipinna spent significantly more time near the larger group when given the choice between two shoals of similar colouration to themselves. When given the choice between a large and a small shoal of dissimilar colouration to themselves, black test fish spent significantly more time with the larger shoal while white test fish showed no preference for either group. Both white and black mollies chose the smaller of two shoals when given the choice between a large dissimilarly coloured shoal and a small similarly coloured shoal. The results indicate that mollies actively discriminate between shoals on the basis of both body colour and shoal size. However, body colour segregation appears to have a stronger influence on shoal choice.
Abstract Past research has shown that angelfish, Pterophyllum scalare, are capable of discriminating between shoals composed of familiar dominant and subordinate companions, whereas they show no preference for shoals of unfamiliar conspecifics. In this study, the relative importance of familiarity and social status (shoal factors) on the shoaling decision of juvenile angelfish, which also differed in social status (individual factor), was investigated as very little is known about such tradeoffs in fishes. Dominant and subordinate individuals were given the choice to shoal with a group of conspecifics composed of familiar dominants vs. unfamiliar dominants and composed of familiar subordinates vs. unfamiliar subordinates. The findings demonstrate that fish with different social status differed in their shoaling preference. Subordinate test fish showed a preferential association with familiar subordinates over unfamiliar subordinates, but preferred the unfamiliar shoal over the familiar one when both shoals were constituted by dominant individuals. The shoaling behaviour shown by dominant test fish, on the other hand, indicated no significant preference for any of the shoals regardless of their composition. A replicate preference test carried out 2 h 30 min after the first one indicated that the association pattern was relatively consistent. Results suggest that angelfish are able to differentiate between the stimulus shoals and demonstrate that the pervasive influence of familiarity on the shoaling decision may be restrained or overridden by the composition of the familiar shoals and the social status of the test fish.
Even though females are usually more selective in choosing their mates, males are also capable of exercising mate choice. Despite the large body of evidence on the individual features preferred in sexual selection, little attention has been devoted to the first stage of male–female interaction. As poeciliid fish are known to be social, in the wild, initially mate choice may concern a preliminary selection among shoals. Only after this primary choice, males may subsequently direct their attention to a specific mate. We observed spontaneous preference of male mosquitofish (Gambusia holbrooki) when choosing between groups differing in size and sex ratio. In partial agreement with our predictions, males preferred to join a group of females rather than an isolated one (expt 1) and the larger group when two female groups were presented (expt 2). An all-female group was preferred to a mixed-sex group (expt 3), whereas no preference was observed when the two mixed-sex groups differed in the number of males (expt 4) or in the size of the males (expt 5). These results suggest that male mosquitofish are capable of discriminating among different quantities of individuals within a group and use such information to select among groups in order to optimize the likelihood of successful matings.
Individuals obtain a range of benefits from aggregating with others. Evidence suggests that group size and the phenotypic characteristics of potential group mates are both important attributes influencing the grouping decisions of prospective group members. Their interaction, however, remains poorly understood. Here we investigate, in a series of dichotomous choice experiments, what happens when a preference with respect to group size is pitted against a preference for assorting on the basis of body size in swordtail fish. When controlled for body size, we found that swordtails preferred to associate with a larger shoal. When controlled for group size, we found that swordtails preferred to associate with similar-sized fish. However, when offered the choice between a single size-matched shoaling partner and four dissimilar-sized shoal mates, we found that females associated randomly. Our results suggest that, depending on context, group size and body size may be equally important in guiding the aggregating decisions of swordtails. Future studies will likely offer valuable insights into shoaling behaviour by considering how, and under what circumstances, different grouping preferences might be prioritized.
Whether predators always attack the most vulnerable prey or simply attack prey that exceeds a minimum vulnerability level is an important question to answer in furthering our understanding of predator and antipredation behaviour. Predators may attack any reasonably vulnerable prey rather than waste time identifying the most vulnerable prey, particularly when prey can respond quickly to alter their vulnerability in response to a predator. We tested whether sparrowhawks always choose to attack the group of prey that maximises their capture probability, or whether they simply attack any group above a minimum vulnerability. We modelled sparrowhawk attack success when hunting redshanks using data from three winters and found that probability of capture increased when group size or distance to predator-concealing cover decreased. We then used this model to predict the relative vulnerability to capture of redshank groups occurring in pairs in a fourth winter and found that sparrowhawks attacked the most vulnerable prey group twice as often as not (66% n=59 pairs). When sparrowhawks attacked the less vulnerable group, there was no tendency for both groups to be particularly vulnerable or for the difference in the vulnerability between the two groups to be relatively small. This suggests that, while sparrowhawks do on average attack the most vulnerable group available, they consider other factors that affect vulnerability or that additional factors lead them to also attack opportunistically. This suggests that there will be selection for the predator to monitor a large number of prey individuals and groups and for prey to have the ability to monitor the behaviour of conspecifics in the same and different groups so that they can assess relative vulnerability.