Biology subjects

van Schaik, C. P.

Publications and source records attributed to van Schaik, C. P..

10 recordsLinked to original sources

bioRxiv2026

Between-group conflict participation is associated with lower long-term cortisol in wild vervet monkeys

Male-biased cooperative behaviours that produce public goods (male services) are widespread in primates. Recent behavioural studies suggest that male services may function beyond paternal investment, either as displays of competitive ability or as honest signals eliciting female choice. Because these hypotheses make contrasting predictions about the physiological state of service providers, endocrine measures offer a complementary approach for distinguishing between them. We investigated whether long-term testosterone and cortisol concentrations, measured from hair samples, were associated with four male services in wild vervet monkeys: alarm calling, participation in between-group conflicts (BGCs), leading river crossings, and sentinelling. Testosterone and cortisol concentrations increased with male competitive status, measured as a combination of dominance rank and recent rank trajectory, particularly during the mating period, establishing the endocrine profile associated with competition. In contrast, most male services showed few relationships with hormone concentrations: participation in BGCs was associated with lower cortisol concentrations and, to a lesser extent, lower testosterone concentrations. Because BGC participation is the strongest predictor of male mating frequency in this population, these findings indicate that males contributing to BGCs present an endocrine profile distinct from that associated with competitive state and instead support the prediction that high-quality males, irrespective of their dominance rank, can perform costly cooperative behaviours while maintaining relatively low physiological costs. More broadly, our results provide physiological support for reputation-based partner choice as a mechanism contributing to the maintenance of male services in primates.

physiology↗

bioRxiv2026

Beyond paternal care: career stage and reproductive opportunities shape male services in vervet monkeys

Why male primates invest in costly behaviours producing public goods remains debated, with two leading explanations, paternal care and reputation-based partner choice (RBPC). Using long-term data from four groups of wild vervet monkeys, we tested: (1) whether males show a bias in four protective "male services" (predator alarm calling, participation in between-group conflicts, leading river crossings and sentinelling); (2) which males contribute most; and (3) whether service provision predicts mating success during the mating season. We confirmed a male bias in all services. Consistent with the paternal care hypothesis, contributions were positively associated with past mating success, independently of rank, although potential fathers did not contribute more than non-fathers. Among non-fathers, service provision varied with rank, suggesting that newly immigrated males adjust their behaviour according to competitive state. Crucially, variation in alarm calling and between-group conflicts predicted future mating success, with between-group conflict emerging as the strongest and most consistent predictor of mating success across years and within mating seasons, whereas rank, tenure and social integration added little explanatory power. In contrast, sentinelling and leading river crossings did not reliably translate into mating benefits. Our findings indicate that male services are shaped by multiple selective pressures operating across different male career stages and that some forms of public goods provision function as signals of quality and cooperativeness to females. By directly linking cooperative investment to mating outcomes in a wild primate, this study provides rare empirical support for reputation-based partner choice beyond humans and highlights female choice as a potentially important force in the evolution of cooperation.

animal behavior and cognition↗

bioRxiv2026

Maternal ranging strategies facilitate offspring social play at energetic cost in the most solitary ape

In most vertebrates, social play among peers is considered essential for behavioral development. Yet in solitary species bearing single offspring, opportunities for social play are inherently scarce. Whether mothers of such species actively facilitate play opportunities for their offspring, and at what cost, remains unknown. We used 15 years of behavioral and movement data ([~]30,000 observation hours) from 31 wild Bornean orangutan (Pongo pygmaeus wurmbii) mother-offspring pairs to test whether mothers adjust ranging behavior to increase their offsprings access to play with neighboring peers. Neighboring mothers with similarly aged offspring showed disproportionately high annual overlap in space use, independent of their relatedness or fruit availability. They intensified use of shared areas within existing range boundaries rather than shifting or expanding their ranges, indicating a fine-scaled ranging strategy. Mothers also incurred energetic costs; on days their offspring played with peers, mothers traveled farther and spent less time feeding. Travel distances were also elevated on the days before and after play, with mothers orienting movement toward play partners core areas before play and back toward their own core areas after play. This suggests these encounters are planned and actively pursued over multiple days rather than arising by chance. These findings reveal that orangutan mothers incorporate their infants social needs into daily ranging decisions, at a cost to their own energy budgets. This points toward an underappreciated form of maternal investment and illustrates how the social requirements of development can be met even near the solitary extreme of animal social organization.

ecology↗

bioRxiv2026

A phylogenetically informed comparative analysis of sexual testosterone dimorphism across mammals in relation to paternal care and sexual size dimorphism

Within species, male testosterone is often linked to mating competition and paternal care, suggesting that sex differences in endogenous testosterone values across mammals may covary with broader reproductive strategies. Using a structured literature search, we compiled 63 studies, spanning 31 non-human species and 9 human populations, reporting endogenous, non-experimentally manipulated testosterone values for both adult males and females within the same population and context. From these studies, we calculated male-to-female testosterone ratios, and analysed these data using Bayesian phylogenetic multilevel models. We tested whether testosterone dimorphism was associated with paternal care and sexual size dimorphism while accounting for sampling matrix, assay method, breeding context, and wild versus captive setting. Across non-human mammals, neither paternal care nor sexual size dimorphism (indexing competition) showed a clear association with testosterone ratios, and the same pattern emerged in the primate-only subset. By contrast, sampling matrix was consistently associated with testosterone dimorphism across all analyses, with lower male-to-female ratios in non-blood than in blood-based measures. In primates, testosterone ratios were also lower in captive than in wild populations, although this pattern was not clearly supported in the broader non-human dataset. In the human-only analysis, testosterone ratios did not clearly differ between industrialized and small-scale societies, whereas the matrix effect remained evident. Overall, our results suggest that sampling matrix is a major source of variation even for ratio-based measures, highlighting the need for caution when inferring between-species endocrine differences from studies using different substrates. More broadly, directly comparable, non-experimentally manipulated testosterone data for both sexes remain rare across mammals, limiting comparative inference.

evolutionary biology↗

bioRxiv2025

Eight years of social and asocial learning synergistically shape orangutan diet profiles

Cultural learning permeates human skill and knowledge acquisition. To understand its significance for nonhuman animals, we must examine how social learning shapes the development of broad behavioral repertoires, and operates alongside other forms of learning. We analyzed nearly seven thousand records of behavioral indicators of social and individual learning across wild orangutans eight-year dependency period to assess their combined effects on ecological competence relevant for post-independence survival. Social learning was five times more frequent than individual learning and influenced subsequent learning for hours. Immatures varied substantially in their learning propensities and those with heightened social and individual learning propensities developed the most expansive diet profiles at independence. Our findings advance the science of cultural evolution by showing that multi-year social and individual learning synergistically shape great apes broad ecological competence.

animal behavior and cognition↗

bioRxiv2025

Observational social learning of "know-how" and "know-what" in wild orangutans: evidence from nest-building skill acquisition

Immature great apes learn how to build their nests over multiple years, yet how they do so has remained largely unclear. We investigated the detailed role of social learning in the acquisition of nest-building skills in wild Sumatran orangutans (Pongo abelii) using data on nest-building, nest practice, and nest peering behaviour from 44 individuals, collected over 17 years. We found that nest peering (but not being close to a nesting individual without peering) is associated with a significant increase in nest practice behaviour, and mostly directed at multi-step nest elements. Dependent immatures mostly peer at their mothers and use nest tree species in common with her, independent immatures peer at a larger range of individuals and use nest tree species in common with them. Our results suggest that orangutans acquire their nest-building skills through observational social learning including selective attention to "know-how" and the transmission of "know-what" information.

animal behavior and cognition↗

bioRxiv2024

Bird brains fit the bill: morphological diversification and the evolution of avian brain size

Brain size varies greatly across and even within lineages. Attempts to explain this variation have mostly focused on the role of specific cognitive demands in the social or ecological domain. However, their predictive power is modest, whereas the effects of additional functions, especially sensory information processing and motor control, on brain size remain underexplored. Here, using phylogenetic comparative models, we show that the socio-cognitive and eco-cognitive demands do not have direct links to relative brain size (that is the residual from a regression against body mass) once morphological features are taken into account. Thus, specific cognitive abilities linked to social life or ecology play a much smaller role in brain size evolution than generally assumed. Instead, parental provisioning, generation length, and especially eye size and beak and leg morphology have a strong direct link to relative brain size. Phylogenetic lability analyses suggest that morphological diversification preceded changes in the rate of brain size evolution and greater visual input, and thus that morphological diversification opened up specialized niches where efficient foraging could produce energy surpluses. Increases in brain size provided general behavioural flexibility, which improved survival by reducing interspecific competition and predation, and was made possible by intense parental provisioning. Indeed, comparative analyses in a subset of species show that thicker beaks are associated with larger size of brain regions involved in behavioural flexibility (telencephalon, pallium). Thus, morphological evolution had a key role in niche diversification, which subsequently may have facilitated the evolution of general cognitive flexibility.

evolutionary biology↗

bioRxiv2021

A farewell to EQ: A new brain size measure for comparative primate cognition

Both absolute and relative brain size vary greatly among and within the major vertebrate lineages. Scientists have long debated how larger brains in primates and hominins translate into greater cognitive performance, and in particular how to control for the relationship between the non-cognitive functions of the brain and body size. One solution to this problem is to establish the slope of cognitive equivalence, that is the line connecting organisms with an identical bauplan but different body sizes. Here, we suggest that intraspecific slopes provide the best available estimate of this measure. This approach was abandoned because slopes were too low by an unknown margin due to estimation error. We control for the error problem by focusing on highly dimorphic primate species with large sample sizes and fitting a line through the mean values for adult females and males. We obtain the best estimate for the slope of ca 0.27, a value much lower than those constructed using all mammal species, and close to the value expected based on the genetic correlation between brain size and body size. We also find that the estimate of cognitive brain size based on cognitive equivalence fits empirical cognitive studies better than the encephalization quotient (EQ), which should therefore be avoided in future studies on primates, and presumably mammals and birds in general. The use of residuals from the line of cognitive equivalence may change conclusions concerning the cognitive abilities of extant and extinct primate species, including hominins.

animal behavior and cognition↗

bioRxiv2021

Wild-captive contrasts in non-vocal communicative repertoires and functional specificity in orang-utans

The creation of novel communicative acts is an essential element of human language. Although some research suggests the presence of this ability in great apes, this claim remains controversial. Here, we use orang-utans (Pongo spp.) to systematically assess the effect of the wild-captive contrast on the repertoire size of communicative acts. We find that individual communicative repertoires are significantly larger in captive compared to wild settings, irrespective of species, age-sex class or sampling effort. Twenty percent of the orang-utan repertoire in captivity were not observed in the wild. In Sumatran orang-utans, the more sociable species, functional specificity was also higher in captive versus wild settings. We thus conclude that orang-utans, when exposed to a more sociable and terrestrial lifestyle, have the behavioural plasticity to invent new communicative behaviours that are highly functionally specific. This productive capacity by great apes is a major prerequisite for the evolution of language and seems to be ancestral in the hominid lineage.

animal behavior and cognition↗

bioRxiv2019

The nose is mightier than the tooth: larger male proboscis monkeys have smaller canines

The uniquely enlarged noses of male proboscis monkeys are prominent adornments, and a sexually selected male trait. A recent study showed significant correlations among nose, body, and testis sizes and clear associations between nose size and the number of females in a males harem. However, to date, the analyses of other common male traits, i.e., canines, are lacking. Whereas male nose size had a positive correlation with body size, we unexpectedly found a negative correlation between body and canine sizes. We explain this by an interaction between sexual and natural selection. Larger noses in males may interfere with the use of canines, thereby reducing their effectiveness as weapons. Additionally, longer canines are opposed by natural selection because the larger gape it imposes upon its bearer reduces foraging efficiency, particularly in folivores. This unique case of decoupling of body and canine size reveals that large canines carry an ecological cost.

evolutionary biology↗