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The Effects of Domestication on Brain Size

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... Carnivorans are divided into two main suborders, Caniformia and Feliformia, both of which include species that were domesticated, which has been suggested to alter the relationship between brain and body size (Kruska, 2007). In phylogenetic terms, carnivorans are closely related to artiodactyls (Bininda-Emonds et al., 2007), animals that the large meat-eating carnivorans prey upon. ...
... Comparisons of the brain mass vs. body mass relationship between domesticated and wild species often yield parallel lines with identical slopes, which have been interpreted as decreased brain size in domesticated animals -that is, a downward shift in the relationship (Kruska, 2007). One should keep in mind, however, that a lateral shift in the relationship is equally possible, with domestication inducing larger body masses rather than decreased brain mass (which would be expected due to greater food availability in captivity). ...
Article
Carnivorans are a diverse group of mammals that includes carnivorous, omnivorous and herbivorous, domesticated and wild species, with a large range of brain sizes. Carnivory is one of several factors expected to be cognitively demanding for carnivorans due to a requirement to outsmart larger prey. On the other hand, large carnivoran species have high hunting costs and unreliable feeding patterns, which, given the high metabolic cost of brain neurons, might put them at risk of metabolic constraints regarding how many brain neurons they can afford, especially in the cerebral cortex. For a given cortical size, do carnivoran species have more cortical neurons than the herbivorous species they prey upon? We find they do not; carnivorans (cat, mongoose, dog, hyena, lion) share with non-primates, including artiodactyls (the typical prey of large carnivorans), roughly the same relationship between cortical mass and number of neurons, which suggests that carnivorans are subject to the same evolutionary scaling rules as other non-primate clades. However, there are a few important exceptions. Carnivorans stand out in that the usual relationship between larger body, larger cortical mass and larger number of cortical neurons only applies to small and medium-sized species, and not beyond dogs: we find that the golden retriever dog has more cortical neurons than the striped hyena, African lion and even brown bear, even though the latter species have up to three times larger cortices than dogs. Remarkably, the brown bear cerebral cortex, the largest examined, only has as many neurons as the ten times smaller cat cerebral cortex, although it does have the expected ten times as many non-neuronal cells in the cerebral cortex compared to the cat. We also find that raccoons have dog-like numbers of neurons in their cat-sized brain, which makes them comparable to primates in neuronal density. Frontiers in Neuroanatomy | www.frontiersin.org 1 December 2017 | Volume 11 | Article 118 Jardim-Messeder et al. Dogs Have the Most Neurons Comparison of domestic and wild species suggests that the neuronal composition of carnivoran brains is not affected by domestication. Instead, large carnivorans appear to be particularly vulnerable to metabolic constraints that impose a trade-off between body size and number of cortical neurons.
... As brain size in animals generally increases with body weight, it could be assumed that body weight and brain size are tightly related. For this reason evolutionary and allometric studies in animals frequently base on relative brain size rather than absolute brain size [41,43]. Because of the difference in the total brain size, comparison of "absolute" volumes of the segmented brain structures was not very informative: virtually all brain structures were bigger in Wistar rats than in WWCPS. ...
... In domesticated animals, the components of the olfactory system (in particular the olfactory bulbs) are usually reduced. [43,47]. ...
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Selective breeding of laboratory rats resulted in changes of their behavior. Concomitantly, the albino strains developed vision related pathologies. These alterations certainly occurred on the background of modifications in brain morphology. The aim of the study was to assess and compare volumes of major structures in brains of wild-captive, laboratory albino and laboratory pigmented rats. High resolution T2-weighted images of brains of adult male Warsaw Wild Captive Pisula-Stryjek rats (WWCPS, a model of wild type), laboratory pigmented (Brown Norway strain, BN) and albino rats (Wistar strain, WI) were obtained with a 7T small animal-dedicated magnetic resonance tomograph. Volume quantification of whole brains and 50 brain structures within each brain were performed with the digital Schwarz rat brain atlas and a custom-made MATLAB/SPM8 scripts. Brain volumes were scaled to body mass, whereas volumes of brain structures were normalized to individual brain volumes. Normalized brain volume was similar in WWCPS and BN, but lower in WI. Normalized neo-cortex volume was smaller in both laboratory strains than in WWCPS and the visual cortex was smaller in albino WI rats than in WWCPS and BN. Relative volumes of phylogenetically older structures, such as hippocampus, amygdala, nucleus accumbens and olfactory nuclei, also displayed certain strain-related differences. The present data shows that selective breeding of laboratory rats markedly affected brain morphology, the neocortex being most significantly altered. In particular, albino rats display reduced volume of the visual cortex, possibly related to retinal degeneration and the development of blindness.
... The problem of the effects of domestication on behavior is of interest as we have learned in the past decade that domesticated animals have reduced brain size and brain regions compared to wild animals of the same species Kruska [29]. This problem is of importance, as it relates to the domestication of ourselves Wilson [30]. ...
... Today, all individuals of this sub-species are born of captive horses. Several authors have shown that domestication generally leads to a decrease in the size of the braincase and the brain in different species, without a return to feralization (Gorgas 1966;Kruska 1973Kruska , 1982Kruska , 1987Kruska , 1988Kruska , 2005Kruska , 2007Ebinger 1993, 1998). Even though Przewalski's horse was never domesticated in a similar way to the common domestic horse, its captivity did last for a century. ...
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Intraspecific variation of endocranial structures is not widely studied in most mammals, particularly fossil mammals, which are mainly represented by a few preserved crania. However, a description of this variation is necessary to be able to study fossil mammals from an ecological and phylogenetic perspective. To facilitate further analyses on fossil equoids, digital reconstructions of the cranial endocast, petrosal bone, and bony labyrinth were created based on CT scans, taken from a wild population of 12 Equus caballus przewalskii currently being monitored. Using descriptive, biometric, and morphometric analyses, an unsuspected range of intraspecific variation for 40 endocranial characters is revealed. Intraindividual variation can be further understood through the comparison of paired organs from a single individual. These results prompt cautious consideration of these characters, as well as an index for the determination of hearing abilities or encephalization quotients. Thanks to this work, more is now known about the intraspecific variation of the external morphology of the most frequently studied structures in the endocranium of mammals and more specifically in equoids, where no such study had been undertaken until now. This will help to improve the resolution of fossil endocranial studies.
... One of the best documented cases is in the platypus lineage, where more highly encephalized Cenozoic fossils indicate reduction of both the olfactory bulb and overall EQ in the evolution of the living platypus Ornithorhynchus (Macrini et al., 2006). Decreases in encephalization are also associated with domestication in various mammalian species (Kruska, 2007). ...
Article
Fossils of mammals and their extinct relatives among cynodonts give evidence of correlated transformations affecting olfaction as well as mastication, head movement, and ventilation, and suggest evolutionary coupling of these seemingly separate anatomical regions into a larger integrated system of ortho-retronasal olfaction. Evidence from paleontology and physiology suggests that ortho-retronasal olfaction played a critical role at three stages of mammalian cortical evolution: early mammalian brain development was driven in part by ortho-nasal olfaction; the bauplan for neocortex had higher-level association functions derived from olfactory cortex; and human cortical evolution was enhanced by ortho-retronasal smell. This article is protected by copyright. All rights reserved. © 2015 Wiley Periodicals, Inc.
... It is therefore remarkable that in the evolution of mammalian brains, which have tended to increase in mass over the last 65 million years (Jerison, 1973), the allometric relationships that apply across species do not also apply across individuals of a single species. As reviewed by Armstrong (1990), while mammalian species with larger bodies tend to have larger brains, larger individuals of a same species do not necessarily have larger brains-or do so with a much smaller allometric exponent (Kruska, 2007). ...
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There is a strong trend toward increased brain size in mammalian evolution, with larger brains composed of more and larger neurons than smaller brains across species within each mammalian order. Does the evolution of increased numbers of brain neurons, and thus larger brain size, occur simply through the selection of individuals with more and larger neurons, and thus larger brains, within a population? That is, do individuals with larger brains also have more, and larger, neurons than individuals with smaller brains, such that allometric relationships across species are simply an extension of intraspecific scaling? Here we show that this is not the case across adult male mice of a similar age. Rather, increased numbers of neurons across individuals are accompanied by increased numbers of other cells and smaller average cell size of both types, in a trade-off that explains how increased brain mass does not necessarily ensue. Fundamental regulatory mechanisms thus must exist that tie numbers of neurons to numbers of other cells and to average cell size within individual brains. Finally, our results indicate that changes in brain size in evolution are not an extension of individual variation in numbers of neurons, but rather occur through step changes that must simultaneously increase numbers of neurons and cause cell size to increase, rather than decrease.
... One of the best documented cases is in the platypus lineage, where more highly encephalized Cenozoic fossils indicate that a 10% reduction of both the olfactory bulb and overall EQ occurred as the living platypus Ornithorhynchus adapted to its semiaquatic habitus (Macrini et al., 2006). Decreases in encephalization are also associated with domestication in various mammalian species (Kruska, 2007). ...
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Paleontology affords a special signal on evolution of neurosensory systems because many parts of that system require rigid skeletal armatures to function properly. This relationship is traced from the ancestral amniote to the origin of Mammalia. Based on inferences about the skeleton and neurosensory system in the ancestral amniote, evolution of correlated traits is traced that culminated in the origin of Mammalia, establishing a bauplan for subsequent diversification. Peripheral sensory arrays profoundly impacted evolution of the highly encephalized mammalian brain and the emergence of neocortex. The dominant pattern is a cascade following an order-of-magnitude increase in olfactory odorant receptor genes. Hypertrophied innervation of the dentition produced a virtually new peripheral array, and origin of the pelt was also influential. The proximate ancestors of mammals were miniaturized and immersed in a rich new source of information from microhabitats dominated to unprecedented degrees by scents and odors. Many peripheral influences became integrated and centralized with the emergence of orthoretronasal olfaction. Several discrete pulses in encephalization are recorded in the fossil record. Once the brain reached an encephalization quotient of ∼0.5, multiple independent increases in encephalization followed. Variational modality of the brain thus shifted from virtual stasis to repeated independent episodes of encephalization. Independent increases in brain size, particularly in neocortical surface area, would become a dominant pattern in mammalian diversification. Numerous mechanisms have been postulated as driving the origin of Mammalia including enlargement of the brain, emergence of neocortex, fur, endothermy, nocturnality, parental care, miniaturization, enhanced olfaction, and hearing. All were influential at certain periods in pan-mammalian history, but most originated prior to the origin of Mammalia, and others only thereafter.
... An example of this was published in Evolutionary Anthropology in 2004 (Neufeld, Conroy, 2004) where a type of human hair was considered to be unique to all mammals. On examination, however, it was demonstrated (Caldararo, 2005) that the authors had overlooked the fact that humans display several types of hair and that the type focused on by the authors (Neufeld, Conroy, 2004) was likely a product of selection due to domestication (Kruska, 2007) or selfdomestication (Caldararo, 2017). Recent traits in humans may also be such products, not only of domestication, as seen in other animals, but of selective pressures of recent patterns of adaptation to sedentary life (e.g. ...
... One conundrum, if we assume that language emerged with early Homo sapiens 250,000 years ago, is what did we do with language until civilizations started to emerge 200,000 years later. Our solution is that the emergence of language may be associated with the reduced brain size in Homo sapiens that started about 50,000 years ago and more markedly 10,000 years ago [75][76][77]. More recent statistical analyses find an even more recent changepoint as close as 3000BP [78], which may be partly due to data selection and using a linear regression model. ...
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We suggest a later timeline for full language capabilities in Homo sapiens, placing the emergence of language over 200,000 years after the emergence of our species. The late Paleolithic period saw several significant changes. Homo sapiens became more gracile and gradually lost significant brain volumes. Detailed realistic cave paintings disappeared completely, and iconic/symbolic ones appeared at other sites. This may indicate a shift in perceptual abilities, away from an accurate perception of the present. Language in modern humans interact with vision. One example is the McGurk effect. Studies show that artistic abilities may improve when language-related brain areas are damaged or temporarily knocked out. Language relies on many pre-existing non-linguistic functions. We suggest that an overwhelming flow of perceptual information, vision, in particular, was an obstacle to language, as is sometimes implied in autism with relative language impairment. We systematically review the recent research literature investigating the relationship between language and perception. We see homologues of language-relevant brain functions predating language. Recent findings show brain lateralization for communicative gestures in other primates without language, supporting the idea that a language-ready brain may be overwhelmed by raw perception, thus blocking overt language from evolving. We find support in converging evidence for a change in neural organization away from raw perception, thus pushing the emergence of language closer in time. A recent origin of language makes it possible to investigate the genetic origins of language.
... All animals bred in captivity were young adults of a few months of age; the age of wild-caught animals could not be determined. While the joint analysis of wild-caught and captivity-bred animals is not ideal, because of possible changes in brain and body size associated with domestication, the reduction of body size-corrected brain mass in Glires has been reported to be very small or even negligible, i.e. 0% in the mouse, 8% in the laboratory rat, and 13% in the lab rabbit [Kruska, 2007]. Because this variation is in the same range of intraspecific variation found in our sample ( table 1 ) and much smaller than the over 1,000-fold variation across species, we believe that the comparison of captive and wild-caught species is therefore warranted. ...
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Brain size scales as different functions of its number of neurons across mammalian orders such as rodents, primates, and insectivores. In rodents, we have previously shown that, across a sample of 6 species, from mouse to capybara, the cerebral cortex, cerebellum and the remaining brain structures increase in size faster than they gain neurons, with an accompanying decrease in neuronal density in these structures [Herculano-Houzel et al.: Proc Natl Acad Sci USA 2006;103:12138-12143]. Important remaining questions are whether such neuronal scaling rules within an order apply equally to all pertaining species, and whether they extend to closely related taxa. Here, we examine whether 4 other species of Rodentia, as well as the closely related rabbit (Lagomorpha), conform to the scaling rules identified previously for rodents. We report the updated neuronal scaling rules obtained for the average values of each species in a way that is directly comparable to the scaling rules that apply to primates [Gabi et al.: Brain Behav Evol 2010;76:32-44], and examine whether the scaling relationships are affected when phylogenetic relatedness in the dataset is accounted for. We have found that the brains of the spiny rat, squirrel, prairie dog and rabbit conform to the neuronal scaling rules that apply to the previous sample of rodents. The conformity to the previous rules of the new set of species, which includes the rabbit, suggests that the cellular scaling rules we have identified apply to rodents in general, and probably to Glires as a whole (rodents/lagomorphs), with one notable exception: the naked mole-rat brain is apparently an outlier, with only about half of the neurons expected from its brain size in its cerebral cortex and cerebellum.
... For almost all domestic mammals, when compared with those of their wild progenitors, the brain size relative to body size is reduced (Röhrs and Ebinger 1978;Hemmer 1990;Kruska 2007). In general, the slopes of the intraspecific allometric lines between body size and brain volume are less steep than those for interspecific comparisons. ...
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Endocranial volume was measured in a large sample (n = 128) of free-ranging dingoes (Canis dingo) where body size was known. The brain/body size relationship in the dingoes was compared with populations of wild (Family Canidae) and domestic canids (Canis familiaris). Despite a great deal of variation among wild and domestic canids, the brain/body size of dingoes forms a tight cluster within the variation of domestic dogs. Like dogs, free-ranging dingoes have paedomorphic crania however, dingoes have a larger brain and are more encephalised than most domestic breeds of dog. The dingo's brain/body size relationship was similar to those of other mesopredators (medium-sized predators that typically prey on smaller animals), including the dhole (Cuon alpinus) and the coyote (Canis latrans). These findings have implications for the antiquity and classification of the dingo, as well as the impact of feralisation on brain size. At the same time, it highlights the difficulty in using brain/body size to distinguish wild and domestic canids.
... This evolutionary transition has occurred relatively quickly on a geological timeframe and poses interesting questions concerning the flexibility and the degree of modularity of the canid nervous system. Previous comparative quantitative investigations have revealed that brain structures in the dog have not undergone a uniform change in size as a result of domestication, but, rather are characterized by a mosaic pattern of reduction with certain regions appearing to be more greatly impacted than others (e.g., hippocampus 42% reduction; limbic lobe, olfactory and cerebellum 30% reductions; mesencephalon a 10% reduction) (Kruska, 2005(Kruska, , 2007; Kruska & Stephan, 1973). Although this similar pattern of mosaic reduction has been observed in at least six other domesticated species (i.e., rat, gerbil, mink, sheep, llama, and pig), it is unknown if this holds true for all domesticated species or if there are particular brain components that are consistently invariant across species. ...
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All domesticated mammals exhibit marked reductions in overall brain size, however, it is unknown whether the corpus callosum, an integral white matter fiber pathway for interhemispheric cortical communication, is affected by domestication differentially or strictly in coordination with changes in brain size. To answer this question, we used quantitative magnetic resonance imaging to compare the mid‐sagittal cross‐sectional areas of the corpus callosum in 35 carnivore species, including eight wild canids and 13 domestic dogs. We segmented rostro‐caudal regions of interest for the corpus callosum and evaluated correlations with brain mass. The results of this study indicate that under the influence of domestication in canids, the corpus callosum scales to brain size in an allometric relationship that is similar to that of wild canids and other carnivores, with relatively high correlation coefficients observed for all regions, except the rostrum. These results indicate that architectural and energetic considerations are likely to tightly constrain variation in caudal components of the corpus callosum relative to overall brain size, however fibers passing through the rostrum, putatively connecting prefrontal cortex, are less constrained and therefore may contribute more towards species‐specific differences in connectivity. Given the species diversity of the Canidae and the resurgence of interest in the brain of the domestic dog, further studies aimed at characterizing the neural architecture in domesticated species is likely to provide new insights into the effects of domestication, or artificial selection, on the brain.
... Domestication represents a particularly valuable resource for understanding how brain structures can be selected at the species level (see Kruska, 2005 for a review and Gleich et al., 2000and Rehkämper et al., 2003, 2008 for recent contributions). What emerged from this body of work are profound differences in brain composition between domesticated and wild forms [review in Kruska (1988Kruska ( , 2005Kruska ( , 2007]. However, the number of individuals per species and per condition (wild vs. domestic) is often very small (typically less than 4-6). ...
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Despite decades of research, some of the most basic issues concerning the extraordinarily complex brains and behavior of birds and mammals, such as the factors responsible for the diversity of brain size and composition, are still unclear. This is partly due to a number of conceptual and methodological issues. Determining species and group differences in brain composition requires accounting for the presence of taxon-cerebrotypes and the use of precise statistical methods. The role of allometry in determining brain variables should be revised. In particular, bird and mammalian brains appear to have evolved in response to a variety of selective pressures influencing both brain size and composition. "Brain" and "cognition" are indeed meta-variables, made up of the variables that are ecologically relevant and evolutionarily selected. External indicators of species differences in cognition and behavior are limited by the complexity of these differences. Indeed, behavioral differences between species and individuals are caused by cognitive and affective components. Although intra-species variability forms the basis of species evolution, some of the mechanisms underlying individual differences in brain and behavior appear to differ from those between species. While many issues have persisted over the years because of a lack of appropriate data or methods to test them; several fallacies, particularly those related to the human brain, reflect scientists' preconceptions. The theoretical framework on the evolution of brain, cognition, and behavior in birds and mammals should be reconsidered with these biases in mind.
... This difference might be attributable to the relaxed selective pressure in captive-bred populations. It is well known that animals bred in captivity often have smaller brains and behave differently than their wild counterparts (e.g., Price 1999;Kruska 2007;Guay and Iwaniuk 2008;Burns et al. 2009;LaDage et al. 2016;Jensen 2017). The mice and geckos were likely kept under low cognitive pressure, whereas guppies in this study were subjected to strong artificial selection on brain size. ...
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Neurons are the basic computational units of the brain, but brain size is the predominant surrogate measure of brain functional capacity in comparative and cognitive neuroscience. This approach is based on the assumption that larger brains harbour higher numbers of neurons and their connections and therefore have a higher information‐processing capacity. However, recent studies have shown that brain mass may be less strongly correlated with neuron counts than previously thought. Till now, no experimental test has been conducted to examine the relationship between evolutionary changes in brain size and the number of brain neurons. Here we provide such a test by comparing neuron number in artificial selection lines of female guppies (Poecilia reticulata) with >15% difference in relative brain mass and numerous previously demonstrated cognitive differences. Using the isotropic fractionator, we demonstrate that large‐brained females have a higher overall number of neurons than small‐brained females, but similar neuronal densities. Importantly, this difference holds also for the telencephalon, a key region for cognition. Our study provides the first direct experimental evidence that selection for brain mass leads to matching changes in number of neurons and shows that brain size evolution is intimately linked to the evolution of neuron number and cognition. This article is protected by copyright. All rights reserved
... Early man, Cro Magnon, as well as Homo neanderthalensis, had larger brains than in modern humans, whose brains are reduced in volume as a result of neoteny and domestication [9,57,63]. Hood [9] sums up research that shows similar effects of domestication of animals compared to the recent shrinkage of the human brain. Domestication may originate in neoteny, which is the retention of phylogenetically early features, which can be thought of as cute features. ...
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Cave Art in the Upper Paleolithic presents a boost of creativity and visual thinking. What can explain these savant-like paintings? The normal brain function in modern man rarely supports the creation of highly detailed paintings, particularly the convincing representation of animal movement, without extensive training and access to modern technology. Differences in neuro-signaling and brain anatomy between modern and archaic Homo sapiens could also cause differences in perception. The brain of archaic Homo sapiens could perceive raw detailed information without using pre-established top-down concepts, as opposed to the common understanding of the normal modern non-savant brain driven by top-down control. Some ancient genes preserved in modern humans may be expressed in rare disorders. Researchers have compared Cave Art with art made by people with autism spectrum disorder. We propose that archaic primary consciousness, as opposed to modern secondary consciousness, included a savant-like perception with a superior richness of details compared to modern man. Modern people with high frequencies of Neanderthal genes, have notable anatomical features such as increased skull width in the occipital and parietal visual areas. We hypothesize that the anatomical differences are functional and may allow a different path to visual perception.
... Research in this field has yielded insights into the rapidity with which selection can alter behavior and (brain) morphology. The influence of domestication on brain morphology has been reviewed in the past, but mostly with a focus on mammals (e.g., Kruska [1988Kruska [ , 2005Kruska [ , 2007). So far, studies on avian species have seldom been taken into account. ...
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The avian class is characterized by particularly strong variability in their domesticated species. With more than 250 breeds and highly efficient commercial lines, domestic chickens represent the outcome of a really long period of artificial selection. One characteristic of domestication is the alterations in brain size and brain composition. The influence of domestication on brain morphology has been reviewed in the past, but mostly with a focus on mammals. Studies on avian species have seldom been taken into account. In this review, we would like to give an overview about the changes and variations in (brain) morphology and behavior in the domestic chicken, taking into consideration the constraints of evolutionary theory and the sense or nonsense of excessive artificial selection.
... Across the five species analyzed here (Figure 2), average body mass varies 18.8-fold (from 25 kg in the springbok to 470 kg in the giraffe), while brain mass varies 9.1-fold, and total number of brain neurons varies only 5.0-fold (Table 1). Brain mass increases as a power function of body mass with a small exponent of 0.555 ± 0.029 (p = 0.0027; Figure 3A, excluding the pig, which is an obvious outlier in the relationship, with a much larger body than expected for its brain mass, a probable consequence of domestication -see Figure 1 in Kruska, 2007). The relationship between brain mass and body mass for artiodactyls does not overlap with any of those found previously for glires, insectivores, afrotherians, or primates ( Figure 3A), but it does overlap with the relationship found for an independent dataset of 22 artiodactyl species (Boddy et al., 2012), with a similarly small exponent of 0.596 ± 0.031 (p < 0.0001, Figure 3B), where again the domestic pig is an obvious outlier. ...
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Quantitative analysis of the cellular composition of rodent, primate, insectivore, and afrotherian brains has shown that non-neuronal scaling rules are similar across these mammalian orders that diverged about 95 million years ago, and therefore appear to be conserved in evolution, while neuronal scaling rules appear to be free to vary in a clade-specific manner. Here we analyze the cellular scaling rules that apply to the brain of artiodactyls, a group within the order Cetartiodactyla, believed to be a relatively recent radiation from the common Eutherian ancestor.We find that artiodactyls share non-neuronal scaling rules with all groups analyzed previously. Artiodactyls share with afrotherians and rodents, but not with primates, the neuronal scaling rules that apply to the cerebral cortex and cerebellum. The neuronal scaling rules that apply to the remaining brain areas are, however, distinct in artiodactyls. Importantly, we show that the folding index of the cerebral cortex scales with the number of neurons in the cerebral cortex in distinct fashions across artiodactyls, afrotherians, rodents, and primates, such that the artiodactyl cerebral cortex is more convoluted than primate cortices of similar numbers of neurons. Our findings suggest that the scaling rules found to be shared across modern afrotherians, glires, and artiodactyls applied to the common Eutherian ancestor, such as the relationship between the mass of the cerebral cortex as a whole and its number of neurons. In turn, the distribution of neurons along the surface of the cerebral cortex, which is related to its degree of gyrification, appears to be a clade-specific characteristic. If the neuronal scaling rules for artiodactyls extend to all cetartiodactyls, we predict that the large cerebral cortex of cetaceans will still have fewer neurons than the human cerebral cortex.
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Bivariate allometric calculations were performed to quantitatively compare skulls of wild cavies with domesticated guinea pigs. Descendents of wild caught Cavia aperea from eastern regions of the species’ distribution area were used, as well as unselected domesticated breeds of guinea pigs differing in outer appearance. The individuals of both groups were kept under similar environmental conditions. Altogether 19 parameters on the skulls and the body weights were used for the analyses. These parameters were studied in relation to greatest skull length and to body size. As a general result the diverse parameters are in most cases significantly different between both groups which is interpreted as a special result of unconsciously selected and genetically determined intraspecific changes concomitant with domestication. The skull does not change in total under the domestication process but in a mosaic manner. However, for the mosaic changes of the diverse parameters in relation to skull length a different picture is valid as related to body weight. This is caused by the fact that the skull of guinea pigs is around 5% shorter independent of the body size, a common effect of domestication also described for other species. Thus, skull length is not an appropriate parameter for body size with respect to such intraspecific investigations, although normally used for the characterization of species in interspecific comparisons of museum materials.
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Self-control, defined as the ability to forgo immediate satisfaction in favor of better pay-offs in the future, has been extensively studied, revealing enormous variation between and within species. Horses are interesting in this regard because as a grazing species they are expected to show low self-control whereas its social complexity might be linked to high self-control abilities. Additionally, self-control may be a key factor in training and/or coping with potentially stressful husbandry conditions. We assessed horses’ self-control abilities in a simplified delay of gratification test that can be easily implemented in a farm setting. In Experiment 1, we gave horses (N = 52) the choice between an immediately available low-quality reward and a delayed high-quality reward that could only be obtained if the horse refrained from consuming the immediate reward. Different experimenters (N = 30) that underwent prior training in the procedures, tested horses in two test phases either with their eyes visible or invisible (sunglasses). Twenty horses waited up to the maximum delay stage of 60 s while all horses performed worse in the second test phase. In Experiment 2, we improved the test procedure (i.e., one experimenter, refined criterion for success), and tested 30 additional horses in a quality and quantity condition (one reward vs. delayed bigger reward). Two horses successfully waited for 60 s (quality: N = 1, quantity: N = 1). Horses tolerated higher delays, if they were first tested in the quantity condition. Furthermore, horses that were fed hay ad libitum, instead of in a restricted manner, reached higher delays. Coping behaviors (e.g., looking away, head movements, pawing, and increasing distance to reward) facilitated waiting success and horses were able to anticipate the upcoming delay duration as indicated by non-random distributions of giving-up times. We found no correlations between owner-assessed traits (e.g., trainability and patience) and individual performance in the test. These results suggest that horses are able to exert self-control in a delay of gratification paradigm similar to other domesticated species. Our simplified paradigm could be used to gather large scale data, e.g., to investigate the role of self-control in trainability or success in equestrian sports.
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Intraspecific allometric calculations of the brain to body size relation revealed distinct differences between 127 (67; 60) ancestral wild cavies and 82 (37; 45) guinea pigs, their domesticated relatives. The dependency of both measures from one another remained the same in both animal groups but the brains of guinea pigs were by 14.22% smaller at any net body weight. Consistent with results in other species the domestication of Cavia aperea is also characterized by a decrease of brain size. Fresh tissue sizes of the five brain parts medulla oblongata, cerebellum, mesencephalon, diencephalon and telencephalon were determined for 6 cavies and 6 guinea pigs by the serial section method. Additionally the sizes of 16 endbrain structures and those of the optic tract, the lateral geniculate body and the cochlear nucleus were measured. Different decrease values resulted for all these structures concomitant with domestication as was calculated from the amount of total brain size decrease and average relative structure values in the wild as well as the domesticated brain. The size decrease of the entire telencephalon (- 13.7%) was within the range of the mean overall reduction as similarly was the case for the total neocortex (- 10.7%) whereas the total allocortex (- 20.9%) clearly was more strongly affected. The size decrease of the olfactory bulb (- 41.9%) was extreme and clearly higher than found for the secondary olfactory structures (around - 11%). The primary nuclei of other sensory systems (vision, audition) were decreased to less extent (lateral geniculate: - 18.1%; cochlear nucleus: - 12.6%). Mass decreases of pure white matter parts were nearly twice as high in contrast to associated grey matter parts (neocortex white versus grey matter; tractus opticus versus lateral geniculate body). The relatively great decrease values found for the limbic structures hippocampus (- 26.9%) and schizocortex (- 25.9%) are especially notable since they are in good conformity with domestication effects in other mammalian species. The findings of this study are discussed with regard to results of similar investigations on wild and domesticated gerbils (Meriones unguiculatus), the encephalization of the wild form, the special and species-specific mode and duration of domestication and in connection with certain behavioral changes as resulted from comparative investigations in ethology, socio-biology, endocrinology and general physiology.
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The red fox (Vulpes vulpes) is the carnivore with the widest distribution in the world. Not much is known about the visual system of these ecologically highly adaptable animals that predominantly are forest‐dwellers. The closely related Arctic fox (Vulpes lagopus) lives in more open tundra habitats. In search for corresponding adaptations, we examined the photoreceptors and retinal ganglion cells (RGCs), using opsin immunohistochemistry, lucifer yellow injections and Nissl staining. Both species possess a majority of middle‐to‐longwave‐sensitive (M/L) and a minority of shortwave‐sensitive (S) cones, indicating dichromatic color vision. Area centralis peak cone densities are 22,600/mm² in the red fox and 44,800/mm² in the Arctic fox. Both have a centro‐peripheral density decrease of M/L cones, and a dorsoventrally increasing density of S cones. Rod densities and rod/cone ratios are higher in the red fox than the Arctic fox. Both species possess the carnivore‐typical alpha and beta RGCs. The RGC topography shows a centro‐peripheral density gradient with a distinct area centralis (mean peak density 7,900 RGCs/mm² in the red fox and 10,000 RGCs/mm² in the Arctic fox), a prominent visual streak of higher RGC densities in the Arctic fox, and a moderate visual streak in the red fox. The estimated upper limit of visual acuity is 8.7 cycles/degree in the red fox and 9.2 cycles/degree in the Arctic fox. The horizontal width of the field of best vision was used to estimate the sound localization ability to be 3° in the red fox and 4° in the Arctic fox. This article is protected by copyright. All rights reserved.
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Humans have the largest brain of any primate. While it seems logical to assume that overall size is very important for generating complex behaviours, brain size relative to body size has been considered to be a major factor in predicting overall brain capacity. It turns out, however, that the absolute number of neurons in the cerebral cortex, regardless of body mass, may be a more relevant factor. Here we review the ways in which brains have increased in size, why absolute brain size is sometimes important, and why the size of the human brain allowed us to have cognitive abilities that exceed those of other primates. We suggest that cognitive functions are largely mediated by the neocortex, and because the human brain scales like a typical primate brain, the large neocortex of humans contains more neurons than any other mammal, even those with larger brains such as elephants. Further, as neurons in primary sensory cortex increase in numbers with brain size at a greater rate than the increase in the number of neurons in thalamic relay nuclei, primates with larger brains and more neocortex also have more neurons to analyze these sensory inputs. As numbers of neurons increase, individual neurons are free to specialize in different ways, generating increasing variability in cell size, shape, dendritic arborization and other features. In addition, an expanded cortical sheet contains more cortical areas, thereby increasing the number of computational levels involved in information processing, decision-making, and information storage. Having more cortical areas allows any given area to become more specialized in terms of laminar and sub-laminar organization, modular organization, connectivity and function. Increases in cortical field number also allow for greater variation in the sizes of areas, and thereby different types of functional specializations. Finally, large brains have more areas that are removed from primary sensory inputs and capable of hemispheric specialization. Of course, the costs of a large brain are considerable in terms of gestation time, postnatal vulnerability, and metabolic costs. Thus, it is not surprising that most mammals have relatively small brains that are constrained in their processing capacity, but are more metabolically efficient, and mature rapidly allowing for early reproduction.
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The evolution of the vertebrate brain along with the function of this fascinating organ must be considered as a particular biological phenomenon many aspects of which have certain implications (Jerison 1973).
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1. The differences in shape and fissuration between brains of wild european pig (Sus scrofa scrofa Linnaeus, 1758) and domesticated pig (Sus scrofa f. dom.) are proved. 2. The relation between brain- and bodyweight clearly shows a reduction of total brain in domestication (34%). 3. The five classical parts of brain differently reduce in domestication. The following row from smallest to strongest reduction is ascertained by means of quantitative analysis: Medulla oblongata, mesencephalon, cerebellum, diencephalon, telencephalon. 4. Single structures of telencephalon were measured. In domestication schizocortex and archicortex reduce to the highest degree. Neocortex only ranges on the third place. 5. A reduction in the various nervous systems is gathered from cytoarchitectonical findings. Here a reduction of the olfactory system is ascertained (ca. 20%) which is half as strong as the reduction of the limbic system (ca. 40%). The small reduction of the septum (23%) in spite of its being a part of limbic system is still inexplicable. Reductions of motoric system as well as of optic and acustic system can also be taken as a fact. 6. A reconstruction of basal and median view from the brain of a european wild pig was made.
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Allometries of the brain to body size relationship in eutherian mammals are examined in this study as they can be used for comparative analyses concerning encephalization. In contrast with some modern presentations of this issue, an older concept is revived and expanded through this author's current study. Three allometries with clearly different slopes are valid and lead to reliable results: interspecific, intraspecific, and ontogenetic allometries. Interspecific allometries follow lines with slope values of 0.56 or 0.63 for larger and smaller species, respectively, and characterize different average encephalization plateaus with rodents and lagomorphs generally more strongly encephalized compared to basal insectivores. Artiodactyls, perissodactyls and carnivores as a whole are again on a higher but rather similar plateau. Several species of carnivores have reached different encephalization levels with respect to their average plateau indicating diverse radiations. A phylogenetic brain size increase from fossil to recent radiations is also evident. Intraspecific allometries have slope values of about 0.25. These are of help in comparing brain sizes of ancestral species with their domesticated relatives. Domestication has generally led to a brain size decrease, but species on higher encephalization plateaus show this trend more strongly than species on a lower level of encephalization. Several brain parts and the sense organs also decrease in size during the domestication process, but vary arbitrarily and to different degrees. Ontogenetic growth allometries are species-specific, but are especially different between altricial and precocial mammals. A very steep 1st phase slope of highly encephalized species is particularly useful for understanding evolutionary and adaptive phenomena. Domesticated mammals that have become feral do not show an increase in brain size despite living many generations in wild habitats.
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SummaryOver several decades American mink (Mustela vison) colonised large parts of northern Eurasia where they occupied species-specific habitats and caused severe problems in indigenous wild life communities. These populations originated from accidental ranch mink escapes or deliberate release. It is of general interest to characterise their taxonomic state in contrast to individuals from North America. Therefore, comparative investigations were accomplished on skulls of adult mink with Canadian and Belarus origin using 18 parameters and the total body weight. The diverse parameters were allometrically analysed in relation to greatest skull length and to body size additionally. As a result the Belarus mink skulls are significantly different from the Canadian and the degree of difference is largely above the level between Canadian subspecies (e. g., M. vison lacustris versus M. v. energumenos). Independent of body size Belarus mink skulls clearly are shorter in the facialis part including tooth row and evidently smaller in brain cavity size. The differences are identical with intraspecific changes due to the process of domestication leading from the wild to the ranch mink. Altogether the Belarus individuals still resemble ranch mink in skull configuration although being feralised for many generations. Zoological consequences of this fact are further discussed and the scientific name Mustela vison f. dom. fera is proposed to characterise the Eurasian wild populations and discriminate these from the autochthonous ancestors in North America.
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The topographical distribution of retinal ganglion cells in seven breeds of dog (Canis lupus f. familiaris) and in the wolf (Canis lupus) was studied in retinal wholemounts stained with cresyl violet or with a reduced silver method. A prominent feature of all wolf retinae was a pronounced “visual streak” of high ganglion cell density, extending from the central area far into both temporal and nasal retina. By contrast, either a pronounced or a moderate visual streak was found in dog retinae. It is hypothesized that a pronounced streak is an archetypal feature of Canis lupus, and that the moderate streak in some dogs is a corollary of breeding during domestication. Irrespective of the differences in streak form and retinal area, the estimated total number of ganglion cells was about 200,000 cells in the wolf and 115,000 in the dog. Ganglion cell density maxima in the central area of the wolf were about 12,000–14,000/mm2, and in the dog they ranged from 6,400/mm2 to 14,400/mm2. This implies individual differences in visual acuity. Alpha ganglion cells constituted 3–14% of all ganglion cells in the dog and 1–18% in the wolf, depending on retinal location. A distinct feature of all dogs and wolves was the absence of alpha cells in a substantial region of temporal peripheral retina. This has not been found in any other mammalian species and suggests corresponding functional deficits.