The Influence of Kinship on Familiar Natal Migrant Rhesus Macaques (Macaca mulatta)

Rhesus macaques build new social connections after a natural disaster

Climate change is increasing the frequency and intensity of weather-related disasters such as hurricanes, wildfires, floods, and droughts. Understanding resilience and vulnerability to these intense stressors and their aftermath could reveal adaptations to extreme environmental change. In 2017, Puerto Rico suffered its worst natural disaster, Hurricane Maria, which left 3,000 dead and provoked a mental health crisis. Cayo Santiago island, home to a population of rhesus macaques (Macaca mulatta), was devastated by the same storm. We compared social networks of two groups of macaques before and after the hurricane and found an increase in affiliative social connections, driven largely by monkeys most socially isolated before Hurricane Maria. Further analysis revealed monkeys invested in building new relationships rather than strengthening existing ones. Social adaptations to environmental instability might predispose rhesus macaques to success in rapidly changing anthropogenic environments.

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.

Social resource foraging is guided by the principles of the Marginal Value Theorem

Optimality principles guide how animals adapt to changing environments. During foraging for nonsocial resources such as food and water, species across taxa obey a strategy that maximizes resource harvest rate. However, it remains unknown whether foraging for social resources also obeys such a strategic principle. We investigated how primates forage for social information conveyed by conspecific facial expressions using the framework of optimal foraging theory. We found that the canonical principle of Marginal Value Theorem (MVT) also applies to social resources. Consistent with MVT, rhesus macaques (Macaca mulatta) spent more time foraging for social information when alternative sources of information were farther away compared to when they were closer by. A comparison of four models of patch-leaving behavior confirmed that the MVT framework provided the best fit to the observed foraging behavior. This analysis further demonstrated that patch-leaving decisions were not driven simply by the declining value of the images in the patch, but instead were dependent upon both the instantaneous social value intake rate and current time in the patch.

Long-term effects of castration on the skeleton of male rhesus monkeys (Macaca mulatta)

While osteopenia (OPE) and osteoporosis (OPO) have been studied in various species of aging nonhuman primates and extensively in ovariectomized rhesus and cynomolgus macaques, there is virtually no information on the effects of castration on the skeleton of male nonhuman primates. Most information on castrated male primates comes from a few studies on the skeletons of eunuchs. This report used a subset of the Caribbean Primate Research Center's (CPRC) Cayo Santiago (CS) rhesus macaque skeletal collection to qualitatively and quantitatively compare the bone mineral density (BMD) of castrated and age-matched intact males and, thereby, determine the long-term effects of castration (orchidectomy) on bone. Lumbar vertebrae, femora, and crania were evaluated using dual-energy X-ray absorptiometry (DEXA or DXA) and digital radiography augmented, when fresh tissues were available, with autoradiography and histology. Results confirmed physical examinations of long bones that castration causes changes in the skeleton of male rhesus macaques similar to those found in eunuchs, including OPE and OPO of the vertebrae and femora, thinning of the skull, and vertebral fractures and kyphosis of the spine more severe than that caused by normal aging alone. Also like eunuchs, some castrated CS male rhesus monkeys had a longer life span than intact males or females. Based on these results and the effects of castration on other tissues and organs of eunuchs, on behavior, hormone profiles and possibly on cognition and visual perception of human and nonhuman primates, and other mammals, castrated male rhesus macaques should be used with caution for laboratory studies and should be considered a separate category from intact males. Despite these caveats, the castrated male rhesus macaque should make an excellent animal model in which to test hormone replacement therapies for boys and men orchidectomized for testicular and prostate cancer.

Male rhesus macaques use vocalizations to distinguish female maternal, but not paternal, kin from non-kin

Recognizing close kin and adjusting one's behavior accordingly (i.e., favor kin in social interactions, but avoid mating with them) would be an important skill that can increase an animals' inclusive fitness. Previous studies showed that philopatric female rhesus macaques (Macaca mulatta) bias their social behavior toward maternal and paternal kin. Benefits gained from selecting kin should, however, not only apply to the philopatric sex, for which the enduring spatial proximity facilitates kin discrimination. Given that dispersal is costly, the dispersing sex may benefit from migrating together with their kin or into groups containing kin. In male rhesus macaques, natal migrants bias their spatial proximity toward familiar male kin rather than familiar non-kin. Here, we set up playback experiments to test if males use the acoustic modality to discriminate familiar female kin from non-kin in a non-sexual context. Males responded differently to the presentation of "coo" calls of related and unrelated females, with their reaction depending on the interaction between kin-line (maternal vs paternal kin) and degree of relatedness (r = 0.5, 0.25). Specifically, males were more likely to respond to close kin compared to more distant kin or unrelated females, with this effect being significant in the maternal, but not paternal kin-line. The present study adds to our knowledge of kin recognition abilities of the dispersing sex, suggesting that male rhesus macaques are also able to identify kin using the acoustic modality. We discuss that the probability of response might be affected by the potential benefit of the social partner.

Genetic studies on the Cayo Santiago rhesus macaques: A review of 40 years of research

Genetic studies not only contribute substantially to our current understanding of the natural variation in behavior and health in many species, they also provide the basis of numerous in vivo models of human traits. Despite the many challenges posed by the high level of biological and social complexity, a long lifespan and difficult access in the field, genetic studies of primates are particularly rewarding because of the close evolutionary relatedness of these species to humans. The free-ranging rhesus macaque (Macaca mulatta) population on Cayo Santiago (CS), Puerto Rico, provides a unique resource in this respect because several of the abovementioned caveats are of either minor importance there, or lacking altogether, thereby allowing long-term genetic research in a primate population under constant surveillance since 1956. This review summarizes more than 40 years of genetic research carried out on CS, from early blood group typing and the genetic characterization of skeletal material via population-wide paternity testing with DNA fingerprints and short tandem repeats (STRs) to the analysis of the highly polymorphic DQB1 locus within the major histocompatibility complex (MHC). The results of the paternity studies also facilitated subsequent studies of male dominance and other factors influencing male reproductive success, of male reproductive skew, paternal kin bias, and mechanisms of paternal kin recognition. More recently, the CS macaques have been the subjects of functional genetic and gene expression analyses and have played an important role in behavioral and quantitative genetic studies. In addition, the CS colony has been used as a natural model for human adult-onset macular degeneration, glaucoma, and circadian rhythm disorder. Our review finishes off with a discussion of potential future directions of research on CS, including the transition from STRs to single nucleotide polymorphism (SNP) typing and whole genome sequencing.

Sex Differences in the Development of Social Relationships in Rhesus Macaques (Macaca mulatta)

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.

Sex differences in kin bias at maturation: Male rhesus macaques prefer paternal kin prior to natal dispersal

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.

Primate Kinship: Contributions from Cayo Santiago

Research on Cayo Santiago and Japan deserves credit for launching the study of primate kinship and for continuing to help shape it in important ways. This review describes the origins of kinship research on Cayo Santiago, beginning with Donald Sade's pioneering work establishing the concepts of kin preferences, matrilineal dominance systems and incest avoidance. It then reviews subsequent research by later Cayo Santiago researchers and alumni, focusing primarily on maternal kinship. Together these researchers have greatly expanded our knowledge of kin preferences in rhesus in terms of (i) what age-sex classes, behaviors and types of kin show them, (ii) the ways in which kinship interfaces with rank, sex, age, and dispersal patterns, and (iii) the graded and variably limited nature of kin preferences in terms of degree of relatedness. Second, the argument for kin selection at least for some types of behavior has survived challenges posed by several alternative explanations, and has been both strengthened by recent findings of paternal kin preferences and narrowed by studies showing that unilateral altruism may extend only to very close kin. Third, work on Cayo Santiago has contributed to an appreciation that both current conditions and inherent social characteristics may influence the strength of kin preferences, and fourth, it has contributed to an understanding of the possible origins of our own species' family systems. Cayo Santiago became a leader in kinship research in large part because of management practices that produce known extended lineages. These lineages have promoted and accelerated research on kinship, prompting other researchers to investigate its importance in other groups and species, where its effects only then became clear. The extended lineages remain valuable tools for research on a species that lives in a broad range of environments in the wild, including those with key parallels to Cayo Santiago.

Long-term effects of tetanus toxoid inoculation on the demography and life expectancy of the Cayo Santiago rhesus macaques

Tetanus was a major cause of mortality in the free-ranging population of rhesus monkeys on Cayo Santiago prior to 1985 when the entire colony was given its first dose of tetanus toxoid. The immediate reduction in mortality that followed tetanus toxoid inoculation (TTI) has been documented, but the long-term demographic effects of eliminating tetanus infections have not. This study uses the Cayo Santiago demographic database to construct comparative life tables 12 years before, and 12 years after, TTI. Life tables and matrix projection models are used to test for differences in: (i) survival among all individuals as well as among social groups, (ii) long-term fitness of the population, (iii) age distribution, (iv) reproductive value, and (v) life expectancy. A retrospective life table response experiment (LTRE) was performed to determine which life cycle transition contributed most to observed changes in long-term fitness of the population post-TTI. Elimination of clinical tetanus infections through mass inoculation improved the health and well-being of the monkeys. It also profoundly affected the population by increasing survivorship and long-term fitness, decreasing the differences in survival rates among social groups, shifting the population's age distribution towards older individuals, and increasing reproductive value and life expectancy. These findings are significant because they demonstrate the long-term effects of eradicating a major cause of mortality at a single point in time on survival, reproduction, and overall demography of a naturalistic population of primates.