Homophily in social and demographic traits predict association patterns in female western and mountain gorillas

Affiliative relationships are a hallmark of social relationships in gregarious mammals, but what drives variation of association patterns when kin are absent remains unknown. Gorillas, where females may disperse multiple times in their lives, provide an interesting counterpoint to female philopatric species to examine the factors influencing variation in association patterns. We examined demographic and social factors that may predict association patterns of female western (Gorilla gorilla gorilla; Loango, Gabon) and mountain gorillas (Gorilla beringei beringei; Bwindi, Uganda). We looked at dyadic and individual strength scores of social proximity (37 group-years). For individuals, high dominance rank increased association scores while newly emigrated females had lower scores than resident females. For dyads, higher mean dominance rank and both partners having a dependent infant increased association scores, whereas a partner being an immigrant decreased scores. Furthermore, time-matched analysis of birth and immigration events confirmed the temporal nature of these associations. Overall, female gorilla association patterns show flexibility in strength based on real-time contingencies, namely social and demographic traits. Association patterns in species with female secondary dispersal may be governed by homophily, like that of modern humans. Understanding female gorilla social structure can enhance our knowledge of the evolutionary origins of sociality.

Paternity success for resident and non-resident males and their influences on paternal sibling cohorts in Japanese macaques (Macaca fuscata) on Shodoshima Island

Reproductive success can be attributed to both resident and non-resident males in non-human primates. However, reproductive success of non-resident males has rarely been investigated at an individual level. As resident males achieve different degree of reproductive success with regard to various factors, such as male dominance relationships or female mate choice, the degree of reproductive success for non-resident males may vary between individuals. As male reproductive success is highly skewed towards specific individuals, the percentage of similar-aged paternal siblings within groups is expected to increase. However, the extent to which each male contributes to the production of cohorts of paternal siblings remains unclear. Here we examined the paternity of offspring born over five consecutive years in a free-ranging group of Japanese macaques Macaca fuscata on Shodoshima Island, Kagawa Prefecture. Genotypes of 87 individuals at 16 autosomal microsatellite loci were analyzed and paternity of 34 offspring was successfully assigned to a single candidate father. We quantitatively assessed paternity success for resident and a few non-resident males whose genetic samples were successfully collected. We quantitatively assessed the percentages of paternal siblings in the same age cohorts produced by those males. Non-resident males sired similar percentage of offspring compared to resident males. A large prime-aged non-resident male was the most successful sire among males in two of the five years. These results provide new insights that male reproductive success could be highly skewed toward a specific non-resident male. Subadult males had a lower percentage of paternity success, which may be because females may prefer physically mature males. Various males, including non-resident males, contributed to the creation of paternal sibling in the same age cohort. The overall results highlighted that not only resident but also non-resident males play an important role in shaping within-group kin structures.

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.

Explaining variation in the kinship composition of mammal groups

Variation in cooperative behavior across mammals is strongly related to the kinship composition of groups. Although the factors affecting average genetic relatedness within groups have been studied, the factors that contribute to the production of different categories of kin remain underexplored. Here, I use a mathematical model to explore the factors that determine the proportion of full siblings, maternal half-siblings, paternal half-siblings, and non-siblings within mammal groups. The results suggest that the production of paternal half-siblings is increased by high male reproductive skew and a female-biased sex ratio, the production of maternal half-siblings is increased by high female reproductive skew and male-biased sex ratio, and that there are two routes to the production of full siblings: either high reproductive skew in both sexes (as seen in cooperatively breeding species) or pair-bond stability within groups of low reproductive skew (as seen in humans). These results broadly correspond to observed variation in sibling composition across mammals.

Social network shrinking is explained by active and passive effects but not increasing selectivity with age in wild macaques

Evidence of social disengagement, network narrowing and social selectivity with advancing age in several non-human animals challenges our understanding of the causes of social ageing. Natural animal populations are needed to test whether social ageing and selectivity occur under natural predation and extrinsic mortality pressures, and longitudinal studies are particularly valuable to disentangle the contribution of within-individual ageing from the demographic processes that shape social ageing at the population level. Data on wild Assamese macaques (Macaca assamensis) were collected between 2013 and 2020 at the Phu Khieo Wildlife Sanctuary, Thailand. We investigated the social behaviour of 61 adult females observed for 13 270 h to test several mechanistic hypotheses of social ageing and evaluated the consistency between patterns from mixed-longitudinal and within-individual analyses. With advancing age, females reduced the size of their social network, which could not be explained by an overall increase in the time spent alone, but by an age-related decline in mostly active, but also passive, behaviour, best demonstrated by within-individual analyses. A selective tendency to approach preferred partners was maintained into old age but did not increase. Our results contribute to our understanding of the driver of social ageing in natural animal populations and suggest that social disengagement and selectivity follow independent trajectories during ageing.

Kinship composition in mammals

Understanding the evolution of group-living and cooperation requires information on who animals live and cooperate with. Animals can live with kin, non-kin or both, and kinship structure can influence the benefits and costs of group-living and the evolution of within-group cooperation. One aspect of kinship structure is kinship composition, i.e. a group-level attribute of the presence of kin and/or non-kin dyads in groups. Despite its putative importance, the kinship composition of mammalian groups has yet to be characterized. Here, we use the published literature to build an initial kinship composition dataset in mammals, laying the groundwork for future work in the field. In roughly half of the 18 species in our sample, individuals lived solely with same-sex kin, and, in the other half, individuals lived with related and unrelated individuals of the same sex. These initial results suggest that it is not rare for social mammals to live with unrelated individuals of the same sex, highlighting the importance of considering indirect and direct fitness benefits as co-drivers of the evolution of sociality. We hope that our initial dataset and insights will spur the study of kinship structure and sociality towards new exciting avenues.

Weak, but not strong, ties support coalition formation among wild female chimpanzees

In social species, individuals may be able to overcome competitive constraints on cooperation by leveraging relationships with familiar, tolerant partners. While strong social ties have been linked to cooperation in several social mammals, it is unclear the extent to which weak social ties can support cooperation, particularly among non-kin. We tested the hypothesis that weakly affiliative social relationships support cooperative coalition formation using 10 years of behavioural data on wild female chimpanzees. Female chimpanzees typically disperse and reside with non-kin as adults. Their social relationships are differentiated but often relatively weak, with few dyads sharing strong bonds. Females occasionally form aggressive coalitions together. Three measures of relationship quality-party association, five-metre proximity and whether a dyad groomed-positively predicted coalitions, indicating that relationship quality influenced coalition partnerships. However, dyads that groomed frequently did not form more coalitions than dyads that groomed occasionally, and kin did not cooperate more than expected given their relationship quality. Thus, strong bonds and kinship did not bolster cooperation. We conclude that cooperative coalitions among female chimpanzees depend on social tolerance but do not require strong bonds. Our findings highlight social tolerance as a distinct pathway through which females can cultivate cooperative relationships. This article is part of the theme issue 'Cooperation among women: evolutionary and cross-cultural perspectives'.

Thermal imaging reveals social monitoring during social feeding in wild chimpanzees

Understanding the affective lives of animals has been a long-standing challenge in science. Recent technological progress in infrared thermal imaging has enabled researchers to monitor animals' physiological states in real-time when exposed to ecologically relevant situations, such as feeding in the company of others. During social feeding, an individual's physiological states are likely to vary with the nature of the resource and perceptions of competition. Previous findings in chimpanzees have indicated that events perceived as competitive cause decreases in nasal temperatures, whereas the opposite was observed for cooperative interactions. Here, we tested how food resources and audience structure impacted on how social feeding events were perceived by wild chimpanzees. Overall, we found that nasal temperatures were lower when meat was consumed as compared to figs, consistent with the idea that social feeding on more contested resources is perceived as more dangerous and stressful. Nasal temperatures were significant affected by interactions between food type and audience composition, in particular the number of males, their dominance status, and their social bond status relative to the subject, while no effects for the presence of females were observed. Our findings suggest that male chimpanzees closely monitor and assess their social environment during competitive situations, and that infrared imaging provides an important complement to access psychological processes beyond observable social behaviours. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

Growth trajectories of wild Assamese macaques (Macaca assamensis) determined from parallel laser photogrammetry

Socioecological factors are associated with life-history patterns and growth trajectories among primates. Under certain conditions, selection may favor a temporal decoupling of growth and major life-history events such as sexual maturation or natal dispersal. Yet, empirical tests of these associations in wild populations remain scarce owing to the lack of non-invasive methods to capture growth trajectories. In this study, we first compared two non-invasive methods of digital photogrammetry. Then, we used parallel laser photogrammetry to investigate forearm growth of wild Assamese macaque males and females in their natural habitat at Phu Khieo Wildlife Sanctuary, Thailand to test life-history and socio-ecological hypotheses. Across 48 males and 44 females, we estimated growth trajectories and pseudo-velocity curves by applying quadratic plateau models and non-parametric LOESS regressions. We assessed the development of sexual dimorphism by comparing the sexes at five different ages. Females had completed 96% of their growth at the age at first birth (5.9 years) and ceased growing at 7.1 years of age. Males, in contrast, grew until well after their average age of natal dispersal: they reached 81% of their size at the age of natal dispersal (4.0 years), and ceased growing only at 9.0 years of age, much later than females. Sexual dimorphism in forearm length was driven by an extended growth period in males, which is expected for males dispersing between multimale and multifemale groups and not facing the risk of being ousted by other larger males. Our results contradict the neonatal investment hypothesis that predicts a desynchronization of investment in growth and reproduction only in female baboons, but not other papionins producing cheaper neonates. Furthermore, male Assamese macaques do not delay natal dispersal until they are fully grown, in accordance with predictions of the male-career-framework for species with low to medium level of direct competition.

No evidence that grooming is exchanged for coalitionary support in the short- or long-term via direct or generalized reciprocity in unrelated rhesus macaques

Abstract

Reciprocity is a prominent explanation for cooperation between non-kin. Studies designed to demonstrate reciprocity often focus on direct reciprocity in the timescale of minutes to hours, whereas alternative mechanisms like generalized reciprocity and the possibility of reciprocation over longer timescales of months and years are less often explored. Using a playback experiment, we tested for evidence of direct and generalized reciprocity across short and longer timescales. We examined the exchange of grooming for coalitionary support between unrelated female rhesus macaques in a population with a complete genetic pedigree. Females that received grooming were not more responsive to calls for coalitionary support from unrelated female group mates compared to control females that received agonism or no interaction — even when the call belonged to a females’ most recent grooming partner. Similarly, females were not more responsive to calls for support from their most frequent unrelated grooming partner of the last two years, nor if they received large amounts of grooming from all other females in their group. We interpret these results as an absence of evidence for direct or generalized reciprocity on any timescale in the exchange of grooming for coalitionary support in rhesus macaques. If grooming is exchanged for support in this population, it is with an intensity below our ability to detect it or over a longer timescale than we examined. We propose by-product explanations may also be at play and highlight the importance of investigating multiple mechanisms when testing apparently cooperative behaviors.

Significance statement

The receipt of help can make some animals more likely to provide help in return, whether it be a singular act, or many acts accumulated over months. Similarly, the receipt of help, be it one act of aid, or a group’s worth of help over time, can make some animals more likely to pay help forward to others. Studies on Old World monkeys suggest females may give grooming and in return receive aid in future physical conflicts. Using a playback experiment, we found female rhesus macaques were not more responsive to calls for intervention in a simulated conflict after being groomed by unrelated females, even if the calling combatant was her most recent, or a long-time grooming partner. Our results suggest females in our study population may be receiving benefits other than support in conflicts for the grooming they provide.

Pathways to paternal care in primates

Natural selection will favor male care when males have limited alternative mating opportunities, can invest in their own offspring, and when care enhances males' fitness. These conditions are easiest to fulfill in pair-bonded species, but neither male care nor stable "breeding bonds" that facilitate it are limited to pair-bonded species. We review evidence of paternal care and extended breeding bonds in owl monkeys, baboons, Assamese macaques, mountain gorillas, and chimpanzees. The data, which span social/mating systems and ecologies, suggest that there are multiple pathways by which conditions conducive to male care can arise. This diversity highlights the difficulty of making inferences about the emergence of male care in early hominins based on single traits visible in the fossil record. We discuss what types of data are most needed and the questions yet to be answered about the evolution of male care and extended breeding bonds in the primate order.

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.