Behavioral Ecology

Published by Oxford University Press (OUP)
Online ISSN: 1465-7279
Print ISSN: 1045-2249
Publications
Stylized phylogenetic tree of Schizocosa species based on a Bayesian analysis (general time reversible [GTR] 1 G model) using data from Hebets and Vink (2007) and sequences of Schizocosa crassipes . Schizocosa ocreata and Schizocosa rovneri are shown as distinct species in this figure; however, this was not supported by the molecular data (see Hebets and Vink 2007). Male foreleg ornamentation is indicated next to each taxon. The graphs next to each foreleg represent the proportion of females that were receptive to isolated courtship signals in prior signal isolation experiments. The y axis is ‘‘proportion of females receptive,’’ and the black bars show female receptivity to visual-only signals, whereas the white bars show female receptivity to seismic-only signals. Asterisk above the graph indicate significant differences in female receptivity to visual-only versus seismic-only stimuli ( P , 0.05). Data are compiled from the following sources: (Scheffer et al. 1996; Hebets and Uetz 1999; Uetz and Roberts 2002). The dashed box around Schizocosa stridulans highlights the species used in the present study. 
(A) Copulation frequencies of female-male pairs under various signaling environments. Pairs were more likely to copulate in the presence versus absence of a seismic signal, whereas the presence/ absence of the visual signal did not influence copulation frequency. (B) Males took longer to initiate courtship in the absence of seismic signals, indicating a male reliance on seismic cues. Shared letters indicate nonsignificant differences (P , 0.05).
Unraveling the function and evolutionary history of multimodal signaling is a difficult, yet common task of much research in animal communication. Here, I investigated multimodal signal function in the visual and seismic courtship display of the wolf spider Schizocosa stridulans and found that only the seismic courtship signal was important for mating success. First, copulation frequency was assessed in the presence/ absence of both visual and seismic courtship signals. The seismic signal was sufficient for successful copulation, whereas the visual signal was neither necessary nor sufficient, suggesting that the signals are not redundant and do not function as backups. Next, female receptivity to video courtship sequences with altered male ornamentation was assessed in the presence of a live male’s seismic signal. Female receptivity did not depend on male foreleg ornamentation. Instead, females performed receptivity displays equally to all video stimuli, demonstrating that in the presence of seismic signaling, receptivity is independent of visual signaling—indicating seismic signal dominance. Finally, female responses to isolated seismic cues from crickets and courting males suggest that seismic courtship signals carry both location and identification information. Schizocosa stridulans represents one of the few examples in which a single component likely dominates a multimodal signal. Includes Supplemental Video of male courtship display, attached as "Related File" (below).
 
Design and results of experiment on spatial scale of conspecific effects. (a) Set up for all arenas in part 2 of experiment 3; 3.8-l box containing two 297-ml cups, one containing 12 female flies and each with 2 model walnuts, 1 inside the cup and 1 outside. Cups were topped with cloth mesh and furnished with vials of water. (b) Number of observations/flies/eggs on either side of the cage. Significant difference (2-tailed paired t -test, a 1⁄4 0.05) indicated with asterisk. 
The effect of social environment and host size on oviposition behavior (experiment 2). Number of ovipositing females estimated as described in text. (a) Mean ( 1 standard error of the mean [SEM]) number of eggs per ovipositing female. Shared letters indicate nonsignificant differences ( t -test, a 1⁄4 0.05; interaction marginally significant: F (social 3 size)1,113 1⁄4 3.50, P 1⁄4 0.06). (b) Mean ( 1 SEM) 
Effect of previous and current social environment on egg-laying decisions (experiment 4). Bars represent proportion of flies in each rearing treatment that attempted to oviposit when presented with a fruit with or without a resident female. Lines represent proportion of females that successfully oviposited. Proportion attempting oviposition was affected by previous ( v 2 1⁄4 5.28, df 1⁄4 1, P , 0.025) and concurrent exposure ( v 2 1⁄4 5.28, df 1⁄4 1, P , 0.025) to 
Mean ( 1 standard error of the mean) number of aggressive encounters (lunges, head butts, and foreleg kicks) in flies differing in social-rearing condition. Shared letters indicate nonsignificant differences (Mann–Whitney U test, a 1⁄4 0.05). Numbers within bars are sample sizes. 
Some animal species increase resource acceptance rates in the presence of conspecifics. Such responses may be adaptive if the presence of conspecifics is a reliable indicator of resource quality. Similarly, these responses could represent an adaptive reduction in choosiness under high levels of scramble competition. Although high resource quality and high levels of scramble competition should both favor increased resource acceptance, the contexts in which the increase occurs should differ. In this paper, we tested the effect of social environment on egg-laying and aggressive behavior in the walnut fly, Rhagoletis juglandis, in multiple contexts to determine whether increased resource acceptance in the presence of conspecifics was better viewed as a response to increased host quality or increased competition. We found that grouped females oviposit more readily than isolated females when provided small (low-quality) artificial hosts but not when provided large (high-quality) artificial hosts, indicating that conspecific presence reduces choosiness. Increased resource acceptance was observed even when exposure to conspecifics was temporally or spatially separate from exposure to the resource. Finally, we found that individuals showed reduced aggression after being housed in groups, as expected under high levels of scramble competition. These results indicate that the pattern of resource acceptance in the presence of conspecifics may be better viewed as a response to increased scramble competition rather than as a response to public information about resource quality.
 
Results of bivariate analyses of factors influencing reproductive skew 
Results of multiple regression models 
Recent studies have uncovered remarkable variation in paternity within primate groups. To date, however, we lack a general understanding of the factors that drive variation in paternity skew among primate groups and across species. Our study focused on hypotheses from reproductive skew theory involving limited control and the use of paternity "concessions" by investigating how paternity covaries with the number of males, female estrous synchrony, and rates of extragroup paternity. In multivariate and phylogenetically controlled analyses of data from 27 studies on 19 species, we found strong support for a limited control skew model, with reproductive skew within groups declining as female reproductive synchrony and the number of males per group increase. Of these 2 variables, female reproductive synchrony explained more of the variation in paternity distributions. To test whether dominant males provide incentives to subordinates to resist matings by extragroup males, that is, whether dominants make concessions of paternity, we derived a novel prediction that skew is lower within groups when threat from outside the group exists. This prediction was not supported as a primary factor underlying patterns of reproductive skew among primate species. However, our approach revealed that if concessions occur in primates, they are most likely when female synchrony is low, as these conditions provide alpha male control of paternity that is assumed by concessions models. Collectively, our analyses demonstrate that aspects of male reproductive competition are the primary drivers of reproductive skew in primates.
 
Novel or changing environments expose animals to diverse stressors that likely require coordinated hormonal and behavioral adaptations. Predicted adaptations to urban environments include attenuated physiological responses to stressors and bolder exploratory behaviors, but few studies to date have evaluated the impact of urban life on codivergence of these hormonal and behavioral traits in natural systems. Here, we demonstrate rapid adaptive shifts in both stress physiology and correlated boldness behaviors in a songbird, the dark-eyed junco, following its colonization of a novel urban environment. We compared elevation in corticosterone (CORT) in response to handling and flight initiation distances in birds from a recently established urban population in San Diego, California to birds from a nearby wildland population in the species' ancestral montane breeding range. We also measured CORT and exploratory behavior in birds raised from early life in a captive common garden study. We found persistent population differences for both reduced CORT responses and bolder exploratory behavior in birds from the colonist population, as well as significant negative covariation between maximum CORT and exploratory behavior. Although early developmental effects cannot be ruled out, these results suggest contemporary adaptive evolution of correlated hormonal and behavioral traits associated with colonization of an urban habitat.
 
Map of all nests of pairs of Neolamprologus caudopunctatus in the studied colony in 2005 at Mpulungu Bay. (●) Marks nests where paternity analysis was carried out. (○) Marks nests of pairs where no genetic sampling was done. (➔) Connect natal nests (base of arrow) and foster nests (tip of arrow) of foreign fry. (->) Connects nests where only the father was identified in the natal nest. 
Parental care of unrelated offspring is widespread but not well understood. We used 11 polymorphic microsatellite loci to investigate the relatedness of fry and parentally caring adults in a 118-nest colony of the socially and genetically monogamous cichlid fish Neolamprologus caudopunctatus in Lake Tanganyika. There was a high proportion of brood mixing, with 59% of 32 broods containing fry unrelated to both parents, and 18% of all 291 sampled fry being unrelated to the breeding pair. There was no evidence of kin selection for adoption because the genetic and foster parents were not more related than expected by chance. Parentage was assigned to 12 adopted fry from 10 broods. Distances traversed by fry varied markedly, from less than one to over 40 meters. The larger distances suggest that at least some brood mixing was instigated by parents transporting portions of their broods in their mouths, as occurs in some cichlids. Further evidence of non-random brood mixing was that foreign fry did not differ in size from their foster siblings within broods, even though they were significantly larger than fry produced by the tending pairs within the colony. These findings suggest that at least some foreign fry had dispersed non-randomly and were adopted by their foster parents. Enlarged broods are known to provide reduced per capita predation, making it potentially adaptive for breeders to adopt unrelated offspring.
 
Outline of the experimental room. The floor was divided into two zones with the log and the butterfly placed in zone 0; zone 1 consisted of the remaining part of the room.
The number of birds that produced at least one monosyllabic alarm call (“cut”) within 30 s from the peacock’s first display, when confronting a butterfly with visible eyespots or a butterfly with its eyespots painted over.
Large conspicuous eyespots have evolved in multiple taxa and presumably function to thwart predator attacks. Traditionally, large eyespots were thought to discourage predator attacks because they mimicked eyes of the predators' own predators. However, this idea is controversial and the intimidating properties of eyespots have recently been suggested to simply be a consequence of their conspicuousness. Some lepidopteran species include large eyespots in their antipredation repertoire. In the peacock butterfly, Inachis io, eyespots are typically hidden during rest and suddenly exposed by the butterfly when disturbed. Previous experiments have shown that small wild passerines are intimidated by this display. Here, we test whether eyespots also intimidate a considerably larger bird, domestic fowl, Gallus gallus domesticus, by staging interactions between birds and peacock butterflies that were sham-painted or had their eyespots painted over. Our results show that birds typically fled when peacock butterflies performed their display regardless of whether eyespots were visible or painted over. However, birds confronting butterflies with visible eyespots delayed their return to the butterfly, were more vigilant, and more likely to utter alarm calls associated with detection of ground-based predators, compared with birds confronting butterflies with eyespots painted over. Because production of alarm calls and increased vigilance are antipredation behaviors in the fowl, their reaction suggests that eyespots may elicit fear rather than just an aversion to conspicuous patterns. Our results, therefore, suggest that predators perceive large lepidopteran eyespots as belonging to the eyes of a potential predator.
 
Examples of behavioral variation among colonies and trials and of consistent behavior across disturbance and foraging situations. Each panel shows the behavior of a single colony in response to 2 stimuli during the 6 trials: responsiveness (A and B) to alarm (solid line) and to food bait (dashed line); speed (C and D) of moving a seedpod (solid line) and toothpicks (dashed line). 
ANOVA results for the effects of colony, trial, and day nested in trial on each of the 4 behaviors 
Consistent behavior across foraging and disturbance situations within each trial. (A) Responsiveness-difference in number of ants on mound (Pearson's correlation: R = 0.28, P value = 0.002); (B) speedcentimeters per minute (Pearson's correlation: R = 0.18, P value = 0.03). 
Consistent behavior across foraging and disturbance situations while a colony occupies a certain nest site. (A) Responsiveness-difference in number of ants on mound (Pearson's correlation: R = 0.35, P value = 0.01); (B) speedcentimeters per minute (Pearson's correlation: R = 0.16, P value = 0.27). 
Environmental conditions and physical constraints both influence an animal's behavior. We investigate whether behavioral variation among colonies of the black harvester ant, Messor andrei, remains consistent across foraging and disturbance situations and ask whether consistent colony behavior is affected by nest site and weather. We examined variation among colonies in responsiveness to food baits and to disturbance, measured as a change in numbers of active ants, and in the speed with which colonies retrieved food and removed debris. Colonies differed consistently, across foraging and disturbance situations, in both responsiveness and speed. Increased activity in response to food was associated with a smaller decrease in response to alarm. Speed of retrieving food was correlated with speed of removing debris. In all colonies, speed was greater in dry conditions, reducing the amount of time ants spent outside the nest. While a colony occupied a certain nest site, its responsiveness was consistent in both foraging and disturbance situations, suggesting that nest structure influences colony personality.
 
Sperm competition occurs when 2 or more males copulate with a particular female during the same reproductive cycle, and their sperm compete to fertilize the female's available eggs. One strategy that male voles use to assess the risk and intensity of sperm competition involves responding to the presence of scent marks of conspecific males found near a sexually receptive female. Previously, we have shown that if a male vole copulated with a female while he was in the presence of the odors of another male he increased his sperm investment relative to his investment if another male's odors were not present. The aim of the present study was to test the hypothesis that males assess differences in the relative quality of competing males and adjust their sperm investment accordingly. We did so by allowing males to copulate when they were exposed to the scent mark of a 24-h food-deprived male (low-quality male) or the scent mark of a male that was not food deprived (high-quality male). The data indicate that male meadow voles did not increase their sperm investment during copulation when exposed to the scent mark of a food-deprived male but did so when they were exposed to the scent mark of a male that was not food deprived. The results support the hypothesis that male voles are able to adjust sperm investment when they encounter the scent marks of males that differ in quality.
 
The experimental setup, demonstrating the encounter sequence of the 3 different treatments and the control during the experimental cycles. “NNC” stands for a non-nestmate Temnothorax longispinosus conspecific; “SM” stands for a slave-making worker of the species Protomognathus americanus and “CGS” stands for congeneric nonparasitic species (Temnothorax curvispinosus). First, all treatments were exposed to non-nestmate conspecifics to evaluate the base aggression of each colony. Second, treatments 1 and 2 were exposed to a slave-making worker, representing a slavemaker scout. Treatment 3 was exposed to a congeneric species, and the control was again confronted with a non-nestmate conspecific. In order to test whether an encounter with a slave-making worker induces elevated aggression, treatments 1 and 2 were exposed to a non-nestmate conspecific and again to a slave-making worker. Treatment 3 and the control investigate whether high aggression is induced by other stimuli (non-nestmate conspecific and a congeneric species). The last cycle aims at estimating the duration of the induced aggression. In addition, the encounter sequence of a raiding event, consisting of the scouting phase (cycle 2) and the raiding event (cycle 3), from the perspective of a host colony, is presented.
Mean aggression per treatment and cycle. Control: circles; treatment 1: triangles (upwards); treatment 2: diamonds; and treatment 3: triangles (downwards). Non-nestmate Temnothorax longispinosus opponent (non-nestmate conspecific, NNC) (gray), slave-making worker of the species Protomognathus americanus (slavemaker, SM) (black), and T. curvispinosus (congeneric species, CGS) (dark gray). Cycle 2, 3, and 4 took place 3, 6, and 20 days after cycle 1, respectively. Means ± 1 standard error are presented.
Slave-making ants reduce the fitness of surrounding host colonies through regular raids, causing the loss of brood and frequently queen and worker death. Consequently, hosts developed defenses against slave raids such as specific recognition and aggression toward social parasites, and indeed, we show that host ants react more aggressively toward slavemakers than toward nonparasitic competitors. Permanent behavioral defenses can be costly, and if social parasite impact varies in time and space, inducible defenses, which are only expressed after slavemaker detection, can be adaptive. We demonstrate for the first time an induced defense against slave-making ants: Cues from the slavemaker Protomognathus americanus caused an unspecific but long-lasting behavioral response in Temnothorax host ants. A 5-min within-nest encounter with a dead slavemaker raised the aggression level in T. longispinosus host colonies. Contrarily, encounters with nonparasitic competitors did not elicit aggressive responses toward non-nestmates. Increased aggression can be adaptive if a slavemaker encounter reliably indicates a forthcoming attack and if aggression increases postraid survival. Host aggression was elevated over 3 days, showing the ability of host ants to remember parasite encounters. The response disappeared after 2 weeks, possibly because by then the benefits of increased aggression counterbalance potential costs associated with it.
 
Rates of female-female agonism across major taxonomic groups of nonhuman primates. Data shown are those from the across-group analysis analyzed with a one-way Anova.
Rates of female-female agonism in relation to the amount of fruits in the diet analyzed by (a) standard statistical methods (least square regression) and (b) phylogenetic methods (PGLS).
Rates of female–female agonism in relation to female party size (number of females per group or average party size in fission–fusion societies) analyzed by (a) standard statistical methods (least square regression) and (b) phylogenetic methods (PGLS).  
Rates of female–female agonism in relation to substrate use analyzed by (a) standard statistical methods (one-way Anova) and (b) phylogenetic methods (PGLS). See Methods for definitions of substrate use.  
Rates of female–female agonism in relation to female party size (number of females per group or average party size in fission–fusion societies) and substrate use analyzed by phylogenetic methods (PGLS). See Methods for definitions of substrate use. For this figure, the categories arboreal-terrestrial and arboreal were lumped.  
Agonism is common in group-living animals, shaping dominance relationships and ultimately impacting individual fitness. Rates of agonism vary considerably among taxa, however, and explaining this variation has been central in ecological models of female social relationships in primates. Early iterations of these models posited a link to diet, with more frequent agonism predicted in frugivorous species due to the presumed greater contestability of fruits relative to other food types. Although some more recent studies have suggested that dietary categories may be poor predictors of contest competition among primates, to date there have been no broad, cross-taxa comparisons of rates of female-female agonism in relation to diet. This study tests whether dietary variables do indeed predict rates of female agonism and further investigates the role of group size (i.e., number of competitors) and substrate use (i.e., degree of arboreality) on the frequency of agonism. Data from 44 wild, unprovisioned groups, including 3 strepsirhine species, 3 platyrrhines, 5 colobines, 10 cercopithecines, and 2 hominoids were analyzed using phylogenetically controlled and uncontrolled methods. Results indicate that diet does not predict agonistic rates, with trends actually being in the opposite direction than predicted for all taxa except cercopithecines. In contrast, agonistic rates are positively associated with group size and possibly degree of terrestriality. Competitor density and perhaps the risk of fighting, thus, appear more important than general diet in predicting agonism among female primates. We discuss the implications of these results for socio-ecological hypotheses.
 
Path analysis representing the causal model that best explained the relationship among abiotic and call variables. Path coefficients are indicated for each path (link) between variables. local veget. 1⁄4 local vegetation, corresponding to the first axis of a PCA for vegetation density; %H 2 O 1⁄4 percentage of open water in the calling site; temp 1⁄4 temperature; and call temporal and call spectral 1⁄4 temporal 
Effect of the styrofoam enclosure on call structure. Boxplots show the result of Student’s t test for paired comparisons. FFM, male free-field recording; ECM, males in the enclosure; FFPB, free-field playback; and ECPB, playback in the enclosure. Individual graphs represent call parameters: DF1, dominant frequency of note 1; DF2, dominant frequency of note 2; DN1, duration of note 1; DN2, duration of note 2; CD, call duration; and INI, internote interval. * for p , 0.05, ** for p , 0.01. 
Scatterplot showing the trajectory of temporal variables for males tested with the styrofoam enclosure. Each starting dot corresponds to a male calling in natural conditions, and the arrowheads show the final position, with that male calling from inside the enclosure. Factor 1 and Factor 2 correspond to the first and second axes, respectively, obtained from the PCA performed for temporal variables. Factor 1 represents all temporal variables with the exception of note 1 duration, which is represented by the second factor. Black circles correspond to males calling in a pond on the night of 26 October 2008; gray circles correspond to males calling in a pond on the night of 5 January 2009; and white circles correspond to males calling in a section of forested stream on the night of 27 October 2008.
loadings and percentage of variance explained for the vegetation PCA
The structure of the environment surrounding signal emission produces different patterns of degradation and attenuation. The expected adjustment of calls to ensure signal transmission in an environment was formalized in the acoustic adaptation hypothesis. Within this framework, most studies considered anuran calls as fixed attributes determined by local adaptations. However, variability in vocalizations as a product of phenotypic expression has also been reported. Empirical evidence supporting the association between environment and call structure has been inconsistent, particularly in anurans. Here, we identify a plausible causal structure connecting environment, individual attributes, and temporal and spectral adjustments as direct or indirect determinants of the observed variation in call attributes of the frog Hypsiboas pulchellus. For that purpose, we recorded the calls of 40 males in the field, together with vegetation density and other environmental descriptors of the calling site. Path analysis revealed a strong effect of habitat structure on the temporal parameters of the call, and an effect of site temperature conditioning the size of organisms calling at each site and thus indirectly affecting the dominant frequency of the call. Experimental habitat modification with a styrofoam enclosure yielded results consistent with field observations, highlighting the potential role of call flexibility on detected call patterns. Both, experimental and correlative results indicate the need to incorporate the so far poorly considered role of phenotypic plasticity in the complex connection between environmental structure and individual call attributes.
 
Time to first capture (in seconds) of one An. gambiae larva during 10 successive and successful attacks by the predator, A. jaczewskii. The box extends between the 25th and 75th percentile, and the median is denoted by a thick line. The whiskers extend up to the most extreme values. 
Activity level (mean 6 standard error) of larvae (third instars) of the M (black circles) and S (white squares) forms in the absence or presence of the predator A. jaczewskii in an experimental arena. 
Disruptive selection mediated by predation on aquatic immature stages has been proposed as a major force driving ecological divergence and fostering speciation between the M and S molecular forms of the African malaria mosquito, Anopheles gambiae. In the dry savannahs of West Africa where both molecular forms co-occur, the S form thrives in temporary pools filled with rainwater, whereas the M form preferentially breeds in permanent freshwater habitats where predator pressure is higher. Here, we explored the proximal mechanisms by which predation may contribute to habitat segregation between molecular forms using progeny of female mosquitoes captured in Burkina Faso. We show that the S form suffers higher predation rates than the M form when simultaneously exposed to the widespread predator, Anisops jaczewskii in an experimental arena. Furthermore, behavioral plasticity induced by exposure to the predator was observed in the M form, but not in the S form, and may partially explain its habitat use and ecological divergence from the S form. We discuss the role of adaptive phenotypic plasticity in allowing successful colonization of a new ecological niche by the M form and highlight further research areas that need to be addressed for a better understanding of the ultimate mechanisms underlying ecological speciation in this pest of major medical importance.
 
Response to change in forager return rate. Shown are smoothed results for 2 trials, each for 1 colony on 1 day. Rates of foraging were smoothed using a Gaussian kernel. Dotted line, returning foragers per second; solid line, outgoing foragers per second. Returning foragers were removed for 3 min beginning at the time indicated by the vertical dotted line. Foraging rates before removals were calculated from 100–350 s, including the time it would have taken for removed ants to return to the nest. Foraging rates after removals were calculated from 350–590 s. (A) Example of a trial with no response; foraging did not change in response to the removal of returning foragers. (B) Example of a trial in which the colony responded to the removal of returning foragers by decreasing the rate of outgoing foragers. 
Foraging activity and response to decrease in forager return rate
Foraging activity and patroller burst size, by colony. Each point shows the average number of patrollers that left the nest per min and the average numbers of outgoing foragers per 30 s for the same colony.
This study investigates variation in collective behavior in a natural population of colonies of the harvester ant, Pogonomyrmex barbatus. Harvester ant colonies regulate foraging activity to adjust to current food availability; the rate at which inactive foragers leave the nest on the next trip depends on the rate at which successful foragers return with food. This study investigates differences among colonies in foraging activity and how these differences are associated with variation among colonies in the regulation of foraging. Colonies differ in the baseline rate at which patrollers leave the nest, without stimulation from returning ants. This baseline rate predicts a colony's foraging activity, suggesting there is a colony-specific activity level that influences how quickly any ant leaves the nest. When a colony's foraging activity is high, the colony is more likely to regulate foraging. Moreover, colonies differ in the propensity to adjust the rate of outgoing foragers to the rate of forager return. Naturally occurring variation in the regulation of foraging may lead to variation in colony survival and reproductive success.
 
Within-pair differences in (a) the median fake-egg incubation temperature and (b) the median constancy of incubation. Each dot represents 1 nest. Incubation temperatures are not comparable between nests because the position of the temperature probe within the fake egg and of the fake egg within a nest is not exactly the same in each nest. Incubation constancy is comparable between nests.  
Increase in incubation bout length over the incubation period. The solid lines represent the model fit, and the shading represents the 95% CIs. Model results are presented in Table 3, and the distribution of the raw data is depicted in Supplementary Figure S5.
Between-and within-pair differences in the median length of incubation bouts. Each dot represents 1 nest. The correlation of the median bouts between sexes: Pearson correlation coefficient (95% CI): r = 0.71 (0.530.83), t 46 = 6.8, P < 0.0001.
Positive relationship between incubation bout length and previous bout length (partner's bout; off-nest bout of the focal bird). The solid lines represent the model fit, and the shading represents the 95% CI. Model results are presented in Table 3.
Shift in the probability of female (vs. male) incubation during specific time of a day over the 21-day incubation period and with respect to the start of incubation within the season. Color lines represent the mean predicted probability of 5000 GLMMs for the 1st (dark blue), the 10th (green), and the 21st (yellow) day of incubation; color shadings represent the nonparametric CIs that contain 95% of the 5000 fits. Left panel: predictions from 6 June (first nest started on 1 June); middle panel: 13 June; right panel: 19 June. The distribution of the nests across the season is in Supplementary Figure S7. The horizontal dashed line indicates an equal share of incubation, and the gray shaded rectangle represents the time when the tundra temperatures were on average above overall median tundra temperature, that is, the warmer period of the Arctic day. Nest-specific incubation patterns for all 48 nests are in Supplementary Actograms. Table 6 Summary of 5000 model (binomial GLMM) estimates of the probability that the female (vs. male) incubates in relation to time of a day, incubation period, and start of incubation within the season Fixed effects Estimate 95% CI Number of iterations (P < 0.05) P (Intercept) 0.059 (−0.021, 0.14) 318 0.94 cos(time) 0.915 (0.757, 1.079) 4998 0.0004 sin(time) 0.755 (0.61, 0.906) 5000 0 Day of incubation 0.007 (−0.01, 0.024) 101 0.98 Start of incubation 0.006 (−0.006, 0.018) 133 0.97 Start of incubation × Day of incubation 0 (−0.002, 0.003) 22 1 cos(time) × Day of incubation −0.066 (−0.098, −0.034) 4494 0.10 cos(time) × Start of incubation 0.02 (−0.003, 0.044) 0 1 cos(time) × Day of incubation × Start of incubation 0.003 (−0.002, 0.008) 284 0.94 sin(time) × Day of incubation 0.109 (0.079, 0.139) 5000 0 sin(time) × Start of incubation 0.026 (0.006, 0.046) 0 1 sin(time) × Day of incubation × Start of incubation −0.012 (−0.017, −0.007) 4928 0.0144 Random effects Variance 95% CI Nest (intercept) 0 (0, 0) cos(time) 4.67 (3.6, 6.0) sin(time) 2.37 (1.84, 3.04) Median (range) N = 1722 (1680–1768) incubation data points from 48 nests. Time of a day (in radians) was decomposed into sin and cos; the remaining fixed effects were mean centered. Estimates are means, and their 95% CIs (nonparametric) are 0.025 and 0.975 quintiles, of the fixed effect estimates of 5000 GLMMs. Variances are mean values, and 95% CIs are 0.025 and 0.975 quintiles, of the random effects from 5000 GLMMs. The median (range) partial autocorrelation coefficient of GLMMs was 0.038 (0.001–0.074) in lag 1 and −0.264 (−0.311 to −0.214) in lag 2.  
In biparental species, parents may be in conflict over how much they invest into their offspring. To understand this conflict, parental care needs to be accurately measured, something rarely done. Here, we quantitatively describe the outcome of parental conflict in terms of quality, amount, and timing of incubation throughout the 21-day incubation period in a population of semipalmated sandpipers (Calidris pusilla) breeding under continuous daylight in the high Arctic. Incubation quality, measured by egg temperature and incubation constancy, showed no marked difference between the sexes. The amount of incubation, measured as length of incubation bouts, was on average 51min longer per bout for females (11.5h) than for males (10.7h), at first glance suggesting that females invested more than males. However, this difference may have been offset by sex differences in the timing of incubation; females were more often off nest during the warmer period of the day, when foraging conditions were presumably better. Overall, the daily timing of incubation shifted over the incubation period (e.g., for female incubation from evening-night to night-morning) and over the season, but varied considerably among pairs. At one extreme, pairs shared the amount of incubation equally, but one parent always incubated during the colder part of the day; at the other extreme, pairs shifted the start of incubation bouts between days so that each parent experienced similar conditions across the incubation period. Our results highlight how the simultaneous consideration of different aspects of care across time allows sex-specific investment to be more accurately quantified.
 
Examples of Dunatothrips aneurae domiciles on Acacia aneura. Squares measure 0.5 × 0.5 cm. 
Characteristics of larval and adult stages of Tubuliferan thrips Biology Predicted sensitivity to reduction in: 
Survival curves for experimental treatments. (a) Experiment 1: (i) high humidity, (ii) low humidity; (b) Experiment 2: (i) high humidity, (ii) low humidity; (c) All larvae exiting intact domiciles (pooled across experiments; X axis shows time since emergence from domicile). Key: Thick line, intact domicile treatment; thin line, destroyed domicile treatment; solid line, adults; dashed line, feeding larval stages (larvae I and II); dotted line, nonfeeding larval stages (propupa, pupa I, pupa II); dot-dashed line, all larvae combined. Crosses show censored data points. (Nb. in (a)(i) and (b)(ii), dashed and dotted lines for intact domicile treatments are superposed, as all larvae survived). 
Desiccation is a particular risk for small animals in arid environments. In response, many organisms "construct niches," favorable microenvironments where they spend part or all of their life cycle. Some maintain such environments for their offspring via parental care. Insect eggs are often protected from desiccation by parentally derived gels, casings, or cocoons, but active parental protection of offspring from desiccation has never been demonstrated. Most free-living thrips (Thysanoptera) alleviate water loss via thigmotaxis (crevice seeking). In arid Australia, Acacia thrips (Phlaeothripidae) construct many kinds of niche. Some thrips induce galls; others, like Dunatothrips aneurae, live and breed within "domiciles" made from loosely glued phyllodes. The function of domiciles is unknown; like other constructed niches, they may 1) create favorable microenvironments, 2) facilitate feeding, 3) protect from enemies, or a combination. To test the first 2 alternatives experimentally, field-collected domiciles were destroyed or left intact. Seven-day survival of feeding and nonfeeding larval stages was monitored at high (70-80%) or low (8-10%, approximately ambient) humidity. Regardless of humidity, most individuals survived in intact domiciles, whereas for destroyed domiciles, survival depended on humidity, suggesting parents construct and maintain domiciles to prevent offspring desiccating. Feeding and nonfeeding larvae had similar survival patterns, suggesting the domicile's role is not nutritional. Outside domiciles, survival at "high" humidity was intermediate, suggesting very high humidity requirements, or energetic costs of wandering outside domiciles. D. aneurae commonly cofound domiciles; cofoundresses may benefit both from shared nestbuilding costs, and from "deferred byproduct mutualism," that is, backup parental care in case of mortality.
 
The "social intelligence hypothesis" states that the need to cope with complexities of social life has driven the evolution of advanced cognitive abilities. It is usually invoked in the context of challenges arising from complex intragroup structures, hierarchies, and alliances. However, a fundamental aspect of group living remains largely unexplored as a driving force in cognitive evolution: the competition between individuals searching for resources (producers) and conspecifics that parasitize their findings (scroungers). In populations of social foragers, abilities that enable scroungers to steal by outsmarting producers, and those allowing producers to prevent theft by outsmarting scroungers, are likely to be beneficial and may fuel a cognitive arms race. Using analytical theory and agent-based simulations, we present a general model for such a race that is driven by the producer-scrounger game and show that the race's plausibility is dramatically affected by the nature of the evolving abilities. If scrounging and scrounging avoidance rely on separate, strategy-specific cognitive abilities, arms races are short-lived and have a limited effect on cognition. However, general cognitive abilities that facilitate both scrounging and scrounging avoidance undergo stable, long-lasting arms races. Thus, ubiquitous foraging interactions may lead to the evolution of general cognitive abilities in social animals, without the requirement of complex intragroup structures.
 
Previous experiments suggest that males spend more time with the more receptive of 2 novel females or the one with the higher fitness potential. However, males often court individual females repeatedly over a season; for example, male lizards sequentially visit familiar females as they patrol territorial boundaries. It may benefit males to vary display intensity as they move between multiple females. In this study, we explored the factors influencing amount of male courtship to familiar females in the sagebrush lizard, Sceloporus graciosus. We tested whether males vary the amount of courtship exhibited due to individual differences among males, female reproductive state, or female fitness potential. Each male was allowed to interact separately, but repeatedly, with 2 females until both females laid eggs. Male courtship behavior with each of the 2 females was assayed at an intermediate point, after 3 weeks of interaction. We found that individual differences among males were considerable. The number of male courtship displays was also positively correlated with female latency to lay eggs, with males displaying more often toward females with eggs that had not yet been fertilized. Courtship behavior was not well predicted by the number of eggs laid or by female width, both measures of female quality. Thus, male S. graciosus appear to alter courtship intensity more in response to signals of female reproductive state than in response to variation in potential female fitness.
 
The SNP (53°N latitude) is shown schematically, along with the birth (B) and weaning (W) times for each of 3 birth cohorts. Food measurements began on week 18, and food clamping (FC) manipulations began on week 24. A subset of hamsters from each cohort was transferred to the winter solstice photoperiod (7.6L; ''SD'') on week 26; all others remained in the SNP. On week 48, a subset of FC hamsters was returned to AL feeding.
Mean 6 standard error of the mean ETV (mm 3 ) (A-C) and body mass (D-F) of photoresponsive male Siberian hamsters born into an SNP 23 weeks (Cohort 1), 15 weeks (Cohort 2), and 112 weeks (Cohort 3) relative to the summer solstice (week 0). Hamsters exhibited either rapid gonadal development (RGD) (B and E) followed by gonadal regression or delayed puberty (DP) (C and F) in the SNP.
Mean 6 standard error of the mean testis volume (ETV [mm 3 ]; A and B) and body mass (C and D) of male Siberian hamsters exhibiting delayed puberty (DP) following birth into the SNP. On week 24, hamsters were provided with a daily ration of food equal to their nadir short-day food intake (food clamp, FC) or were fed AL. On week 26, hamsters either (A and C) remained in the SNP or (B and D) were maintained at the solstice photoperiod of 7.6L (SD) for the next 26 weeks. *P , 0.05 versus AL value. #(below symbol) P , 0.05 versus solstice value, within group.
Mean 6 standard error of the mean testis volume (ETV [mm 3 ]; A and B) and body mass (C and D) of male Siberian hamsters exhibiting rapid gonadal development (RGD) following birth into the SNP. On week 24, hamsters were provided with a daily ration of food equal to their nadir shortday food intake (food clamp, FC) or were fed AL. On week 26, hamsters either (A and C) remained in the SNP or (B and D) were maintained at the solstice photoperiod of 7.6L (SD) for the next 26 weeks. *P , 0.05 versus AL value. #(below symbol) P , 0.05 versus solstice value, within group.
Mean 6 standard error of the mean testis volume (ETV [mm 3 ]; A and C) and body mass (B and D) of male Siberian hamsters exhibiting delayed puberty (DP). From weeks 24 to 48, hamsters were provided with daily food rations (FC) or AL access to food. On week 48, a subset of previously FC hamsters was allowed AL access to food (FC/AL), whereas the remainder of the FC hamsters remained on food rationing (FC/FC). *P , 0.05 versus FC/FC; 1P , 0.05 versus AL; #P , 0.05 versus week 48 value, within group.
In seasonally breeding mammals, vernal reproductive development is not directly triggered by increases in day length, rather, an endogenous program of photorefractoriness to short winter days initiates spontaneous development in advance of spring. The transition to the reproductive phenotype is energetically demanding. How food availability in late winter and early spring impacts the onset and expression of photorefractoriness is not known. In this study, male Siberian hamsters were born into a simulated natural photoperiod, and at the winter solstice, they were subjected to a restricted feeding protocol in which a daily food ration was provided in an amount equal to ad libitum (AL) intake during the weeks preceding the solstice. Over the next several months, AL-fed control hamsters exhibited spontaneous recrudescence or spontaneous development. In contrast, vernal reproductive development was abolished in most food-rationed hamsters. In food-rationed hamsters that did exhibit recrudescence, conspicuous delays in the onset of gonadal development and decreases in the magnitude of growth were evident. In all hamsters, the termination of food rationing triggered rapid gonadal development. The data indicate that late winter/early spring increases in environmental food availability are required for the normal manifestation of photorefractoriness-induced reproductive development and suggest that a function of photorefractoriness may be merely to disinhibit the reproductive axis from photoperiodic suppression. Vernal gonadal development or recrudescence appears to be strongly affected by proximate energy availability.
 
Human visible (HV) and ultraviolet (UV) images of typical male and female Podarcis erhardii backs and ventrolateral flanks from the 3 focal Aegean island populations (Syros, Folegandros, and Skopelos). Images: Marshall K (unpublished). 
Top: an outline of an aerial view of an Aegean wall lizard (Podarcis erhardii) showing the body regions measured that are more visible to avian predators hunting from above (lower and upper backs). Bottom: an outline of a lateral view showing the body region measured that is potentially more visible to conspecifics on the ground, and less visible to avian predators hunting from above (ventrolateral flanks). Figure adapted from images of P. erhardii (Marshall K, unpublished). 
Distributions of avian-and conspecific-perceived coloration of male and female Aegean wall lizards (Podarcis erhardii) and their corresponding natural backgrounds in tetrahedral color space. Figures show different lizard body regions and backgrounds to which they were compared to measure their conspicuousness to avian predators and lizard conspecifics: (A) lower backs and (B) upper backs (Each N = 166; avian-perceived = 83, conspecific-perceived = 83) and (C) ventrolateral flanks (N = 124; avian perceived = 62, conspecific-perceived = 62). Alternative dark backgrounds were selected for comparison with lizard body regions cast in shadow (relative to the reflectance standard) in nine cases overall (lower backs = 5/9 cases, upper backs = 4/9 cases, flanks = 6/9 cases). Each color is a point in the tetrahedron determined by the relative stimulation of the four cone color channels, UV/V, SW, MW, and LW where the V channel refers to avian predator violet-sensitive vision and the UV channel refers to lizard conspecific ultraviolet-sensitive vision. 
Visual signals are often under conflicting selection to be hidden from predators while being conspicuous to mates and rivals. Here, we investigated whether 3 different island populations of Aegean wall lizards (Podarcis erhardii) with variable coloration among diverse island habitats exhibit simultaneous camouflage and sexual signals. We examined whether signals appear better tuned to conspecific vision as opposed to that of avian predators, and whether background-matching camouflage and sexual signals are partitioned to specific body regions. This could facilitate both covert sexual signaling and camouflage according to the viewing perspectives of predators and conspecifics. We found that lizards typically appeared twice as conspicuous to conspecifics than to avian predators against the same visual background, largely due to lizards' enhanced sensitivity to ultraviolet, suggesting that P. erhardii signals are tuned to conspecific vision to reduce detection by predators. Males were more conspicuous than females to both predators and conspecifics. In 2 populations, male backs were relatively more camouflaged to predators compared to signaling flanks, whereas in females, exposed and concealed surfaces were camouflaged to predators and generally did not differ in background matching. These findings indicate that lizard coloration evolves under the competing demands of natural and sexual selection to promote signals that are visible to conspecifics while being less perceptible to avian predators. They also elucidate how interactions between natural and sexual selection influence signal detectability and partitioning to different body regions, highlighting the importance of considering receiver vision, viewing perspectives, and signaling environments in studies of signal evolution.
 
We tested the signal value of song overlapping in banded wrens (Thryothorus pleurostictus), using interactive playback to either overlap or alternate with their songs. Males shortened song duration and decreased variability in song length when their songs were overlapped by playback, suggesting that they were attempting to avoid being overlapped and perhaps being less aggressive. A novel finding was an effect of long-term prior experience: song lengths remained relatively short in alternating trials that followed two or more days after overlapping trials. Approach responses to the two treatments did not differ overall, but there was a parallel effect of prior experience: males tended to stay further from the speaker during alternating treatments if they had previously been overlapped by playback. Some females paired to the male subjects sang in response to playback and were also influenced by prior experience, singing more during alternating trials that had not been preceded by an overlapping trial. Male overlappers may signal dominance over a rival to other male or female receivers in a communication network, but it is currently unclear whether overlapping indicates motivation to escalate an aggressive interaction, or whether this singing strategy is related to male quality. Banded wrens are long-lived and maintain year-round territories, so modifying responses to rivals based on prior experience is likely to be important for success.
 
Interpreting receiver responses to on-territory playback of aggressive signals is problematic. One solution is to combine such receiver-perspective experiments with a sender-perspective experiment that allows subjects to demonstrate how their choice of singing strategies is associated with their approach behavior. Here we report the results of a sender-perspective study on the banded wren (Thryothorus pleurostictus), and combine information on context and results of previous receiver-perspective experiments to clarify function. Territorial males were presented with a 5-min playback consisting of song types present in their repertoire. We assessed the degree to which the subjects' song matching rate, overlapping rate, and song-type versatility were correlated with their approach latency, closeness of approach, latency to first retreat, and time spent close to the speaker. Male age, breeding stage, and features of the playback stimuli were also considered. Song matching was associated with rapid and close approach, consistent with the receiver-perspective interpretation of type matching as a conventional signal of aggressive motivation. Overlapping was associated with earlier retreat, and together with the aversive receiver response to our previous overlapping playback experiment suggests that overlapping is a defensive withdrawal signal. High versatility was associated with slower first retreat from the speaker and high levels of reciprocal matching between subject and playback. Males with fledglings sang with particularly low versatility and approached the speaker aggressively, whereas males with nestlings overlapped more and retreated quickly. Finally, older males matched more but overlapped less.
 
Arena for experimental contests. Escape holes remained covered until after the first interaction. 
Effect of social experience on male contest behavior during a male's first contest. (A) Male encounter rate and (B) male aggressive behavior. Mean ± standard error. 
Logistic relationships showing the effect of relative pronotum width ([focal male-opponent]/focal male) on contest outcome. (A) A male's first contest and (B) a male's second contest. Mean ± standard error. 
Contest behavior forms an important part of reproductive investment. Life-history theory predicts that as individuals age and their residual reproductive value decreases, they should increase investment in contest behavior. However, other factors such as social experience may also be important in determining age-related variation in contest behavior. To understand how selection acts on contest behavior over an individual’s lifetime, it is therefore important to tease apart the effects of age per se from other factors that may vary with age. Here, we independently manipulate male age and social experience to examine their effects on male contest behavior in the burying beetle Nicrophorus vespilloides. We found that social experience, but not age, influenced male contest behavior but that these changes in behavior did not alter contest outcomes. Male size (relative to his opponent) was overwhelmingly the most important factor determining contest outcome. Our results suggest that in systems with high variation in fighting ability among males, there may be little opportunity for selection to act on factors that influence contest outcomes by altering motivation to win.
 
Although there has been extensive research on the evolution of individual decision making under risk (when facing variable outcomes), little is known on how the evolution of such decision-making mechanisms has been shaped by social learning and exploitation. We presented socially foraging house sparrows with a choice between scattered feeding wells in which millet seeds were hidden under 2 types of colored sand: green sand offering ~80 seeds with a probability of 0.1 (high risk-high reward) and yellow sand offering 1 seed with certainty (low risk-low reward). Although the expected benefit of choosing variable wells was 8 times higher than that of choosing constant wells, only some sparrows developed a preference for variable wells, whereas others developed a significant preference for constant wells. We found that this dichotomy could be explained by stochastic individual differences in sampling success during foraging, rather than by social foraging strategies (active searching vs. joining others). Moreover, preference for variable or constant wells was related to the sparrows' success during searching, rather than during joining others or when picking exposed seeds (i.e., they learn when actively searching in the sand). Finally, although for many sparrows learning resulted in an apparently maladaptive risk aversion, group living still allowed them to enjoy profitable variable wells by occasionally joining variable-preferring sparrows.
 
Composite facial skin color profiles throughout the ovarian cycle. Values represent the mean (6standard error of the mean) percentage of the maximum value reached for each cycle for: (a) CameraR:G (from Dubuc et al. 2009), (b) RhesusR:G, (c) RhesusLuminance, and (d) CameraLuminance.
Spectral sensitivities of the MW and LW receptors for: (a) a Fuji digital camera (includes SW sensitivity) and (b) a rhesus macaque (data from Bowmaker et al. 1978). Note that the available data for rhesus macaques do not extend to wavelengths below 440 nm. 
Composite JND profiles. Values represent the mean ( 6 standard error of the mean) difference per day relative to ovulation between the maximum luminance value reached for each cycle for face images. 
Animal coloration has provided many classical examples of both natural and sexual selection. Methods to study color signals range from human assessment to models of receiver vision, with objective measurements commonly involving spectrometry or digital photography. However, signal assessment by a receiver is not objective but linked to receiver perception. Here, we use standardized digital photographs of female rhesus macaque (Macaca mulatta) face and hindquarter regions, combined with estimates of the timing of the female fertile phase, to assess how color varies with respect to this timing. We compare objective color measures (camera sensor responses) with models of rhesus vision (retinal receptor stimulation and visual discriminability). Due to differences in spectral separation between camera sensors and rhesus receptors, camera measures overestimated color variation and underestimated luminance variation compared with rhesus macaques. Consequently, objective digital camera measurements can produce statistically significant relationships that are probably undetectable to rhesus macaques, and hence biologically irrelevant, while missing variation in the measure that may be relevant. Discrimination modeling provided results that were most meaningful (as they were directly related to receiver perception) and were easiest to relate to underlying physiology. Further, this gave new insight into the function of such signals, revealing perceptually salient signal luminance changes outside of the fertile phase that could potentially enhance paternity confusion. Our study demonstrates how, even for species with similar visual systems to humans, models of vision may provide more accurate and meaningful information on the form and function of visual signals than objective color measures do.
 
Schematic illustrations of more (A) and less (B) problematic cases for the estimation of fixed-effect covariates in random-intercept models. (a) Regression lines for several individuals with high (A) and low (B) between-individual variation in slopes (σb). (b) Two individual regression slopes with low (A) and high (B) scatter around the regression line (σr). (c) Regression lines with (A) many and (B) few measurements per individual (independent of the number of levels of the covariate).
Type I error rate (proportion of estimated 95% confidence intervals for fixed effects not including the true value of zero) in a split-plot design with treatment applied to individuals and slopes measured within individuals as estimated from a random intercept model. The 3 figures in one row show the type I error rates for tests for the treatment main effect (left), the slope main effect (centre), and the slope-by-treatment interaction (right). The 2 rows of figures show simulation with 2 different numbers of measurements within individuals. Type I error rates are indicated by shades of orange (the darker, the higher) and isolines. Error rates depend on the amount of within-individual scatter around the individual regression line (σr, x axis), and the between-individual variation in regression slopes (σb, y axis).
Type I error rates (proportion of estimated 95% confidence intervals for fixed effects not including the true value) in a split-plot design with treatment applied to individuals and slopes measured within individuals as estimated from a random slope model. For details, see Figure 2.
Mixed-effect models are frequently used to control for the nonindependence of data points, for example, when repeated measures from the same individuals are available. The aim of these models is often to estimate fixed effects and to test their significance. This is usually done by including random intercepts, that is, intercepts that are allowed to vary between individuals. The widespread belief is that this controls for all types of pseudoreplication within individuals. Here we show that this is not the case, if the aim is to estimate effects that vary within individuals and individuals differ in their response to these effects. In these cases, random intercept models give overconfident estimates leading to conclusions that are not supported by the data. By allowing individuals to differ in the slopes of their responses, it is possible to account for the nonindependence of data points that pseudoreplicate slope information. Such random slope models give appropriate standard errors and are easily implemented in standard statistical software. Because random slope models are not always used where they are essential, we suspect that many published findings have too narrow confidence intervals and a substantially inflated type I error rate. Besides reducing type I errors, random slope models have the potential to reduce residual variance by accounting for between-individual variation in slopes, which makes it easier to detect treatment effects that are applied between individuals, hence reducing type II errors as well.
 
No major effects of heating (A and B) or nest insulation (A, gray area) on the length of incubation bouts. In (A), the red dots show the combined effect size for the heating and insulation experiment; filled circles: simple model estimates (statistical details are in Supplementary 1: Section 6, Tables S3 and S5); open circles: estimates from complex models with covariates (statistical details are in Supplementary 1: Section 6, Tables S4 and S6); in white: the estimate from the original insulation study (Cresswell et al. 2003). In (B), the red ellipse highlights the main comparison of interest, namely the difference between treated bouts in treated nests (heated bouts) and control nests (nonheated bouts). The values depicted in (B) are derived from the simple model using the "allEffects" function of the R package "effects" (Fox 2003); shown are bout lengths relative to the before-treatment bout length (i.e., for each individual, the mean beforebout length was calculated and subtracted from all incubation bouts). This allowed direct comparison of the treatment effects among nests, and controlled for changes in bout length with date and with day of incubation. 
Incubation is energetically demanding, but it is debated whether these demands constrain incubation-scheduling (i.e., the length, constancy, and timing of incubation bouts) in cases where both parents incubate. Using 2 methods, we experimentally reduced the energetic demands of incubation in the semipalmated sandpiper, a biparental shorebird breeding in the harsh conditions of the high Arctic. First, we decreased the demands of incubation for 1 parent only by exchanging 1 of the 4 eggs for an artificial egg that heated up when the focal bird incubated. Second, we reanalyzed the data from the only published experimental study that has explicitly tested energetic constraints on incubation-scheduling in a biparentally incubating species (Cresswell et al. 2003). In this experiment, the energetic demands of incubation were decreased for both parents by insulating the nest cup. We expected that the treated birds, in both experiments, would change the length of their incubation bouts, if biparental incubation-scheduling is energetically constrained. However, we found no evidence that heating or insulation of the nest affected the length of incubation bouts: the combined effect of both experiments was an increase in bout length of 3.6min (95% CI: -33 to 40), which is equivalent to a 0.5% increase in the length of the average incubation bout. These results demonstrate that the observed biparental incubation-scheduling in semipalmated sandpipers is not primarily driven by energetic constraints and therefore by the state of the incubating bird, implying that we still do not understand the factors driving biparental incubation-scheduling.
 
A hypothetical relationship between signal performance level (x axis) and response intensity (y axis). Stimulus set A, with one stimulus taken from the lower and one from the median part of the performance range, represents the situation in which 2 signals cover the upward slope of the response intensity curve. In this case, the higher performance level signal (Median) is expected to elicit the strongest response intensity. Stimulus set B represents the situation in which 2 signals cover the downward slope of the curve. In this case, the higher performance level signal (High) is expected to elicit a weaker response intensity than the median performance level signal.
Waveform (top panel) and spectrogram (bottom panel) of a song manipulated to simulate a low-performance (Low), median-performance (Median), and high-performance (High) signal by changing the frequency bandwidth in the trills. Median was manipulated to have the median value of the frequency bandwidth of the trill notes in the natural population ( N 1⁄4 10). The highest frequency was manipulated to 1 kHz lower or higher than the median high frequency for Low and High, respectively. The top horizontal line indicates the median frequency level of the highest frequency in natural banded wren song. The bottom horizontal line indicates the lowest frequency in the song, which is equal in each stimulus. Note that the frequency range in the trill falls within the frequency range of the entire song. 
An example of a manipulated trill note. On the left is a trill note manipulated to the Low, in the middle to the Median, and on the right to the High-frequency range. In this case, the last 3 ms of a trill note (indicated by the vertical lines) was selected and the sampling frequency altered while the remainder of the note stayed unchanged. The top dashed horizontal line indicates the highest, the middle the median, and the bottom the lowest frequency value for the maximum frequency in natural trills for this song type in the population. The bottom continuous line highlights the fact that the lower part of each trill note was not altered. See text for further details. 
The behavioral response of banded wrens to playback of a conspecific song of low, median, and high performance. Top panel: time spent within 15 m of the playback speaker; middle panel: number of songs produced by the individuals that entered the experimental circle (r ¼ 15 m) surrounding the speaker; bottom panel: number of calls produced during the trial.
Latency to approach to within 15 m of the experimental speaker in response to low-performance (Low), median-performance (Median), or high-performance (High) stimuli, using a survival analysis approach to show the cumulative number of males close to the speaker with increasing time since the start of playback.
Using the responses of territory owners to playback to infer the territorial function of acoustic signals is common practice. However, difficulties with interpreting the results of such experiments have obscured our understanding of territorial signalling. For instance, a stronger response to playback is often interpreted as more aggressive, but there is no consensus as to whether this should be in response to the least or most threatening simulated intruder. Rather than following a gradual increase or decrease, the relationship between signal intensity and response strength may instead describe a peaked curve. We manipulated banded wren (Thryophilus pleurostictus) songs to simulate low-, median-, and high-performance singers and used these songs as stimuli in playback experiments. Banded wrens were less likely to approach the high-performance stimulus compared with the low- and median-performance stimuli. However, the birds that did approach the high-performance stimulus sang more than those that approached the low-performance stimulus. In addition, birds were more likely to match the songs when exposed to the median- and high-performance stimuli compared with the low-performance stimuli, and song matching predicted approach behavior. These results are in accordance with theoretical models of aggressive encounters in which low-performance opponents are challenged without further assessment. Median- and high-performance opponents, however, may require further assessment, and the latter may be perceived as too intimidating for approach.
 
Structure and similarity of superb starling flight calls. (A) Time–frequency spectrogram of a representative flight call bout. Darker shades indicate higher power, with the total dynamic range scaled to 50 dB. The bout consists of a series of temporally discrete motifs, which are marked by horizontal bars. (B) Detail of the last 3 motifs from (A), with overlaid F0 estimates (red line). Note the tonal, harmonic structure, and rapid modulations of F0. (C) Similar motifs from another starling in the same social group. (D) Ranked plot of highest similarity scores for each of the birds in the study to the reference motif (B, bottom). Similarity (arbitrary units) is measured using DTW of the F0 traces. Insets show F0 estimates for selected exemplars with high and low similarity. Color indicates whether the comparison motif is from the same bird, a bird in the same social group, or a bird in a different social group.  
In many complex societies, intricate communication and recognition systems may evolve to help support both direct and indirect benefits of group membership. In cooperatively breeding species where groups typically comprise relatives, both learned and innate vocal signals may serve as reliable cues for kin recognition. Here, we investigated vocal communication in the plural cooperatively breeding superb starling, Lamprotornis superbus, where flight calls-short, stereotyped vocalizations used when approaching conspecifics-may communicate kin relationships, group membership, and/or individual identity. We found that flight calls were most similar within individual repertoires but were also more similar within groups than within the larger population. Although starlings responded differently to playback of calls from their own versus other neighboring and distant social groups, call similarity was uncorrelated with genetic relatedness. Additionally, immigrant females showed similar patterns to birds born in the study population. Together, these results suggest that flight calls are learned signals that reflect social association but may also carry a signal of individuality. Flight calls, therefore, provide a reliable recognition mechanism for groups and may also be used to recognize individuals. In complex societies comprising related and unrelated individuals, signaling individuality and group association, rather than kinship, may be a route to cooperation.
 
Study area and radar reflectivity measured on New Year’s Eve, 2008; (a) Overview of the study area in the Netherlands, including general land cover. The radar at De Bilt (lat 52.103 ° N, long 5.179 ° E) is indicated by a red dot and the blue lines and polygons show major rivers, lakes, and wetlands in the area. The lake and wetland area Oostelijke Vechtplassen (lat 52.2 ° N, long 5.05 ° E) is indicated by a red polygon. (b) Overlaid radar reflectivity (square centimeter/cubic kilometer) at 1 January 2009 00:00 LT(GMT 1 1). The area where the beam center exceeds 750 m altitude is shaded in gray, as birds mainly fly up to about 500 m (Figure 2e), not many echoes are expected outside this area. (c) Radar reflectivity (square centimeter/cubic kilometer) at 1 January 2009 00:15 LT. Red and orange areas indicate hot spots of bird activity predominantly over water bodies. Supplementary Material, Supplementary movie 1 shows the reflectivity in 5-min intervals for region shown in this figure. 
Time series of bird movements over Loosdrechtse Plassen. VIR (square centimeter/square kilometer) from 30 December 00:00 to 3 January 00:00 for (a) 2007/2008, (b) 2008/2009, and (c) 2009/2010. (d) VIR (square centimeter/square kilometer) from 31 December 23:00 to 1 January 02:00 (2007/2008 green, 2008/2009 blue, and 2009/2010 red). (e) Altitude density profile from 31 December 2008 23:00 to 1 January 2009 02:00 over Oostelijke Vechtplassen. Altitude (kilometer) is shown on the y axis and time on the x axis. Colors represent measured reflectivity (square centimeter/cubic kilometer). Gray shaded areas in 2a–d indicate the time between sunset and sunrise. 
Anthropogenic disturbances of wildlife, such as noise, human presence, hunting activity, and motor vehicles, are becoming an increasing concern in conservation biology. Fireworks are an important part of celebrations worldwide, and although humans often find fireworks spectacular, fireworks are probably perceived quite differently by wild animals. Behavioral responses to fireworks are difficult to study at night, and little is known about the negative effects fireworks may have on wildlife. Every year, thousands of tons of fireworks are lit by civilians on New Year's Eve in the Netherlands. Using an operational weather radar, we quantified the reaction of birds to fireworks in 3 consecutive years. Thousands of birds took flight shortly after midnight, with high aerial movements lasting at least 45 min and peak densities measured at 500 m altitude. The highest densities were observed over grasslands and wetlands, including nature conservation sites, where thousands of waterfowl rest and feed. The Netherlands is the most important winter staging area for several species of waterfowl in Europe. We estimate that hundreds of thousands of birds in the Netherlands take flight due to fireworks. The spatial and temporal extent of disturbance is substantial, and potential consequences are discussed. Weather radar provides a unique opportunity to study the reaction of birds to fireworks, which has otherwise remained elusive.
 
Experimental feeding site locations across London. Black circles represent parakeet free sites, white circles represent sites where parakeets were present, the solid gray area represents the area of Greater London, the lined polygon represents the area of the 2009 parakeet range (this is the extent of the Breeding Bird Survey 1 km 2 squares where parakeets were recorded present in 2009). The cross represents the location of St. Paul's Cathedral (sites were chosen within a 50-km radius of this).  
Differences between sites per treatments outside and inside parakeet range for (a) mean total visits, and (b) the proportion of visits resulting in a feed (visits outside range n = 4027, inside range n = 6826). Black lines show controls and gray lines show treatments. Dotted lines denote differences between range sites within treatments which are not significant, solid lines are significant (P values, *P < 0.05). Significant difference refers to differences within treatments between sites outside and inside the parakeet range.  
Resource competition is one potential behavioral mechanism by which invasive species can impact native species, but detecting this competition can be difficult due to the interactions that variable environmental conditions can have on species behavior. This is particularly the case in urban habitats where the disturbed environment can alter natural behavior from that in undisturbed habitats. The rose-ringed parakeet (Psittacula krameri), is an increasingly common invasive species, predominantly associated with large urban centers. Using an experimental approach, we tested the behavioral responses of native garden birds in response to the presence of a rose-ringed parakeet versus the presence of a similarly sized and dominant native bird, the great spotted woodpecker (Dendrocopos major). Parakeet presence significantly reduced feeding rates and increased vigilance among native birds compared with our control treatments. Of visits made by native birds in the presence of a parakeet, feeding was more likely to occur in sites within the parakeet range compared with sites outside, suggesting some habituation of native birds has occurred following prior exposure to parakeets but overall foraging behavior is still disrupted. The results of our study suggest that nonnative species can have complex and subtle impacts on native fauna and show that a nonnative competitor can impact native species simply through their presence near resources.
 
Average reflectance curves for unmanipulated juvenile male eastern bluebirds ( 6 SD, solid black line), violet marker treated (relatively brightened) juvenile males (dashed gray line), black marker treated (relatively darkened) juvenile males ( 6 SD, dashed black line), and 8 naturally dull juvenile males (circles). 
The relationship between female eastern bluebird feeding behavior during the control period (proportion of total feeding attempts toward naturally brighter sons) and the number of feeding attempts that fathers directed toward naturally brighter sons during the same period. To facilitate a more accurate interpretation of the relative weights of each trial to the final model, symbol sizes are proportional to the number of maternal feeding attempts in each trial. 
The relationship between male eastern bluebird feeding behavior during the control period (proportion of total feeding attempts toward naturally brighter sons) and the relative plumage brightness of the sons being fed. Symbol sizes are proportional to the number of paternal feeding attempts in each trial. 
The relationship between female eastern bluebird feeding behavior during the experimental period (proportion of total feeding attempts toward experimentally brightened sons) and the relative parental investment during the nestling stage. Symbol sizes are proportional to the number of maternal feeding attempts in each trial. 
The relationship between male eastern bluebird feeding behavior during the experimental period (proportion of total feeding attempts toward experimentally brightened sons) and the relative plumage brightness of the sons being fed. Symbol sizes are proportional to the number of paternal feeding attempts in each trial. 
The relative amount of resources that avian parents provide to individual offspring within a brood represents a strategy that can have large effects on reproductive success. We tested whether parental feeding decisions of eastern bluebirds Sialia sialis are influenced by offspring plumage color by presenting pairs of differently colored fledglings side by side and observing how they were provisioned by parents. After a control period, we manipulated blue plumage color so that one sibling in each trial became relatively dark and one became relatively bright. During neither the control nor the experimental periods did either parent consistently feed naturally brighter or experimentally brightened sons more than drab sons. Under specific circumstances, however, both parents directed a higher proportion of their feeding attempts to more brightly colored sons. Paternal feeding attempts to brighter offspring during both the control and experimental periods increased in relation to the brightness of these fledglings relative to their brothers. Maternal feeding decision, on the other hand, were influenced by numerous variables during control and experimental periods including the date of the trial, the difference in mass between fledglings, the feeding behavior of fathers during the trial, the relative investment by fathers during the nestling stage, and the amount of UV chroma in fledgling plumage. Taken together, these results suggest that equal provisioning of offspring is the strategy most commonly adopted by eastern bluebirds but more brightly colored offspring will be fed preferentially when resources for offspring are limited.
 
How do territorial neighbors resolve the location of their boundaries? We addressed this question by testing the predictions of 2 nonexclusive game theoretical models for competitive signaling: the sequential assessment game and the territorial bargaining game. Our study species, the banded wren, is a neotropical nonmigratory songbird living in densely packed territorial neighborhoods. The males possess repertoires of approximately 25 song types that are largely shared between neighbors and sequentially delivered with variable switching rates. Over 3 days, boundary disputes among pairs of neighboring males were synchronously recorded, their perch positions were marked, and their behavioral interactions were noted. For each countersinging interaction between 2 focal males, we quantified approach and retreat order, a variety of song and call patterns, closest approach distance, distance from the territorial center, and female presence. Aggressors produced more rattle-buzz songs during the approaching phase of interactions, whereas defenders overlapped their opponent's songs. During the close phase of the interaction, both males matched frequently, but the key determinant of which one retreated first was song-type diversity-first retreaters sang with a higher diversity. Retreaters also produced more unshared song types during the interaction, and in the retreating phase of the interaction, they overlapped more. A negative correlation between song-type diversity asymmetry and contest duration suggested sequential assessment of motivational asymmetry. The use of this graded signal, which varied with distance from the center and indicated a male's motivation to defend a particular position, supported the bargaining model. The bargaining game could be viewed as a series of sequential assessment contests.
 
Frequency histogram of individual jumping spiders' (Phidippus clarus) latency to scale (s) a clear plastic vial (height = 8.5 cm). 
Consistent interindividual differences in behavior (i.e., "behavioral types") may be a key factor in determining the outcome of species interactions. Studies that simultaneously account for the behavioral types of individuals in multiple interacting species, such as predator-prey systems, may be particularly strong predictors of ecological outcomes. Here, we test the predator-prey locomotor crossover hypothesis, which predicts that active predators are more likely to encounter and consume prey with the opposing locomotor tendency. We test this hypothesis using intraspecific behavioral variation in both a predator and prey species as predictors of foraging outcomes. We use the old field jumping spider, Phidippus clarus (Araneae, Salticidae), and the house cricket, Acheta domesticus (Orthoptera, Gryllidae), as a model predator-prey system in laboratory mesocosm trials. Stable individual differences in locomotor tendencies were identified in both P. clarus and A. domesticus, and the outcome of foraging bouts depended neither on the average activity level of the predator nor on the average activity level of prey. Instead, an interaction between the activity level of spiders and crickets predicted spider foraging success and prey survivorship. Consistent with the locomotor crossover hypothesis, predators exhibiting higher activity levels consumed more prey when in an environment containing low-activity prey items and vice versa. This study highlights 1) the importance of intraspecific variation in determining the outcome of predator-prey interactions and 2) that acknowledging behavioral variation in only a single species may be insufficient to characterize the performance consequences of intraspecific trait variants.
 
Although many herbivores and omnivores have been shown to balance their intake of macronutrients when faced with nutritionally variable foods, study of this ability has been relatively neglected in carnivores, largely on the assumption that prey are less variable in nutrient composition than the foods of herbivores and omnivores and such mechanisms therefore unnecessary. We performed diet selection studies in 5 breeds of adult dog (Canis lupus familiaris) to determine whether these domesticated carnivores regulate macronutrient intake. Using nutritional geometry, we show that the macronutrient content of the diet was regulated to a protein:fat:carbohydrate ratio of approximately 30%:63%:7% by energy, a value that was remarkably similar across breeds. These values, which the analysis suggests are dietary target values, are based on intakes of dogs with prior experience of the respective experimental food combinations. On initial exposure to the diets (i.e., when naive), the same dogs self-selected a diet that was marginally but significantly lower in fat, suggesting that learning played a role in macronutrient regulation. In contrast with the tight regulation of macronutrient ratios, the total amount of food and energy eaten was far higher than expected based on calculated maintenance energy requirements. We interpret these results in relation to the evolutionary history of domestic dogs and compare them to equivalent studies on domestic cats.
 
Examples of how sexual swelling size varies in the Amboseli baboon population in southern Kenya (A) within one cycle of an individual, (B) between cycles of one individual, and (C) across individuals. (A) Changes in swelling size over the course of one nonconceptive cycle for one female. Each point represents the mean width ( 6 standard deviation [SD]) of the swelling measured from multiple images on a single day. The vertical dashed line represents the first day of deturgescence. (B) Changes in maximum swelling size for 3 consecutive estrous cycles within one female; cycles are numbered consecutively from when the female resumed cycling (after the birth of the previous offspring). Each point represents the mean width ( 6 SD) of the swelling measured from multiple images on the day prior to deturgescence in each cycle. The female’s fourth cycle after resumption was a conceptive cycle. (C) Differences in maximum swelling size across females; each point represents the mean maximum swelling size ( 6 SD) for one conceptive cycle of one female. 
The exaggerated sexual swellings exhibited by females of some primate species have been of interest to evolutionary biologists since the time of Darwin. We summarize existing hypotheses for their function and evolution and categorize these hypotheses within the context of 3 types of variation in sexual swelling size: 1) variation within a single sexual cycle, 2) variation between the sexual cycles of a single female, and 3) differences between females. We then propose the Paternal Care Hypothesis for the function of sexual swellings, which posits that exaggerated sexual swellings function to elicit the right quantity and quality of male care for a female's infant. As others have noted, swellings may allow females to engender paternity confusion, or they may allow females to confer relative paternal certainty on one male. Key to our hypothesis is that both of these scenarios create an incentive for one or more males to provide care. This hypothesis builds on previous hypotheses but differs from them by highlighting the elicitation of paternal care as a key function of swellings. Our hypothesis predicts that true paternal care (in which males accurately differentiate and provide assistance to their own offspring) will be most common in species in which exaggerated swellings accurately signal the probability of conception, and males can monopolize females during the window of highest conception probability. Our hypothesis also predicts that females will experience selection to behave in ways that either augment paternity confusion or enhance paternal certainty depending on their social and demographic contexts.
 
Long-lived iteroparous species often show aging-related changes in reproduction that may be explained by 2 non-mutually exclusive hypotheses. The terminal investment hypothesis predicts increased female reproductive effort toward the end of the life span, as individuals have little to gain by reserving effort for the future. The senescence hypothesis predicts decreased female reproductive output toward the end of the life span due to an age-related decline in body condition. Nonhuman primates are ideal organisms for testing these hypotheses, as they are long lived and produce altricial offspring heavily dependent on maternal investment. In this study, we integrated 50 years of continuous demographic records for the Cayo Santiago rhesus macaque (Macaca mulatta) population with new morphometric and behavioral data to test the senescence and terminal investment hypotheses. We examined relationships between maternal age and activity, mother and infant body condition, interbirth intervals, measures of behavioral investment in offspring, and offspring survival and fitness to test for age-associated declines in reproduction that would indicate senescence, and for age-associated increases in maternal effort that would indicate terminal investment. Compared with younger mothers, older mothers had lower body mass indices and were less active, had longer interbirth intervals, and spent more time in contact with infants, but had infants of lower masses and survival rates. Taken together, our results provide strong evidence for the occurrence of reproductive senescence in free-ranging female rhesus macaques but are also consistent with some of the predictions of the terminal investment hypothesis.
 
Mating trial results 
In unicellular organisms like yeast, mating with the right partner is critical to future fitness because each individual can only mate once. Because cell size is important for viability, mating with a partner of the right size could be a significant advantage. To investigate this idea, we manipulated the size of unmated yeast cells and showed that their viability depended on environmental conditions; large cells do better on rich medium and small cells do better on poor medium. We also found that the fitness of offspring is determined by the size of their parents. Finally, we demonstrated that when a focal cell of one mating type was placed with a large and a small cell of the opposite mating type, it was more likely to mate with the cell that was closer to the optimum size for growth in a given environment. This pattern was not generated by differences in passive mating efficiency of large and small cells across environments but by competitive mating behavior, mate preference, or both. We conclude that the most likely mechanism underlying this interesting behavior is that yeast cells compete for mates by producing pheromone signals advertising their viability, and cells with the opportunity to choose prefer to mate with stronger signalers because such matings produce more viable offspring.
 
(A) A TIC profile obtained from male UCSD junco preen oil, (B) a corresponding postrun single ion current (SIC) profile of methyl ketones (m/z 58) from the time range 25–50 min (1: 2-dodecanone, 2: 2-tridecanone, 3: 2-tetradecanone, 4: 2-pentadecanone, 5: 2- hexadecanone, 5: 2-heptadecanone), and (C) the mass spectrum of 2-pentadecanone from the SIC profile (B). 
Repeatability estimates for volatile compounds in junco preen oil (n 5 26 individuals)
Principal component 1 and principal component 3 scores derived from relative proportions of volatile compounds. (See Table 2 for principal component loadings.) 
Chemical signaling has been documented in many animals, but its potential importance in avian species, particularly songbirds, has received far less attention. We tested whether volatile compounds in the preen oil of a songbird (Junco hyemalis) contain reliable information about individual identity, sex, or population of origin by repeated sampling from captive male and female juncos originating from 2 recently diverged junco populations in southern California. One of the populations recently colonized an urban environment; the other resides in a species-typical montane environment. The birds were field-caught as juveniles, housed under identical conditions, and fed the same diet for 10 months prior to sampling. We used capillary gas chromatography–mass spectrometry to quantify the relative abundance of 19 volatile compounds previously shown to vary seasonally in this species. We found individual repeatability as well as significant sex and population differences in volatile profiles. The persistence of population differences in a common environment suggests that preen oil chemistry likely has a genetic basis and may thus evolve rapidly in response to environmental change. These finding suggest that songbird preen oil odors have the potential to function as chemosignals associated with mate recognition or reproductive isolation.
 
Male responses to 3 female-mediated cues of sperm competition (origin, mating history, and oestrus stage) at 3 different levels of reproductive effort: (a) mating propensity, (b) ejaculation frequency, and (c) sperm allocation per ejaculate. Males were significantly more likely to mate with unfamiliar females and significantly less likely to mate with previously mated females (a, see Table 1 for details) but did not significantly alter either their ejaculation frequency (b) or sperm allocation (c) according to these cues (see main text for test statistics). Bars in (a) represent the percentage of trials resulting in mating and in (b) and (c) means ± standard error of the mean. 
Male eagerness to mate is a central paradigm of sexual selection theory. However, limited sperm supplies mean that male sexual restraint might sometimes be favored under promiscuous mating. Here, we demonstrate dynamic plasticity in male mating effort when females are encountered sequentially under varying sperm competition risk. Rather than showing consistent eagerness to mate, male house mice (Mus musculus domesticus) instead tailor their mating effort according to likely reproductive payoffs. They are significantly less likely to mate when sperm competition is certain and potential reproductive payoffs low, but dramatically increase investment if they do choose to mate under such circumstances. By contrast, male mice are significantly more likely to mate in situations simulating extra-territorial copulations, where future risk of competition is high but so too are potential reproductive rewards. Differential mating propensity appears to be the primary mechanism by which male house mice allocate sperm adaptively under sperm competition risk because we find no evidence for facultative adjustment of sperm numbers per ejaculate or ejaculation frequency in response to female-related cues. We conclude that sequential male mate choice under sperm competition risk could be a widespread but often unappreciated mechanism of strategic sperm allocation.
 
Theoretical predictions for the relationship between number of competitors and fight intensity; a) increasing encounter rate is counteracted by increased cost of fighting as the number of competitors increases (Murray, 1987); b) fight intensity varies with competitor number and the relatedness of competitors, where competitors are either closely related (1 foundress female; solid line), a mixture of related and unrelated males (2 foundresses; dotted line) giving intermediate average relatedness, or have low average relatedness (3 foundresses; dashed line) (Reinhold, 2003). In both cases, the y axis corresponds to increasing fight intensity. 
Mean fighting rate between male pairs within 3 treatment combinations: both virgins (VV), both mated (MM), and a mated versus a virgin male (MV). Error bars indicate standard errors. 
Although most animals employ strategies to avoid costly escalation of conflict, the limitation of critical resources may lead to extreme contests and fatal fighting. Evolutionary theories predict that the occurrence and intensity of fights can be explained by resource value and the density and relatedness of competitors. However, the interaction between these factors and their relative importance often remains unclear; moreover, few systems allow all variables to be experimentally investigated, making tests of these theoretical predictions rare. Here, we use the parasitoid wasp Melittobia to test the importance of all these factors. In contrast to predictions, variation in contested resource value (female mates) and the relatedness of competitors do not influence levels of aggression. However, as predicted, fight intensity increased with competitor density and was not influenced by the greater cost of fighting at high density. Our results suggest that in the absence of kin recognition, indirectly altruistic behavior (spite) is unlikely to evolve, and in such circumstances, the scale of competition will strongly influence the amount of kin discrimination in the form of level of aggression as observed in Melittobia species.
 
A migrating bird's response to wind can impact its timing, energy expenditure, and path taken. The extent to which nocturnal migrants select departure nights based on wind (wind selectivity) and compensate for wind drift remains unclear. In this paper, we determine the effect of wind selectivity and partial drift compensation on the probability of successfully arriving at a destination area and on overall migration speed. To do so, we developed an individual-based model (IBM) to simulate full drift and partial compensation migration of juvenile Willow Warblers (Phylloscopus trochilus) along the southwesterly (SW) European migration corridor to the Iberian coast. Various degrees of wind selectivity were tested according to how large a drift angle and transport cost (mechanical energy per unit distance) individuals were willing to tolerate on departure after dusk. In order to assess model results, we used radar measurements of nocturnal migration to estimate the wind selectivity and proportional drift among passerines flying in SW directions. Migration speeds in the IBM were highest for partial compensation populations tolerating at least 25% extra transport cost compared to windless conditions, which allowed more frequent departure opportunities. Drift tolerance affected migration speeds only weakly, whereas arrival probabilities were highest with drift tolerances below 20°. The radar measurements were indicative of low drift tolerance, 25% extra transport cost tolerance and partial compensation. We conclude that along migration corridors with generally nonsupportive winds, juvenile passerines should not strictly select supportive winds but partially compensate for drift to increase their chances for timely and accurate arrival.
 
In spite of recent interest in sexual selection in females, debate exists over whether traits that influence female-female competition are sexually selected. This review uses female-female aggressive behavior as a model behavioral trait for understanding the evolutionary mechanisms promoting intrasexual competition, focusing especially on sexual selection. I employ a broad definition of sexual selection, whereby traits that influence competition for mates are sexually selected, whereas those that directly influence fecundity or offspring survival are naturally selected. Drawing examples from across animal taxa, including humans, I examine 4 predictions about female intrasexual competition based on the abundance of resources, the availability of males, and the direct or indirect benefits those males provide. These patterns reveal a key sex difference in sexual selection: Although females may compete for the number of mates, they appear to compete more so for access to high-quality mates that provide direct and indirect (genetic) benefits. As is the case in males, intrasexual selection in females also includes competition for essential resources required for access to mates. If mate quality affects the magnitude of mating success, then restricting sexual selection to competition for quantity of mates may ignore important components of fitness in females and underestimate the role of sexual selection in shaping female phenotype. In the future, understanding sex differences in sexual selection will require further exploration of the extent of mutual intrasexual competition and the incorporation of quality of mating success into the study of sexual selection in both sexes.
 
The proportion of male Phidippus clarus that won a contest with a naive size-matched rival at 4 time intervals after first winning (black) or losing (white) an initial contest. In P. clarus, relative size predicts competitive success, so if there is no experience effect, 50% of previous winners or losers should win subsequent contests (dotted line). Stars indicate proportions significantly different from 50% (see text). 
In many animal taxa, prior contest experience affects future performance such that winning increases the chances of winning in the future (winner effect) and losing increases the chances of losing in the future (loser effect). It is, however, not clear whether this pattern typically arises from experience effects on actual or perceived fighting ability (or both). In this study, we looked at winner and loser effects in the jumping spider Phidippus clarus. We assigned winning or losing experience to spiders and tested them against opponents of similar fighting ability in subsequent contests at 1-, 2-, 5-, and 24-h intervals. We examined the strength of winner and loser effects, how long effects persist, as well as how experience affected perceived and actual fighting ability. Our results demonstrate that winner and loser effects are of approximately the same magnitude, although loser effects last longer than winner effects. Our results also demonstrate that previous experience alters actual fighting ability because both the assessment and escalation periods were affected by experience. We suggest that the retention time of experience effects depends on expected encounter rates as well as other behavioral and ecological factors. In systems with short breeding seasons and/or rapidly fluctuating populations, context-dependent retention of experience effects may allow males to track their status relative to the fluctuating fighting ability of local competitors without paying the costs necessary to recall or assess individual competitors.
 
Mean, standard deviation, and ranges for cortisol concentrations in mother's milk at early and peak lactation (1 month and 3–4 months postpartum, respectively) and the calculated change (Δ) per day between the 2 time points for rhesus macaques (N = 108) 
Concentration of cortisol in milk (nmol/L) for each individual rhesus macaque mother at early and peak lactation (1 month and 3-4 months, respectively). Each line represents an individual female (N = 107 shown, y-axis truncated at 425 nmol/L excludes one subject) and variable color for better visualization of individuals.
Scatter plot of cortisol concentrations in milk at early (A) and peak (B) lactation as well as the calculated change (Δ) per day between the 2 time points (C) by maternal parity. Fitted regression line in black.  
Mean ± SEM of cortisol concentrations in milk of mothers of sons (N = 47) and daughters (N = 61) at early and peak lactation and the mean calculated change (Δ) per day between the 2 time points.
Residualized scatter plots and fitted regression line for milk cortisol parameters and infant temperament factor by infant sex. Nervous temperament of daughters in relation to cortisol concentrations in milk at early lactation (A) and for sons in relation to the change in cortisol concentrations in milk from early to peak lactation (B). Confident temperament of daughters in relation to cortisol concentrations in milk at peak lactation (C) and for sons in relation to the change in cortisol concentrations in milk from early to peak lactation (D).
The maternal environment exerts important influences on offspring mass/growth, metabolism, reproduction, neurobiology, immune function, and behavior among birds, insects, reptiles, fish, and mammals. For mammals, mother's milk is an important physiological pathway for nutrient transfer and glucocorticoid signaling that potentially influences offspring growth and behavioral phenotype. Glucocorticoids in mother's milk have been associated with offspring behavioral phenotype in several mammals, but studies have been handicapped by not simultaneously evaluating milk energy density and yield. This is problematic as milk glucocorticoids and nutrients likely have simultaneous effects on offspring phenotype. We investigated mother's milk and infant temperament and growth in a cohort of rhesus macaque (Macaca mulatta) mother-infant dyads at the California National Primate Research Center (N = 108). Glucocorticoids in mother's milk, independent of available milk energy, predicted a more Nervous, less Confident temperament in both sons and daughters. We additionally found sex differences in the windows of sensitivity and the magnitude of sensitivity to maternal-origin glucocorticoids. Lower parity mothers produced milk with higher cortisol concentrations. Lastly, higher cortisol concentrations in milk were associated with greater infant weight gain across time. Taken together, these results suggest that mothers with fewer somatic resources, even in captivity, may be "programming" through cortisol signaling, behaviorally cautious offspring that prioritize growth. Glucocorticoids ingested through milk may importantly contribute to the assimilation of available milk energy, development of temperament, and orchestrate, in part, the allocation of maternal milk energy between growth and behavioral phenotype.
 
One of the most fascinating examples of parasite-induced host manipulation is that of hairworms, first, because they induce a spectacular "suicide" water-seeking behavior in their terrestrial insect hosts and, second, because the emergence of the parasite is not lethal per se for the host that can live several months following parasite release. The mechanisms hairworms use to increase the encounter rate between their host and water remain, however, poorly understood. Considering the selective landscape in which nematomorph manipulation has evolved as well as previously obtained proteomics data, we predicted that crickets harboring mature hairworms would display a modified behavioral response to light. Since following parasite emergence in water, the cricket host and parasitic worm do not interact physiologically anymore, we also predicted that the host would recover from the modified behaviors. We examined the effect of hairworm infection on different behavioral responses of the host when stimulated by light to record responses from uninfected, infected, and ex-infected crickets. We showed that hairworm infection fundamentally modifies cricket behavior by inducing directed responses to light, a condition from which they mostly recover once the parasite is released. This study supports the idea that host manipulation by parasites is subtle, complex, and multidimensional.
 
Field observation of a wild population of Lymnaea stagnalis. The top scatter plot depicts the shell length of the snails that were collected monthly over a 2-year period (data are depicted according to month of the year). Based on a K-means clustering on collection date and body size, the collected samples could be separated into 2 age cohorts (white and black circles). The top histograms represent the bimodal distribution of 2 age cohorts over the year (respectively, white and black), but their shell length distributions are similar, as shown by the histogram on the right side. As abiotic factors affecting their reproductive activity, the middle graph shows the daylight hours over the year and the bottom graph indicates the water temperature measured during the field sampling. The bar at the bottom of the figure indicates the period during which reproductive activities, that is, mating and egg laying, were observed (black). 
Mating role frequency in the sex role choice experiment. Primary donor rate is shown as proportion of individuals that inseminate first in a pair of snails with same size but different age (age-different group) and same age but different size (size-different group). The numbers in parentheses indicate sample size of each treatment, and error bars indicate 95% CIs. 
Mating behavior in the sex role choice experiment. Courtship and insemination duration are shown in comparison to mating order (A and B), age (C and D), and size (E and F). Because each pair includes primary and secondary donors, we used paired Wilcoxon rank sum tests. Asterisks indicate significant differences between groups (*P < 0.05; **P < 0.01; ***P < 0.001). The whiskers indicate minimum and maximum values, the box is for quartiles, and the thick line in the box stands for median. The open circles show outliners. 
Sex allocation proxies of Lymnaea stagnalis of different ages and sizes. Dry weight of prostate glands, seminal vesicles (male investment: A and B), albumen glands (female investment: C), and whole-body weight are represented. The lines in the first figure represent significant regressions with whole-body weight. A closed symbol indicates a small individual and an open symbol a large one. The 2 types of symbols, square and circle, show age difference, although we did not detect any difference between ages (see Results). 
Contrasting with separate-sexed animals, simultaneous hermaphrodites display unique reproductive strategies as they are male and female at the same time. Simultaneous hermaphrodites that copulate unilaterally, for instance, make a decision to mate as a male or female. Previous studies have demonstrated that sex role preference in hermaphrodites is flexible and is controlled by several, often confounding, factors. We examined the relationship between sex role decisions and 3 life-history traits (age, size, and mating history) in the great pond snail, Lymnaea stagnalis. Based on our field observations, which indicate that adult individuals show overlapping generations and large variation in body size during the breeding season, we performed a sex role choice experiment in the laboratory. We found that young and small snails mate as males first. Both age and size significantly affected sex role decision, with age having a stronger effect. Furthermore, we tested whether L. stagnalis becomes reluctant to inseminate a mate after being inseminated because it is known that after insemination, male investment substantially reduces. Contrary to expectations, our results indicate that the receipt of seminal fluid does not seem to reduce male motivation. In sum, sex role decisions in L. stagnalis are largely determined by age and size but not by having received seminal fluid. This mating pattern, however, does not fully support the size-advantage model because large or old individuals did not perform better as females in our experiment. These results imply a conflicting mating interest, rather than harmonious agreement, between age- and size-different hermaphrodites.
 
Visual illustration of the size of menstrual cycle shifts in preferences documented in Penton-Voak et al. 1999. Photos show mean masculinity preferred at high fertility (L) and low fertility (R).
In the literature on human mate choice, masculine facial morphology is often proposed to be an intersexual signal of heritable immunocompetence, and hence an important component of men's attractiveness. This hypothesis has received considerable research attention, and is increasingly treated as plausible and well supported. In this article, we propose that the strength of the evidence for the immunocompetence hypothesis is somewhat overstated, and that a number of difficulties have been under-acknowledged. Such difficulties include (1) the tentative nature of the evidence regarding masculinity and disease in humans, (2) the complex and uncertain picture emerging from the animal literature on sexual ornaments and immunity, (3) the absence of consistent, cross-cultural support for the predictions of the immunocompetence hypothesis regarding preferences for masculinized stimuli, and (4) evidence that facial masculinity contributes very little, if anything, to overall attractiveness in real men. Furthermore, alternative explanations for patterns of preferences, in particular the proposal that masculinity is primarily an intrasexual signal, have been neglected. We suggest that immunocompetence perspectives on masculinity, whilst appealing in many ways, should still be regarded as speculative, and that other perspectives-and other traits-should be the subject of greater attention for researchers studying human mate preferences.
 
Top-cited authors
Anders Pape Moller
  • French National Centre for Scientific Research
Wolfgang Forstmeier
  • Max Planck Institute for Ornithology
Holger Schielzeth
  • Friedrich Schiller University Jena
Niels J Dingemanse
  • Ludwig-Maximilians-University of Munich
Geoffrey E. Hill
  • Auburn University