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ABSTRACT In ecological contexts natural selection might favor individuals with a larger body mass to monopolize resources; however, there is wide variation in body mass within populations and potential factors. In this study, we evaluated whether fruit production limits body mass in one group of Sapajus apella, its effects on behavior, and whether there is a relationship between social status and body mass. We recorded activity patterns using focal follows (20 min), body mass (using a modified Ohaus scale), and community wide fruit production estimates (from 60 fruit traps). Body mass remained relatively stable during periods of food scarcity, but in periods of abundance most individuals gained weight, as indicated by their relative growth rates (RGR). Subordinate subadults showed the highest RGR, as expected by their age. In periods of high fruit production activities as traveling, grooming, and playing were more frequent than during fruit scarcity, suggesting energy maximization and potential energetic constraints. We found differences in behavior within the group, as the alpha male was observed feeding more frequently (and for longer periods of time), was more aggressive (e.g., feeding trees), and received more grooming than other individuals. In addition, the alpha male was 60% heavier than the group's average body mass. Our study supports the hypothesis that body weight gain is related to fruit abundance and that body size is associated with social dominance; however, large individuals (both males and females) seem to incur in high metabolic or reproductive costs, as they do not gain much weight as smaller individuals. [ABSTRACT FROM AUTHOR]

The ability to quickly perceive others' rank minimizes costs by helping individuals behave appropriately when interacting with strangers. Indeed, humans and at least some other species can quickly determine strangers' rank or dominance based only on physical features without observing others' interactions or behavior. Nonhuman primates can determine strangers' ranks by observing their interactions, and some evidence suggests that at least some cues to dominance, such as facial width‐to‐height ratio (fWHR), are also present in other primates. However, it is unknown whether they can determine strangers' rank simply by looking at their faces, rather than observing their interactions. If so, this would suggest selective pressure across the primates on both cues to dominance and the ability to detect those cues accurately. To address this, we examined the ability of male and female tufted capuchin monkeys (Sapajus [Cebus] apella) to categorize images of the faces of unknown conspecifics (Sapajus from different colonies) and humans (computer‐generated and real) as dominant or nondominant based only on still images. Capuchins' categorization of unknown conspecific faces was consistent with fWHR, a cue to dominance, although there was a strong tendency to categorize strangers as dominant, particularly for males. This was true despite the continued correct categorization of known individuals. In addition, capuchins did not categorize human strangers in accordance with external pre‐ratings of dominance by independent human raters, despite the availability of the same cue, fWHR. We consider these results in the context of capuchin socio‐ecology and what they mean for the evolution of rapid decision‐making in social contexts. Research Highlights: Tufted capuchins categorize unknown male faces as dominant or not consistent with facial width‐to‐height ratios.Categorizations are biased toward dominance, suggesting that all strangers are initially treated as a threat.This suggests that capuchins do best by assuming dominance in unknown males. [ABSTRACT FROM AUTHOR]

Facial expressions are key to navigating social group life. The Power Asymmetry Hypothesis of Motivational Emancipation predicts that the type of social organization shapes the meaning of communicative displays in relation to an individual's dominance rank. The bared‐teeth (BT) display represents one of the most widely observed communicative signals across primate species. Studies in macaques indicate that the BT display in despotic species is often performed unidirectionally, from low‐ to high‐ranking individuals (signaling submission), whereas the BT display in egalitarian species is usually produced irrespective of dominance (mainly signaling affiliation and appeasement). Despite its widespread presence, research connecting BT displays to the power asymmetry hypothesis outside the Macaca genus remains scarce. To extend this knowledge, we investigated the production of BT in relation to social dominance in dyadic interactions (N = 11,377 events) of 11 captive bonobos (Pan paniscus). Although adult bonobos were more despotic than previously suggested in the literature, BT displays were produced irrespective of dominance rank. Moreover, while adults produced the BT exclusively during socio‐sexual interactions, especially during periods of social tension, immature bonobos produced the BT in a wider number of contexts. As such, the results indicate that the communicative meaning of the BT display is consistent with signaling appeasement, especially in periods of social tension. Moreover, the BT display does not seem to signal social status, supporting the prediction for species with a high degree of social tolerance. These results advance our understanding of the origins of communicative signals and their relation to species' social systems. Research Highlight: Adult bonobos use bared‐teeth (BT) displays irrespectively of dominance rank, during sexual behaviors, when tensions are high.Juvenile bonobos use the BT in wider contexts, suggesting an adjustment of display rules over bonobo development. [ABSTRACT FROM AUTHOR]

Female dominance over males is more frequent in Malagasy lemurs than in other primate clades, but patterns of female-male dominance vary among species, and few data are available for one particularly species-rich genus, Lepilemur. We investigated the types, temporal distribution, and outcome of female-male agonistic conflicts in the Milne-Edwards' sportive lemur (Lepilemur edwardsi). Ten L. edwardsi belonging to five social units were equipped with radio collars and observed during 79 focal follows between May and November 1998. We quantified agonistic conflicts, monthly conflict rates, and documented the winner and context for all conflicts. Female-male agonistic conflicts (N = 162) occurred at a mean rate of 0.21 conflicts/hour within groups. Agonistic conflicts peaked during the birth season and occurred mostly in unknown or infant proximity contexts. Females won 96% of all decided agonistic conflicts (N = 154). Other outcomes occurred only when females were with infants during the birth season. In that context, one female sometimes withdrew from her pair partner, and another female withdrew or fled from an extra-group male whose attacks eventually led to infanticide. Our results suggest female dominance in pair-living L. edwardsi. We hypothesize that elevated female aggression may convey fitness benefits to female Lepilemur because it generally allows females to intervene quickly and efficiently on behalf of their infants against nonfather males who may approach the newborn infants when they are parked in vegetation., (© 2024 The Author(s). American Journal of Primatology published by Wiley Periodicals LLC.)

Enhanced survival and reproduction are associated with an individual's direct and indirect social connections with members of a group. Yet, the role of these connections is little known in a vast range of primate species. We studied female Central Himalayan Langur (CHL) to investigate the link between four specific attributes (dominance rank, age, genetic relatedness, and the presence of females carrying infants) and a female's direct and indirect social relationships. By analyzing grooming networks, we revealed different behavioral strategies: high-ranking females form relationships with many females (high degree), whereas females with dependent infants have strong relationships (high strength and eigenvector). Subadult females are important individuals that hold the social network together (high betweenness), while an immigrant female strategy is to integrate herself into the group by forming strong bonds with females who themselves have strong bonds (high eigenvector). Our study sheds light on how behavioral strategies shape female CHL grooming networks, which may help them to secure fitness and survival advantages., (© 2024 Wiley Periodicals LLC.)

Strong, stable social bonds in primates are characterized by high levels of social affiliation, low levels of aggression, minimal stress, and affiliative reciprocity within the dyad. In relatively well‐studied catarrhine monkeys, these bonds tend to form most frequently between kin, animals close in age, and animals close in rank. This results in patterns of affiliation in which kin, similarly aged animals, and like‐ranked animals tend to affiliate and patterns of aggression and submission where animals tend to aggress more toward nonkin and closely ranked animals, and submit more toward distantly ranked animals. However, literature on how affiliative and agonistic relationships are organized in platyrrhine primate species like brown capuchin monkeys is limited and conflicting. In this study, we used social network analyses to characterize how age, sex, maternal kinship, and dominance rank relate to the patterns of submissive, aggressive, contact, and grooming interactions in a group of captive brown capuchin monkeys. Like catarrhine monkeys, brown capuchin monkeys showed a steep linear dominance hierarchy, tended to affiliate with kin, similarly aged animals, and like‐ranked animals, and tended to aggress more toward nonkin. However, our monkeys showed a pattern of affiliation and grooming down the hierarchy that is inconsistent with grooming up the hierarchy patterns often seen in catarrhine monkey groups, suggesting that brown capuchins do not compete for access to higher ranking social partners. Higher ranking monkeys were most central to the aggression network, and lower ranking monkeys were most central to the submission network. Mid‐ranking monkeys were the most central to the contact network, suggesting that they may play an important role in the affiliative cohesion of the group. These results inform our understanding of brown capuchin social behavior specifically, and of how demographic factors relate to social organization in platyrrhine primates generally. Highlights: Captive brown capuchins show a steep linear dominance hierarchy.Rank differentially predicts who is the most central across networks: High‐rankers for aggression, low‐rankers for submission, & mid‐rankers for contact.They affiliate with kin, similarly aged and like‐ranked animals, & aggress to nonkin. [ABSTRACT FROM AUTHOR]

Performance in cognitive tasks has been linked to differences in species' social organization, yet to understand its function its relationship to within-species variation in behavior should also be explored. One important cognitive capacity, the ability to inhibit impulses, is typically better in egalitarian than despotic primate species and in primate species with strong fission-fusion dynamics. A different line of research indicates that a high ability to inhibit impulses is related to less aggressive behavior and more socio-positive behavior. However, within species the relationship between performance on cognitive inhibition tasks and variation in social behavior remains to be explored. Here we investigate how performance in a typical inhibition task in cognitive research is related to aggressive and socio-positive behavior in despotic long-tailed macaques. Twenty individuals living in two naturalistic mixed-sex groups were tested with the Plexiglass Hole Task. Aggressive behavior and three types of socio-positive behavior (neutral/friendly approaches, socio-positive signaling, and grooming others) among group members were measured. Individuals differed in their ability to inhibit impulses. Individuals that were not good at inhibiting impulses showed higher rates of aggressive behavior, but also more socio-positive signals, whereas inhibition was not related to neutral/friendly approaches and grooming. These results confirm the positive link between impulsiveness and aggression. In addition, the results indicate that some social-positive behavior may be enhanced when inhibition is limited. In this species, benefits potentially derived from aggression and socio-positive signals match a low ability to inhibit impulses, suggesting that a low ability to inhibit impulses may actually be advantageous. To understand differences between species in cognitive skills, understanding the benefits of variation in a cognitive capacity within a species is crucial., (© 2023 The Authors. American Journal of Primatology published by Wiley Periodicals LLC.)

Though lion-tailed macaques are managed ex situ as an assurance population, little information is available on the relationship between behavior and reproductive physiology to ensure successful management and evaluate welfare. To facilitate social and reproductive management in a group of one male and three female lion-tailed macaques at Disney's Animal Kingdom®, we observed behaviors associated with dominance and reproduction, rated genital swellings, recorded menses, tracked ovarian cycles as determined by measurement of fecal immunoreactive progesterone (P4) and estradiol (E2), and monitored fecal cortisol (CORT). We analyzed the dominance hierarchy of females, compared hormone patterns of ovarian cycles with genital swellings and copulations relative to menses, and tested for correlations between hormones and behavior. The strength of the dominance hierarchy indicated moderate linearity, but relaxed after the dominant female received anxiolytic medications to reduce intraspecific aggression. Fecal P4 and E2 patterns revealed ovarian cycle lengths averaging 29.7 ± 0.8 days. The timing of single-day E2 peaks varied between and within females' cycles. However, on average, E2 peaks occurred 6-10 days after the first day of menses, with maximum genital swellings and copulations occurring slightly thereafter in uncontracepted females. Female-to-female aggression and sexual behaviors with the male were related to ovarian cycle phase, but correlations between behaviors and hormones varied between females. Initiated aggression was positively correlated with E2 in the dominant and mid-ranked females, while aggression received was positively correlated with P4 or negatively correlated with E2 in the mid-ranked and subordinate females. In one uncontracepted female, sexual behaviors with the male were negatively correlated to P4. CORT was positively correlated to aggression in the dominant and mid-ranked females and negatively correlated to grooming in the subordinate female. Results indicate observations of visual and behavioral cues can be used to track ovarian cycles and evaluate social dynamics and welfare in zoo-housed lion-tailed macaques., (© 2022 Wiley Periodicals LLC.)

The self-initiated split of a social group, known as fission, is a challenge faced by many group-living animals. The study of group fission and the social restructuring process in real time provides insights into the mechanism of this biologically important process. Previous studies on fission in Japanese macaques (Macaca fuscata) assigned individuals to newly reorganized groups mainly using behavioral observations and group attendance records based on periods before or after fission itself. Here, we present a novel framework for group classification during the process of fission that uses quantifiable behavioral variables and statistical analyses. The framework was tested on a group fission process at Affenberg Landskron (Austria), a park that housed around 160 semi-free-ranging Japanese macaques. The behavioral data were collected for 26 days during fission. We analyzed three behavioral developments recurrent in fissions in Japanese macaques, that is, independence of behavior, participation in group movements, and separation of nomadic ranges. These analyses were combined to assign individuals to different groups. Our study resulted in one main group (N = 33), one subgroup (N = 36) and 56 individuals whose group membership was still undefined. The demographic characteristics of these newly formed groups were comparable with those of fissioned groups in wild populations. Furthermore, we found that these newly forming groups showed early social dynamics of fission five months before group level movements, that is: grouping based on spatial proximity and spatial withdrawal of the subgroup to the periphery. These results underline the validity of our novel framework to study social dynamics in Japanese macaques during the process of fission. It represents an important addition to existing methods, and we recommend testing its scope in other primate societies., (© 2023 The Authors. American Journal of Primatology published by Wiley Periodicals LLC.)