Several studies have documented the importance of social bonding for the enhancement of individual fitness. However, little is known about how social relationships develop through ontogeny, and whether their development follows the same trajectory in males and females. Here we analyzed affiliative interactions (proximity, social grooming, play) combined with demographic and genetic data in semi-free-ranging rhesus macaques (Macaca mulatta) on Cayo Santiago over their first 4 yr of life (from birth to sexual maturation) to understand how these interactions change through development in both sexes. Generalized linear mixed models revealed that social behaviors mostly followed different developmental trajectories in males and females and were highly dependent on the social context. In particular, sex differences in social behavior varied through development depending on the partner’s sex and age. Females engaged in more social interactions than males, especially with other females, and were more involved in grooming around the time of maturation. In contrast, males interacted more with males and age peers, especially around maturation. Sex differences in social behavior varied through development, but also depended on rank, partner’s rank, and kin line, although not consistently. High-ranking individuals, especially older females, were generally preferred as social partners. Moreover, both male and female individuals interacted mostly with maternal kin, although males also preferred paternal kin over nonkin. Importantly, most developmental changes in sociality happened when individuals were ca. 2 yr old, suggesting that this might be a milestone in the development of sociality in rhesus macaques. The only notable exception to this pattern was play, which was more pronounced in males from the beginning of their lives. We propose that play might serve as a trigger of sex differences in social behavior, with sex differences emerging early in development and increasing through time as males and females gradually grow into their adult social roles.

In multi-male groups where females mate promiscuously, male-infant associations have rarely been studied. However, recent studies have shown that males selectively support their offspring during agonistic conflicts with other juveniles and that father’s presence accelerates offspring maturation. Furthermore, it was shown that males invest in unrelated infants to enhance future mating success with the infant’s mother. Hence, infant care might provide fitness gain for males. Here we investigate male-infant associations in rhesus macaques (Macaca mulatta), a primate with low paternity certainty as females mate with multiple partners and males ensure paternity less efficiently through mate-guarding. We combined behavioral data with genetic paternity analyses of one cohort of the semifree-ranging population of Cayo Santiago (Puerto Rico) and recorded affiliative and aggressive interactions between focal subjects and adult males from birth to sexual maturation (0–4 years) of focal subjects. Our results revealed, that 9.6% of all interactions of focal subjects involved an adult male and 94% of all male-infant interactions were affiliative, indicating the rareness of male-infant aggression. Second and most interestingly, sires were more likely to affiliate with their offspring than non-sires with unrelated infants. This preference was independent of mother’s proximity and emphasized during early infancy. Male-infant affiliation rose with infant age and was pronounced between adult males and male rather than female focal subjects. Overall our results suggest that male-infant affiliation are also an important component in structuring primate societies and affiliation directed towards own offspring presumably represent low cost paternal care.

Preferential affiliative relationships, or social bonds, play a crucial role in primate social life, but little is known about their development. Here, we review macaque studies investigating the social development of both sexes. Firstly, we highlight the emergence of sex differences in mother-offspring bonds, as macaque mothers form stronger bonds with daughters, while being more aggressive towards sons, possibly contributing to maintain female philopatry and/or male dispersal. Secondly, despite paternity uncertainty, we discuss studies reporting that fathers of several macaque species preferentially engage with their offspring, but less than mothers and only in periods of high infant mortality. Thirdly, we show that immature females, the philopatric sex in macaques, already form stronger bonds with close maternal kin than immature males, mirroring social patterns during adulthood. However, this bias seems not caused by kin availability, as kin availability is similar for both sexes prior to male dispersal. Moreover, immature males might preferentially affiliate with paternal kin over non-kin, possibly because of lower maternal integration in their maternal family and/or in preparation of dispersal. Fourthly, we discuss how immature females engage in grooming and proximity with female partners as they grow older, while immature males preferentially interact with adult males and peers, playing more than females from early on. Finally, we show that most developmental changes in sociality happen around 2-3 years of age, probably representing a milestone in macaque social development. We conclude that sex differences in sociality emerge early in development and increase through time, with sexes gradually growing into their adult roles.

Aggressive behavior plays a central role in primate life, having a crucial effect on their reproductive performance and survival and possibly affecting the formation and maintenance of social bonds. Although aggressive behavior might serve a different function in males and females, and sex differences in aggressive behavior seem to emerge early during development, very few studies have investigated whether aggressive patterns follow different developmental trajectories in male and female primates. However, the developmental perspective is crucial to understanding when differences in adults' aggression emerge and which factors trigger them. We here analyzed aggressive interactions in rhesus macaques from birth to sexual maturation (before male dispersal), including male and female focal subjects. We further considered the partner's sex, age, and rank, as well as maternal and paternal kinship, and used powerful multivariate statistical analysis. The probability to initiate aggression was largely similar for both sexes and throughout development. Both males and females were more aggressive toward partners of the same sex and similar age. In contrast, the probability of receiving aggression mostly differed between sexes across development and depended on the social context. The probability of receiving aggression increased through development. Finally, important developmental changes appeared between 2 and 3 yr of age, indicating that this period is crucial for the development of adult social roles. Our results suggest that aggressive behavior largely serves a similar function for Int J Primatol

Among mammals, individuals form strong social bonds preferentially with their kin. Differences in these relationships are linked to differential kin availability due to sex-specific dispersal patterns, but there is some indication that differential bonding among sexes already occurs prior to maturation. However, little is known about how these patterns arise during individual development. Here we investigated sex differences in the development of mother-offspring bonds in rhesus macaques (Macaca mulatta). Our results revealed that mothers showed sex-biased bonding toward their offspring. Sons had a distinctly higher probability of receiving aggression from their mothers than did daughters in the first year of life, while no differences were found with respect to affiliative interactions. After the first year, probabilities of all affiliative and aggressive behaviours investigated were higher for daughters than for sons, although generally declining. Furthermore, sons spending less time with their mother and receiving more maternal aggression tended to disperse earlier. The results of our study suggest that mothers influence their bonding strength with offspring by interacting less affiliative with sons than daughters.

Dispersal and mating patterns are known to affect the availability of both maternal and paternal kin within social groups, with important effects on the evolution of sociality. It is generally assumed that the philopatric sex forms stronger social bonds than the dispersing sex, possibly as a result of reduced kin availability for the dispersing sex after departure. However, few primate studies have directly compared whether sex differences in association patterns, particular with kin, are already present prior to dispersal when kin availability should be the same for both sexes. Here, we compared affiliative and aggressive interactions in a female philopatric species, the rhesus macaque (Macaca mulatta), to test whether sex differences in kin bias already occur around the age of maturation, when both sexes still live together with kin in their natal group. Our data confirmed that kin availability was the same for both sexes prior to male dispersal. Similar kin availability was partially reflected by comparable association patterns, as both females and males preferentially interacted with maternal kin. However, females had stronger affiliative bonds with maternal kin than males of the same age, indicating that kin associations not only depended upon kin availability, but were also sex-specific. Similarly, males were significantly more likely to affiliate with paternal kin than non-kin, as compared to females, suggesting that males discriminated paternal kin from non-kin earlier in life than females. Males might have a stronger need than females to affiliate with paternal kin due to a reduced integration in the matrilineal family prior to dispersal and the high availability of paternally related age-peers, with whom males could potentially migrate. Females, in contrast, form stronger affiliations with maternal kin, which may enhance their offspring's survival. More comparative studies are needed to understand the impact of different dispersal regimes on patterns of kin associations.

Among many mammals, maternal care strongly impacts infant survival; however, less is known about whether adult males also affect infant fitness. Paternal care is expected when providing care enhances offspring survival and reproduction, which likewise increases fathers’ fitness. Males might also care for unrelated immature individuals to increase their mating probability with the immature individuals’ mothers. Studies in multimale primate groups showed that sires enhance food access for offspring and provide protection in conflicts. Furthermore, fathers’ presence during infancy has been suggested to accelerate offspring sexual maturation. However, no study has yet directly linked the degree of father-offspring bonds to offspring fitness in primates. We previously reported father-offspring affiliation in rhesus macaques, pronounced during early infancy and independent of mothers’ presence. The present study aims at investigating whether affiliation with fathers or other males affects proxies of immature fitness (body mass gain, body fat and testis size). First, we combined behavioral, genetic and morphometric data from 55 subjects of one group. Second, using demographic and genetic data, we investigated for 92 individuals of the population whether mother- and father-offspring co-residence during immaturity influenced offspring lifetime reproductive success (LRS). Our results show that focal rank and higher amounts of affiliation with high-ranking males during infancy tend to positively impact body mass gain of female, but not male focal animals. In contrast, body mass gain of male focal individuals, but not females’, appeared to be higher when affiliation of male immature individuals was evenly distributed across their adult male partners. Moreover, we found mothers’, but not fathers’, presence during immaturity to predict offspring LRS. Our results suggest that male-immature affiliation, but not father-offspring co-residence, potentially impacts proxies of immature fitness. However, future studies should investigate the underlying mechanisms of male-immature relationships and their impact on immature fitness in more detail.

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