Photoregulatory functions drive variation in eye coloration across macaque species

Primates, the most colorful mammalian radiation, have previously served as an interesting model to test the functions and evolutionary drivers of variation in eye color. We assess the contribution of photo-regulatory and communicative functions to the external eye appearance of nine macaque species representing all the branches of their radiation. Macaques' well described social structure and wide geographical distribution make them interesting to explore. We find that (1) the posterior option of the anterior eyeball is more pigmented closer to the equator, suggesting photoprotective functions. We also find that (2) the temporal side of the eyeball is more heavily pigmented than the nasal side. This suggests that eyeball pigmentation in macaques is distributed to reduce damage to the corneal limbus. The inclusion of a translocated population of M. fuscata in our analyses also suggests that external eye appearance may change quickly, perhaps owing to phenotypic plasticity. We find no evidence that communicative functions drive variation in external eye appearance in macaques. These results suggest that the amount of light in a species' environment drives variation in eye coloration across macaque species. Furthermore, the geographical distribution of macaques hints at important factors that have yet to be accounted for, such as the reflectivity of the terrain a given species inhabits.

The dynamics of sociality and glucocorticoids in wild male Assamese macaques

For males of gregarious species, dominance status and the strength of affiliative relationships can have major fitness consequences. Social dynamics also impose costs by affecting glucocorticoids, mediators of homeostasis and indicators of the physiological response to challenges and within-group competition. We investigated the relationships between dominance, social bonds, seasonal challenges, and faecal glucocorticoid metabolite (fGC) measures in wild Assamese macaques (Macaca assamensis) at Phu Khieo Wildlife Sanctuary, Thailand, combining behavioural data with 4129 samples from 62 adult males over 15 years. Our previous work on this population suggested that increased competition during the mating season was associated with elevated fGC levels and that, unusually for male primates, lower rank position correlated with higher fGC levels. With a much larger dataset and dynamic measures of sociality, we re-examined these relationships and additionally tested the potentially fGC-attenuating effect of social support. Contrary to our previous study, yet consistent with the majority of work on male primates, dominance rank had a positive relationship with fGC levels, as high status correlated with elevated glucocorticoid measures. fGC levels were increased at the onset of the mating season. We demonstrated an fGC-reducing effect of supportive relationships in males and showed that dynamics in affiliation can correlate with dynamics in physiological responses. Our results suggest that in a system with intermediate contest potential, high dominance status can impose physiological costs on males that may potentially be moderated by social relationships. We highlight the need to consider the dynamics of sociality and competition that influence hormonal processes.

Animal behavior: A tale of two apes

A new paper shows that rates of aggression are higher, and rates of coalition formation are lower, among male bonobos than among male chimpanzees. These findings are noteworthy because they challenge the view that female bonobos' preferences for less aggressive males favored a reduction in male aggression and an increase in social tolerance.

Sex differences in cooperative coalitions: a mammalian perspective

In group-living species, cooperative tactics can offset asymmetries in resource-holding potential between individuals and alter the outcome of intragroup conflicts. Differences in the kinds of competitive pressures that males and females face might influence the benefits they gain from forming intragroup coalitions. We predicted that there would be a female bias in intragroup coalitions because females (1) are more like to live with kin than males are, and (2) compete over resources that are more readily shared than resources males compete over. We tested this main prediction using information about coalition formation across mammalian species and phylogenetic comparative analyses. We found that for nearly all species in which intragroup coalitions occur, members of both sexes participate, making this the typical mammalian pattern. The presence and frequency of female or male coalitions were not strongly associated with key socio-ecological factors like resource defensibility, sexual dimorphism or philopatry. This suggests that once the ability to form intragroup coalitions emerges in one sex, it is likely to emerge in the other sex as well and that there is no strong phylogenetic legacy of sex differences in this form of cooperation. This article is part of the theme issue 'Cooperation among women: evolutionary and cross-cultural perspectives'.

The economics of brain size evolution in vertebrates

Across the animal kingdom, we see remarkable variation in brain size. This variation has even increased over evolutionary time. Traditionally, studies aiming to explain brain size evolution have looked at the fitness benefits of increased brain size in relation to its increased cognitive performance in the social and/or ecological domain. However, brains are among the most energetically expensive tissues in the body and also require an uninterrupted energy supply. If not compensated, these energetic demands inevitably lead to a reduction in energy allocation to other vital functions. In this review, we summarize how an increasing number of studies show that to fully comprehend brain size evolution and the large variation in brain size across lineages, it is important to look at the economics of brains, including the different pathways through which the high energetic costs of brains can be offset. We further show how numerous studies converge on the conclusion that cognitive abilities can only drive brain size evolution in vertebrate lineages where they result in an improved energy balance through favourable ecological preconditions. Cognitive benefits that do not directly improve the organism's energy balance can only be selectively favoured when they produce such large improvements in reproduction or survival that they outweigh the negative energetic effects of the large brain.

The ecology of wealth inequality in animal societies

Individuals vary in their access to resources, social connections and phenotypic traits, and a central goal of evolutionary biology is to understand how this variation arises and influences fitness. Parallel research on humans has focused on the causes and consequences of variation in material possessions, opportunity and health. Central to both fields of study is that unequal distribution of wealth is an important component of social structure that drives variation in relevant outcomes. Here, we advance a research framework and agenda for studying wealth inequality within an ecological and evolutionary context. This ecology of inequality approach presents the opportunity to reintegrate key evolutionary concepts as different dimensions of the link between wealth and fitness by (i) developing measures of wealth and inequality as taxonomically broad features of societies, (ii) considering how feedback loops link inequality to individual and societal outcomes, (iii) exploring the ecological and evolutionary underpinnings of what makes some societies more unequal than others, and (iv) studying the long-term dynamics of inequality as a central component of social evolution. We hope that this framework will facilitate a cohesive understanding of inequality as a widespread biological phenomenon and clarify the role of social systems as central to evolutionary biology.

Mating and Reproductive Success in Free-Ranging Stump-Tailed Macaques: Effectiveness of Male–Male Coalition Formation as a Reproductive Strategy

Male coalition-like formation, recently found in stump-tailed macaques (Macaca arctoides), occurs when several top-ranking males collaboratively guard females to prevent mating with other rival males and actively share secured mating opportunities with their allies. We lack a comprehensive understanding of the proximate mechanisms underlying such male coalitions, e.g., the effect of genetic relatedness. Such cooperative partner choice among males is particularly interesting in animals, such as M. arctoides, that live in matrilineal (female philopatry/male dispersal) society. Theoretically, in such a social system, females, and not males, are often related to each other by kinship. Thus, the occurrence of cooperative behavior between related females are generally common, but it would be rare between unrelated/non-kin males in such matrilineal society. Herein, for the first time, we report detailed copulatory behaviors, including mating and reproductive success, in relation to male coalition-like formation in free-ranging M. arctoides following direct behavioral observation and genetic analysis. We found that coalition-forming male–male alliances often occur among both individuals that are highly related and those that are less related. We identified two groups with different mating strategies, i.e., single-male monopoly and coalition-male monopoly groups. In both groups, nearly 80% of copulations were monopolized by a single male or by coalition males. However, the single-male monopoly strategy allows opportunistic/sneaky copulations by other males with a relatively high probability. Thus, the degree of reproductive success did not reflect mating success. In contrast, the males employing a coalition strategy successfully shared their mating and reproductive success, particularly in the largest group. Compared with single-male monopolized groups, the coalition-male monopoly groups copulated with a considerably more number of females, suggesting that coalition males can effectively guard against opportunistic/sneaky copulation by rival males. We also found that coalition-forming male–male alliances often occur regardless of the degree of kinship/relatedness, indicating the complexity and flexibility inherent in the male social bond of M. arctoides.

How socio-ecological factors influence the differentiation of social relationships: an integrated conceptual framework

The extent of differentiation of social relationships within groups is a means to assess social complexity, with greater differentiation indicating greater social complexity. Socio-ecological factors are likely to influence social complexity, but no attempt has been made to explain the differentiation of social relationships using multiple socio-ecological factors. Here, we propose a conceptual framework based on four components underlying multiple socio-ecological factors that influence the differentiation of social relationships: the extent of within-group contest competition to access resources, the extent to which individuals differ in their ability to provide a variety of services, the need for group-level cooperation and the constraints on social interactions. We use the framework to make predictions about the degree of relationship differentiation that can be expected within a group according to the cumulative contribution of multiple socio-ecological factors to each of the four components. The framework has broad applicability, since the four components are likely to be relevant to a wide range of animal taxa and to additional socio-ecological factors not explicitly dealt with here. Hence, the framework can be used as the basis for the development of novel and testable hypotheses about intra- and interspecific differences in relationship differentiation and social complexity.

Bonds of bros and brothers: Kinship and social bonding in postdispersal male macaques

Group-living animals often maintain a few very close affiliative relationships-social bonds-that can buffer them against many of the inevitable costs of gregariousness. Kinship plays a central role in the development of such social bonds. The bulk of research on kin biases in sociality has focused on philopatric females, who typically live in deeply kin-structured systems, with matrilineal dominance rank inheritance and life-long familiarity between kin. Closely related males, in contrast, are usually not close in rank or familiar, which offers the opportunity to test the importance of kinship per se in the formation of social bonds. So far, however, kin biases in male social bonding have only been tested in philopatric males, where familiarity remains a confounding factor. Here, we studied bonds between male Assamese macaques, a species in which males disperse from their natal groups and in which male bonds are known to affect fitness. Combining extensive behavioural data on 43 adult males over a 10-year period with DNA microsatellite relatedness analyses, we find that postdispersal males form stronger relationships with the few close kin available in the group than with the average nonkin. However, males form the majority of their bonds with nonkin and may choose nonkin over available close kin to bond with. Our results show that kinship facilitates bond formation, but is not a prerequisite for it, which suggests that strong bonds are not restricted to kin in male mammals and that animals cooperate for both direct and indirect fitness benefits.

Male-male social bonds predict tolerance but not coalition formation in wild Japanese macaques

Social bonds have been construed as differentiated and enduring affiliative relationships. Strong bonds will improve fitness through interchanging with coalition formation or tolerance over resources. Social bonds have been found in a variety of taxa and predict the formation of coalitions even amongst males. However, in species exhibiting steeply linear dominance hierarchies, coalitions are hypothesized to be suppressed due to severe competition amongst males, and thus strong bonds may manifest in other forms of behavior, notably social tolerance. The aim of this study was to examine the effects of male-male social bonds and dominance on agonistic supports and aggressive interaction in one of the most despotic primate species, Japanese macaques. I conducted focal samples on male individuals, recording their grooming, proximity to other members, agonistic supports and membership, and aggressive interactions over a 2-year observation period. Male macaques formed differentiated affiliative relationships across dyads and those relationships showed positive relations between the non-mating and the mating seasons. Steep dominance hierarchies were found amongst males. The occurrence of agonistic supports was not explained by the strength of social bonds but by the dominance of the participants, whereas strong bonds predicted less frequent aggressive interaction. These results are in line with the hypothesis that dominance is a major mechanism underlying coalition formation amongst males. Unlike more egalitarian species, strong bonds do not predict coalition formation but rather tolerance in despotic species. These results suggest male-male social bonds will bring alternative consequences according to dominance structures.

Dominance relationships and coalitionary aggression against conspecifics in female carrion crows

Cooperation is a prevailing feature of many animal systems. Coalitionary aggression, where a group of individuals engages in coordinated behaviour to the detriment of conspecific targets, is a form of cooperation involving complex social interactions. To date, evidence has been dominated by studies in humans and other primates with a clear bias towards studies of male-male coalitions. We here characterize coalitionary aggression behaviour in a group of female carrion crows consisting of recruitment, coordinated chase, and attack. The individual of highest social rank liaised with the second most dominant individual to engage in coordinated chase and attack of a lower ranked crow on several occasions. Despite active intervention by the third most highly ranked individual opposing the offenders, the attack finally resulted in the death of the victim. All individuals were unrelated, of the same sex, and naïve to the behaviour excluding kinship, reproduction, and social learning as possible drivers. Instead, the coalition may reflect a strategy of the dominant individual to secure long-term social benefits. Overall, the study provides evidence that members of the crow family engage in coordinated alliances directed against conspecifics as a possible means to manipulate their social environment.

Zoon politikon: The evolutionary origins of human socio-political systems

We deploy the most up-to-date evidence available in various behavioral fields in support of the following hypothesis: The emergence of bipedalism and cooperative breeding in the hominin line, together with environmental developments that made a diet of meat from large animals adaptive, as well as cultural innovations in the form of fire, cooking, and lethal weapons, created a niche for hominins in which there was a significant advantage to individuals with the ability to communicate and persuade in a moral context. These forces added a unique political dimension to human social life which, through gene-culture coevolution, became Homo ludens-Man, the game player-with the power to conserve and transform the social order. Homo sapiens became, in the words of Aristotle's Nicomachean Ethics, a zoon politikon.

Male dominance relationships in an extremely large chimpanzee community at Ngogo, Kibale National Park, Uganda

Male dominance hierarchies occur in many group-living primates and some non-primate mammals. Variation in aspects of agonistic relationships such as how many dyads show bidirectionality in aggression leads to variation in dominance hierarchies along a continuum from egalitarian (relatively small agonistic power differences between adjacently-ranked individuals, shallow hierarchies) to despotic (relatively large differences, steep hierarchies). Ranks usually depend mostly or entirely on individual characteristics that influence fighting ability (e.g., body size) and show inverse-U shaped relationships to age. However, coalitionary support sometimes also influences ranks. Chimpanzees (Pan troglodytes) form multi-male, multi-female communities within which males compete for status. Males typically form dominance hierarchies, and data from multiple study show that rank is positively related to paternity success. Males also often form coalitions and some dyads form long-term alliances. Effective coalitionary support can help individuals improve and maintain their ranks, and some evidence supports the hypothesis that coalitionary aggression generally, and the positions that males hold in coalitions networks specifically, influences paternity success. Hierarchy steepness varies among communities and within communities over time; variation in the number of prime-aged males per community is a likely source of this variation. Long-term data from an extremely large chimpanzee community with unusually many males, at Ngogo, Kibale National Park, Uganda, are largely consistent with previous analyses of male chimpanzee dominance hierarchies, but show several notable contrasts. Males at Ngogo formed significantly linear hierarchies and hierarchy steepness was greater than expected if the outcomes of agonistic interactions were random. However, variation in steepness did not show the significant inverse relationship to the number of “prime-aged” males documented for other chimpanzee communities and average steepness was high given the large number of males. Ranks showed an inverse-U shaped relationship to age, although individual rank trajectories varied considerably, but males attained their highest lifetime ranks at later ages and maintained relatively high ranks to later ages than those at other chimpanzee research sites. Two measures of coalition networks, strength and Bonacich power, showed significant positive relationships with male ranks. Strength is the rate at which males joined coalitions. Bonacich power is a measure of network centrality that assesses a male’s relational power, or influence (Bonacich, 1987): a male with high Bonacich power formed coalitions with relatively many other males who were also central in the coalition network, i.e., he was strongly connected to powerful others. On average, males also attained maximum values for these and other network measures relatively late and maintained relatively high values to relatively late ages. High coalition network strength, Bonacich power, and eigenvector centrality early in adulthood were associated with high peak ranks at later ages. However, the direction of causality between participation in coalition networks and ranks is not yet clear, and the effects of body size on dominance ranks and individual rank trajectories remains to be explored. Ngogo is a favourable habitat for chimpanzees and survivorship there is unusually high; this presumably facilitates the ability of males to maintain high competitive ability longer than at other sites and shifts rank trajectories toward older ages and leads to relatively steep hierarchies despite the fact that many male dyads have similar competitive ability. Future work will assess the impact of coalitions on dominance relationships in more detail and the relationship of coalitionary aggression to paternity success.

Selfish partners: resource partitioning in male coalitions of Asiatic lions

Behavioral plasticity within species is adaptive which directs survival traits to take multiple pathways under varying conditions. Male-male cooperation is an evolutionary strategy often exhibiting an array of alternatives between and within species. African male lions coalesce to safeguard territories and mate acquisition. Unique to these coalitions is lack of strict hierarchies between partners, who have similar resource securities possibly because of many mating opportunities within large female groups. Skewed mating and feeding rights have only been documented in large coalitions where males were related. However, smaller modal prey coupled with less simultaneous mating opportunities for male Asiatic lions in Gir forests, India would likely result in a different coalition structure. Observations on mating events (n = 127) and feeding incidents (n = 44) were made on 11 male coalitions and 9 female prides in Gir, to assess resource distribution within and among different sized male coalitions. Information from 39 males was used to estimate annual tenure-holding probabilities. Single males had smaller tenures and appropriated fewer matings than coalition males. Pronounced dominance hierarchies were observed within coalitions, with one partner getting more than 70% of all matings and 47% more food. Competition between coalition partners at kills increased with decline in prey size, increase in coalition size and the appetite states of the males. However, immediate subordinates in coalitions had higher reproductive fitness than single males. Declining benefits to partners with increasing coalition size, with individuals below the immediate subordinates having fitness comparable to single males, suggest to an optimal coalition size of 2 lions. Lions under different competitive selection in Gir show behavioral plasticity to form hierarchical coalitions, wherein partners utilize resources asymmetrically, yet coalesce for personal gains.

Age-dependent change of coalitionary strategy in male Barbary macaques

Inter- and intra-specific variation in the propensity to form coalitions has been explained by variation in the availability of suitable partners, distribution of fighting ability, coalition profitability, and costs of coordination. Male coalition formation can be an alternative reproductive strategy to one-on-one competition to maximize male reproductive success. Here we focus on age as a state variable to explain within-group variation in individual propensity to form coalitions against other group members. We specifically test the prediction that males conditionally switch from a solo strategy for achievement of high mating success to a cooperative strategy after reaching post-prime age in male Barbary macaques (Macaca sylvanus). We combined new observations with data collected in 2006 and 2008 on the same individuals from one captive group living in semi-natural conditions at Affenberg Salem, Germany, and found that in all years males between 5 and 13 years formed significantly fewer coalitions than males 14 years and older (post-prime). More importantly, we found those males that aged into the post-prime phase to have switched their reproductive strategy and to form significantly more coalitions in 2014 compared to 2008. These first longitudinal data together with earlier cross-sectional analyses in this and other primate species suggest that group-level measures of coalition propensity may be strongly affected by the age composition of groups and that male coalition formation can be a conditional reproductive strategy.

Self-organizing dominance hierarchies in a wild primate population

Linear dominance hierarchies, which are common in social animals, can profoundly influence access to limited resources, reproductive opportunities and health. In spite of their importance, the mechanisms that govern the dynamics of such hierarchies remain unclear. Two hypotheses explain how linear hierarchies might emerge and change over time. The 'prior attributes hypothesis' posits that individual differences in fighting ability directly determine dominance ranks. By contrast, the 'social dynamics hypothesis' posits that dominance ranks emerge from social self-organization dynamics such as winner and loser effects. While the prior attributes hypothesis is well supported in the literature, current support for the social dynamics hypothesis is limited to experimental studies that artificially eliminate or minimize individual differences in fighting abilities. Here, we present the first evidence supporting the social dynamics hypothesis in a wild population. Specifically, we test for winner and loser effects on male hierarchy dynamics in wild baboons, using a novel statistical approach based on the Elo rating method for cardinal rank assignment, which enables the detection of winner and loser effects in uncontrolled group settings. Our results demonstrate (i) the presence of winner and loser effects, and (ii) that individual susceptibility to such effects may have a genetic basis. Taken together, our results show that both social self-organization dynamics and prior attributes can combine to influence hierarchy dynamics even when agonistic interactions are strongly influenced by differences in individual attributes. We hypothesize that, despite variation in individual attributes, winner and loser effects exist (i) because these effects could be particularly beneficial when fighting abilities in other group members change over time, and (ii) because the coevolution of prior attributes and winner and loser effects maintains a balance of both effects.

Ecophysiology of cognition: How do environmentally induced changes in physiology affect cognitive performance?

Cognitive performance is based on brain functions, which have energetic demands and are modulated by physiological parameters such as metabolic hormones. As both environmental demands and environmental energy availability change seasonally, we propose that cognitive performance in free-living animals might also change seasonally due to phenotypic plasticity. This is part of an emerging research field, the 'ecophysiology of cognition': environmentally induced changes in physiological traits, such as blood glucose and hormone levels, are predicted to influence cognitive performance in free-living animals. Energy availability for the brain might change, and as such cognition, with changing energetic demands (e.g. reproduction) and changes of energy availability in the environment (e.g. winter, drought). Individuals spending more energy than they can currently obtain from their environment (allostatic overload type I) are expected to trade off energy investment between cognition and other life-sustaining processes or even reproduction. Environmental changes reducing energy availability might thus impair cognition. However, selection pressures such as predation risk, mate choice or social demands may act on the trade-off between energy saving and cognition. We assume that different environmental conditions can lead to three different trade-off outcomes: cognitive impairment, resilience or enhancement. Currently we cannot understand these trade-offs, because we lack information about changes in cognitive performance due to seasonal changes in energy availability and both the resulting changes in homeostasis (for example, blood glucose levels) and the associated changes in the mechanisms of allostasis (for example, hormone levels). Additionally, so far we know little about the fitness consequences of individual variation in cognitive performance. General cognitive abilities, such as attention and associative learning, might be more important in determining fitness than complex and specialized cognitive abilities, and easier to use for comparative study in a large number of species. We propose to study seasonal changes in cognitive performance depending on energy availability in populations facing different predation risks, and the resulting fitness consequences.

Association patterns and affiliative relationships outside a troop in wild male Japanese macaques, Macaca fuscata, during the non-mating season

In many mammals, males disperse more than females. Although males in some male-dispersing species form all-male groups, little is known about the association patterns or social relationships among them. Studies on male–male social relationships have primarily focused on competition over fertile females, but affiliative relationships among males have attracted much interest recently. I suggested a novel method for the classification of males based on their behaviour by principal component analysis (PCA), and investigated association patterns, and affiliative relationships in male Japanese macaques, Macaca fuscata, during the non-mating season. I observed 12 wild male Japanese macaques for 809 h during the non-mating season. The number of other animals within visual range, the amount of time that males spent in the vicinity of other animals, and the frequency of grooming interactions were examined. I classified males into two distinctive clusters (Cluster 1 and 2) according to their association and interaction patterns. Cluster 1 males associated with females and participated in grooming with them. Cluster 2 males had less visual encounters with females and did not groom them. Cluster 2 males showed proximity to other Cluster 2 males in all possible dyads. Although Cluster 2 males showed less proximity to each other than Cluster 1 males did, they frequently exchanged grooming among themselves. Cluster 2 males groomed Cluster 1 males more frequently than they were groomed by them. These results suggested that Cluster 1 were troop males, and Cluster 2 were non-troop males. Cluster 2 males had less opportunity for social interaction than Cluster 1 (troop) males, and they may form all-male groups. Males in all-male groups engaged in more frequent grooming than troop males. In addition, they groomed troop males more frequently than they were groomed. These results suggest that males could be separated by their behaviour. Male–male affiliative relationships might be influenced by within-group potential competition and imbalanced grooming appears to cause troop males to tolerate non-troop males which might be immigrated in near future.

Sociality and health: impacts of sociality on disease susceptibility and transmission in animal and human societies

This paper introduces a theme issue presenting the latest developments in research on the impacts of sociality on health and fitness. The articles that follow cover research on societies ranging from insects to humans. Variation in measures of fitness (i.e. survival and reproduction) has been linked to various aspects of sociality in humans and animals alike, and variability in individual health and condition has been recognized as a key mediator of these relationships. Viewed from a broad evolutionary perspective, the evolutionary transitions from a solitary lifestyle to group living have resulted in several new health-related costs and benefits of sociality. Social transmission of parasites within groups represents a major cost of group living, but some behavioural mechanisms, such as grooming, have evolved repeatedly to reduce this cost. Group living also has created novel costs in terms of altered susceptibility to infectious and non-infectious disease as a result of the unavoidable physiological consequences of social competition and integration, which are partly alleviated by social buffering in some vertebrates. Here, we define the relevant aspects of sociality, summarize their health-related costs and benefits, and discuss possible fitness measures in different study systems. Given the pervasive effects of social factors on health and fitness, we propose a synthesis of existing conceptual approaches in disease ecology, ecological immunology and behavioural neurosciences by adding sociality as a key factor, with the goal to generate a broader framework for organismal integration of health-related research.

Home range overlap as a driver of intelligence in primates

Various socioecological factors have been suggested to influence cognitive capacity in primates, including challenges associated with foraging and dealing with the complexities of social life. Alexander [Alexander, 1989]. Evolution of the human psyche. In: Mellars P, Stringer C, editors. The human revolution: Behavioural and biological perspectives on the origins of modern humans. Princeton: Princeton University Press. p 455-513] proposed an integrative model for the evolution of human cognitive abilities and complex sociality that incorporates competition among coalitions of conspecifics (inter-group conflict) as a major selective pressure. However, one of the premises of this model, i.e., that when confronted with inter-group conflict selection should favor enhanced cognition, has remained empirically untested. Using a comparative approach on species data, I aimed to test the prediction that primate species (n = 104) that face greater inter-group conflict have higher cognitive abilities (indexed by endocranial volume). The degree of inter-group conflict/complexity was approximated via the variable home range overlap among groups. I found a significant relationship between home range overlap and endocranial volume, even after controlling for other predictor variables and covariates such as group size and body mass. I conclude that brain size evolution cannot be attributed exclusively to social factors such as group size, but likely reflects a variety of social and ecological determinants including inter-group conflict which poses cognitive demands on monitoring both the wider social milieu as well as spatial attributes of the habitat.