The evolution of female social relationships in nonhuman primates

Considerable interspecific variation in female social relationships occurs in gregarious primates, particularly with regard to agonism and cooperation between females and to the quality of female relationships with males. This variation exists alongside variation in female philopatry and dispersal. Socioecological theories have tried to explain variation in female-female social relationships from an evolutionary perspective focused on ecological factors, notably predation and food distribution. According to the current “ecological model”, predation risk forces females of most diurnal primate species to live in groups; the strength of the contest component of competition for resources within and between groups then largely determines social relationships between females. Social relationships among gregarious females are here characterized as Dispersal-Egalitarian, Resident-Nepotistic, Resident-Nepotistic-Tolerant, or Resident-Egalitarian. This ecological model has successfully explained differences in the occurrence of formal submission signals, decided dominance relationships, coalitions and female philopatry. Group size and female rank generally affect female reproduction success as the model predicts, and studies of closely related species in different ecological circumstances underscore the importance of the model. Some cases, however, can only be explained when we extend the model to incorporate the effects of infanticide risk and habitat saturation. We review evidence in support of the ecological model and test the power of alternative models that invoke between-group competition, forced female philopatry, demographic female recruitment, male interventions into female aggression, and male harassment. Not one of these models can replace the ecological model, which already encompasses the between-group competition. Currently the best model, which explains several phenomena that the ecological model does not, is a “socioecological model” based on the combined importance of ecological factors, habitat saturation and infanticide avoidance. We note some points of similarity and divergence with other mammalian taxa; these remain to be explored in detail. Received: 30 September 1996 / Accepted after revision: 20 July 1997     

When females should contest for food – testing hypotheses about resource density, distribution, size, and quality with Hanuman langurs (Presbytis entellus)

Predictions of ecological models on female social relationships (van Schaik 1989) and their links with food distribution and the potential competitive regime are used to analyze the feeding and spatial behavior, and resource density, size, distribution, and quality in a forest population of Hanuman langurs (Presbytis entellus). In contrast to other populations, and assumptions on folivorous primates, the females of this population exhibit a linear dominance hierarchy. The langurs concentrated their feeding on three key resources with a low density and clumped distribution. Two out of the three key resources contained significantly higher levels of extractable protein and soluble sugar than other food plants, indicating high spatial variability of food quality. Even the mature leaves of the most preferred food plant were about twice as nutritious as those from other food plants. Group spread was small and only a single high-quality resource was used at a time. Finally, even rich resources could accommodate only a subset of a group. These findings fit predictions made for the prevalence of within-group contest competition. Given the observed food distribution and phytochemical heterogeneity of mature foliage, even females of folivorous species should contest for food. The effect of female dominance rank on size and composition of feeding parties also agrees with this prediction. A comparison with data from another forest population, where female dominance relations are weakly developed, revealed a clear-cut difference in the use and abundance of resources. It is argued that between-population differences in female social relationships within a species may be viewed as adaptive responses to local habitat conditions. Received: 1 August 1997 / Accepted after revision: 7 December 1997     

Proximate mechanisms of contest competition among female Bwindi mountain gorillas (Gorilla beringei beringei)

Socioecological models provide a framework for predicting how animals respond competitively to the abundance and distribution of food resources. Testing predictions of socioecological models requires analysis of relationships among food resource characteristics, competitive behaviors, and measures of rank-related skew in energy balance or reproductive success. A positive relationship between dominance rank and energy balance has been observed among female mountain gorillas in Bwindi Impenetrable National Park, Uganda. This study examines the proximate mechanisms underlying feeding competition among those females. To assess the contestability of food resources, we measured the time a female spent feeding at a food site (food site residence time). We also examined the relationship between dominance rank and the access to resources, as well as the rate, context, and direction of aggression, and the number of neighbors in close proximity. As predicted, females had longer food site residence times and higher aggression rates with fruit and decaying wood than with herbaceous vegetation, suggesting that those resources may be contestable. Aggression was predominantly directed down the dominance hierarchy, although against expectation, rank was not significantly correlated with aggression rates or the time spent feeding on contestable foods. Higher-ranking females had significantly fewer neighbors, suggesting that lower-ranking females avoid higher-ranking ones. This study provides additional support for the claim that there is variability in how primates respond to the quality and distribution of food resources and that avoidance as a strategy to cope with feeding competition may result in similar skew in energy balance as rank-related aggression.     

Competitive regimes and female bonding in two species of squirrel monkeys (Saimiri oerstedi and S. sciureus)

Summary Ecological and behavioral data from long-term field studies of known individuals in two closely related squirrel monkey species (Saimiri oerstedi and S. sciureus) were used to examine hypotheses about the source of variation in female bonding among group-living primates. Social relationships in species which live in cohesive groups are thought to depend on the nature of competition for resources. S. oerstedi and S. sciureus both live in large groups and are subject to intense predation. Direct feeding competition both between and within groups is extremely low in S. oerstedi; in this species female relationships are undifferentiated, no female dominance hierarchy is evident and females disperse from their natal group. S. sciureus also experiences very low levels of between-group competition, but within-group direct competition for resources is frequent; this species demonstrates differentiated female relationships, a female dominance hierarchy, and female philopatry. The correlated ecological and social variables found in these two congeners further minimize the minor effects of phylogenetic differences and emphasize the importance of food distribution in determining social characteristics. Offprint requests to: S. Boinski     

Within-group spatial position in ring-tailed coatis: balancing predation, feeding competition, and social competition

A variety of factors can influence an individual’s choice of within-group spatial position. For terrestrial social animals, predation, feeding success, and social competition are thought to be three of the most important variables. The relative importance of these three factors was investigated in groups of ring-tailed coatis (Nasua nasua) in Iguazú, Argentina. Different age/sex classes responded differently to these three variables. Coatis were found in close proximity to their own age/sex class more often than random, and three out of four age/sex classes were found to exhibit within-group spatial position preferences which differed from random. Juveniles were located more often at the front edge and were rarely found at the back of the group. Juveniles appeared to choose spatial locations based on feeding success and not predation avoidance. Since juveniles are the most susceptible to predation and presumably have less prior knowledge of food source location, these results have important implications in relation to predator-sensitive foraging and models of democratic group leadership. Subadults were subordinate to adult females, and their relationships were characterized by high levels of aggression. This aggression was especially common during the first half of the coati year (Nov–April), and subadults were more peripheralized during this time period. Subadults likely chose spatial positions to avoid aggression and were actively excluded from the center of the group by adult females. In the Iguazú coati groups, it appeared that food acquisition and social agonism were the major determinants driving spatial choice, while predation played little or no role. This paper demonstrates that within-group spatial structure can be a complex process shaped by differences in body size and nutritional requirements, food patch size and depletion rate, and social dominance status. How and why these factors interact is important to understanding the costs and benefits of sociality and emergent properties of animal group formation.     

Seasonal variation in feeding behavior, competition and female social relationships in a forest dwelling guenon, the blue monkey (Cercopithecus mitis stuhlmanni), in the Kakamega Forest, Kenya

Socioecological models relate differences in feeding strategies to variation in the nature of female social relationships. Among the African forest guenons, females consume large quantities of fruit and other plant reproductive parts, resources which are thought to promote contest competition, yet these monkeys have been characterized as having agonistically undifferentiated relationships in which rank, if discernible at all, does not correlate with fitness benefits. To determine whether female relationships become more hierarchical under relevant ecological conditions, we monitored the adult females of two blue monkey groups (Cercopithecus mitis stuhlmanni) over a complete annual cycle in the Kakamega Forest, Kenya. Females competed aggressively for plant reproductive parts more often than any other resource type, and in both groups we detected linear dominance hierarchies. Nonetheless, agonism rates remained low throughout our study, and did not vary with changes in ecological conditions. Rather, when plant reproductive parts were scarce, subordinate females spent more time feeding and less time resting in an apparent attempt to compensate for a reduced efficiency of food intake. The effects of rank and food abundance were not reflected, however, in the distribution of grooming. The use of alternative feeding strategies appeared to blunt competition – females of all ranks were unlikely to be near others while feeding and spent more time consuming alternative resources when plant reproductive parts were scarce. The diverse diet of this species may allow females to avoid conflict so that dominance has only subtle effects that are difficult to detect. While socioecological models often simplify the connection between resources and female interactions, our results emphasize that the behavior of animals consuming particular resources, and not the resources themselves, are critical predictors of social patterns.     

Socioecological influences on the reproductive success of female mountain gorillas (<Emphasis Type="Italic">Gorilla beringei beringei</Emphasis>)

Over the past few decades, socioecological models have been developed to explain the relationships between the ecological conditions, social systems, and reproductive success of primates. Feeding competition, predation pressures, and risk of infanticide are predicted to influence how female reproductive success (FRS) depends upon their dominance rank, group size, and mate choices. This paper examines how those factors affected the reproductive success of female mountain gorillas (Gorilla beringei beringei) of the Virunga Volcanoes, Rwanda from 1967–2004. Reproductive success was measured through analyses of interbirth intervals, infant survival, and surviving infant birth rates using data from 214 infants born to 67 females. Mountain gorillas were predicted to have “within-group scramble” feeding competition, but we found no evidence of lower FRS in larger groups, even as those groups became two to five times larger than the population average. The gorillas are considered to have negligible “within-group contest” competition, yet higher ranked mothers had shorter interbirth intervals. Infant survival was higher in multimale groups, which was expected because infanticide occurs when the male dies in a one-male group. The combination of those results led to higher surviving birth rates for higher ranking females in multimale groups. Overall, however, the socioecological factors accounted for a relatively small portion of the variance in FRS, as expected for a species that feeds on abundant, evenly distributed foliage.     

Behavioral correlates of non-random mortality among free-ranging female vervet monkeys

Summary Over a 3.5 year period, illness and predation operated in a non-random manner on free-ranging vervet monkeys in Amboseli National Park. As a result, there was no correlation among adult females between dominance rank and reproductive success. Deaths due to illness were concentrated among low-ranking individuals, and appeared to occur as a result of restricted access to food and water during the dry season. In contrast, deaths due to predation were concentrated among high-ranking individuals. The precise cause of such increased vulnerability could not be determined.High-ranking females alarm-called at higher frequencies than low-ranking females, and were also more aggressive than low-ranking females during intergroup encounters. In contrast, low-ranking females were more likely to initiate friendly interactions with the members of other groups. The non-random distribution of causes of mortality suggests that individuals living in the same social group may confront different selective pressures. Perhaps as a result, individuals appear to respond differently to similar social and environmental variables.     

Divided destinies: group choice by female savannah baboons during social group fission

Group living provides benefits to individuals while imposing costs on them. In species that live in permanent social groups, group division provides the only opportunity for nondispersing individuals to change their group membership and improve their benefit to cost ratio. We examined group choice by 81 adult female savannah baboons (Papio cynocephalus) during four fission events. We measured how each female’s group choice was affected by several factors: the presence of her maternal kin, paternal kin, age peers, and close social partners, her average kinship to groupmates, and her potential for improved dominance rank. Maternal kin, paternal kin, and close social partners influenced group choice by some females, but the relative importance of these factors varied across fissions. Age peers other than paternal kin had no effect on group choice, and average kinship to all groupmates had the same effect on group choice as did maternal kin alone. Most females were subordinate to fewer females after fissions than before, but status improvement did not drive female group choice; females often preferred to remain with social superiors who were their close maternal kin, rather than improving their own social ranks. We suggest that during permanent group fissions, female baboons prefer to remain with close maternal kin if those are abundant enough to influence their fitness; if they have too few close maternal kin then females prefer to remain with close paternal kin, and social bonds with nonkin might also become influential. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hierarchical dominance structure in reintroduced California condors: correlates, consequences, and dynamics

Populations of reintroduced California condors (Gymnogyps californianus) develop complex social structures and dynamics to maintain stable group cohesion, and birds that do not successfully integrate into group hierarchies have highly impaired survivability. Consequently, improved understanding of condor socioecology is needed to inform conservation management strategies. We report on the dominance structure of free-ranging condors and identify the causes and consequences of rank in condor populations by matching social status with the behavioral and physical correlates of individual birds. We characterized the hierarchical social structure of wild condor populations as mildly linear, despotic, and dynamic. Condor social groups were not egalitarian and dominance hierarchies regulated competitive access to food resources. Absence of kin-based social groups also indicated that condor social structure is individualistic. Agonistic interactions among condors were strongly unidirectional, but the overall linearity and steepness of their hierarchies was low. Although one aggressive male maintained the highest dominance rank across the 3-year observation period, there was considerable fluidity in social status among condors within middle and lower rank orders. Older condors were more dominant than younger birds and younger males supplanted older females over time to achieve higher status. Dominance rank did not predict the amount of time that a bird spent feeding at a carcass or the frequency that a bird was interrupted while feeding. Thus, younger, less dominant birds are able to obtain sufficient nutrition in wild social populations.     

Dominance, access to colonies, and queues for mating opportunities by male boat-tailed grackles

Species characterized by female-defense polygyny have extreme variance in male mating success. Many studies have considered alternative male strategies for access to females, but few have considered age-specific strategies. Male boat-tailed grackles (Quiscalus major) compete for access to colonies of females and form linear dominance hierarchies. I observed two groups of males that competed for females at seven colonies. Dominance rank was significantly correlated with mass, but not after controlling for age. In contrast, the correlation between dominance rank and age remained significant after controlling for mass. Older males dominated younger males and dominance relationships were very stable. Thus, dominance hierarchies represent queues for mating opportunities. A male's rank in the hierarchy determined how closely he approached a colony. Furthermore, males of all ranks prevented lower-ranked individuals from approaching the colonies. Dominance rank thus determined access to nesting females. One top-ranking male's loss of mass over the course of the breeding season presumably reflected the energetic cost of defending females, but he maintained his position in the hierarchy despite the small loss of mass. One alpha male held a colony for at least 4 years, and the ages of males from two queues indicated that males wait 6 or more years before becoming an alpha male. Therefore, most males die before acquiring a colony of females. Spatial structure such as that documented here could obscure recognition of queues and explain why they have not been documented in more species. Received: 13 May 1996 / Accepted after revision: 26 April 1997     

Dominance,kinship and reproductive value in female Barbary macaques (Macaca sylvanus) at Affenberg Salem

Summary In the course of a long-term study on social organization of semi-free-ranging Barbary macaques (Macaca sylvanus) at Affenberg Salem, genealogical and cross-genealogical rank relations of adult and adolescent females in three social groups were studied. Female rank was highly dependent on maternal rank, but the process of rank acquisition was also affected by age/size differences between members of different families. Mother-daughter rank reversal was rare, but all old, postreproductive matriarchs were outranked by their adult daughters. Contrary to findings of other studies on macaques, younger sisters seldom outranked older sisters. There was no genealogy with a strict age-inversed hierarchy among adult sisters as described for rhesus and Japanese macaques. Rank reversals between sisters were more frequent in genealogies with old or dead matriarchs, in large clans, and in dyads with an age difference of more than 1 year, indicating that demographic variables influence intra-genealogical dominance relations. It is suggested that close, long-lasting relationships between sisters and mothers and doughters impede rank reversals. Previous evolutionary models of female dominance relations in primates that explain rank relations among sisters as a function of their reproductive value or as a strategy of the mother to maintain her own status are not supported by the data. The results of this and other studies indicate that rank reversal between sisters is not as universal for Old World monkeys, or even macaques, as frequently proposed.     

Is male rhesus macaque red color ornamentation attractive to females?

Male sexually selected traits can evolve through different mechanisms: conspicuous and colorful ornaments usually evolve through intersexual selection, while weapons usually evolve through intra-sexual selection. Male ornaments are rare among mammals in comparison to birds, leading to the notion that female mate choice generally plays little role in trait evolution in this taxon. Supporting this view, when ornaments are present in mammals, they typically indicate social status and are products of male-male competition. This general mammalian pattern, however, may not apply to rhesus macaques (Macaca mulatta). Males of this species display conspicuous skin coloration, but this expression is not correlated to dominance rank and is therefore unlikely to have evolved due to male-male competition. Here, we investigate whether male color expression influences female proceptivity toward males in the Cayo Santiago free-ranging rhesus macaque population. We collected face images of 24 adult males varying in dominance rank and age at the peak of the mating season and modeled these to rhesus macaque visual perception. We also recorded female sociosexual behaviors toward these males. Results show that dark red males received more sexual solicitations, by more females, than pale pink ones. Together with previous results, our study suggests that male color ornaments are more likely to be a product of inter- rather than intra-sexual selection. This may especially be the case in rhesus macaques due to the particular characteristics of male-male competition in this species.     

Are faecal hormone levels linked to winter progression,diet quality and social rank in young ungulates ? An experiment with white-tailed deer (<Emphasis Type="Italic">Odocoileus virginianus</Emphasis>) fawns

Hormones play a central role in the physiology and behaviour of animals. The recent development of noninvasive techniques has increased information on physical and social states of individuals through hormone measurements. The relationships among hormones, life history traits and behaviours are, however, still poorly known. For the first time, we evaluated natural winter glucocorticoid and testosterone levels in young ungulates in relation to winter progression, diet quality and social rank. Overwinter, levels of glucocorticoid and testosterone decreased, possibly due to the decline of fawns’ body mass. The relationships between hormone levels and diet quality were surprising: Fawns fed the control diet presented higher glucocorticoid and lower testosterone levels then fawns fed the poor diet, suggesting that control fawns faced a higher nutritional stress than those on the poor diet. Similarly to other studies on social mammals, we found no relationship between faecal glucocorticoid levels and social rank, suggesting that social stress was similar for dominant and subordinate fawns during winter. Testosterone levels were not correlated to social rank as found previously in groups of individuals forming stable social hierarchies and maintaining stable dominance relationships. The simultaneous suppression of glucocorticoid and testosterone levels suggests for the first time that young ungulates present a hormonal strategy to prevent fast depletion of limited proteins and fat resources during winter.     

Competitive regimes in forest-dwelling Hanuman langur females (Semnopithecus entellus)

Van Schaik’s socioecological model predicts interrelations among food distribution, competitive regimes, and female social relationships. To test the internal consistency of the model, feeding competition was examined in three differently sized groups of a forest-dwelling population of Hanuman langurs (Semnopithecus entellus). The nutritional condition of females was used as a direct indicator of feeding competition and related to the seasonal variation in resource distribution and abundance. Female dominance hierarchies were characterized by displacements. Dominance hierarchies were significantly linear and relatively stable, but less so with increasing group size. Physical condition correlated with dominance rank and high-ranking females were in the best condition, indicating within-group contest competition. The strength of this relationship became less pronounced with increasing group size. The females of the medium-sized group were in the best physical condition indicating between-group contest plus within-group scramble competition. Closer examination revealed variable costs and benefits of group foraging with a predominance of within-group scramble competition when food was more abundant. The results support some basic predictions of the model. Limiting food abundance was bound to ubiquitous within-group scramble competition. The use of clumped resources translated into differences in net energy gain based on dominance. In contrast to the predictions, group-size-related costs and benefits were related to food abundance instead of food distribution. As predicted, within-group contest competition was linked to a linear dominance hierarchy. The absence of nepotism and coalitions in Hanuman langurs may be attributed to dominance hierarchies that are unstable through time, probably minimizing fitness gain via kin support. Received: 25 May 1999 / Received in revised form: 18 February 2000 / Accepted: 25 February 2000     

Tradeoffs between courtship, fighting, and antipredatory behavior by a lizard, Eumeces laticeps

Male fitness in many species depends strongly on social behaviors needed to obtain fertilizations and prevent loss of fertilizations to other males, but courtship, copulation, and fighting may incur increased risk of predation. When demands for reproductive and antipredatory behaviors conflict, fitness may be maximized by accepting some degree of risk to enhance reproductive success. To examine such tradeoffs, I introduced tethered conspecific males or females to adult male broad-headed skinks, Eumeces laticeps, in the field and observed how close they allowed a simulated predator (me) to approach before fleeing, or their latency to approach an introduced female located at different distances from the predator. When conspecific males were introduced, isolated and mate-guarding males initiated agonistic behaviors and permitted closer approach than control males, and mate-guarding males permitted closer approach than isolated males. When females were introduced, both isolated and mate-guarding males courted the introduced females and isolated males permitted closer approach than did mate-guarding males. These results for introduced males and females suggest that increasing risk was accepted when reproductive benefits were greater. Latency for isolated males to approach a conspecific female was greater when the predator was closer to the female, further suggesting sensitivity to predation risk during a reproductive opportunity. Relationships between reproductive and antipredatory behaviors have been studied much less than those between feeding and antipredatory behaviors, but this study indicates that animals balance increased risk of predation with the opportunity to perform several reproductively important behaviors. Received: 5 March 1999 / Received in revised form: 15 July 1999 / Accepted: 25 July 1999     

Androgen and glucocorticoid levels reflect seasonally occurring social challenges in male redfronted lemurs (Eulemur fulvus rufus)

Intense reproductive competition and social instability are assumed to increase concentrations of glucocorticoids and androgens in vertebrates, as a means of coping with these challenges. In seasonally breeding redfronted lemurs (Eulemur fulvus rufus), the mating and the birth season and the associated increased male competition are predicted to pose such reproductive challenges. In this paper, we investigate seasonal variation in hormone excretion in male redfronted lemurs, and examine whether this variation is associated with social or ecological factors. Although dominance status has been shown to affect individual stress levels across many taxa, we predicted no rank-related differences in glucocorticoids for redfronted lemurs because relatively equal costs are associated with both high and low rank positions (based on patterns of rank acquisition/maintenance and threats toward subordinates). Over a 14-month period, we collected behavioral data (1843 focal hours) and 617 fecal samples from 13 redfronted lemur males in Kirindy Forest/Madagascar. We found no general rank-related pattern of testosterone or glucocorticoid excretion in this species. Both hormones were excreted at significantly higher levels during the mating and the birth season, despite social stability during both periods. The elevated mating season levels may be explained by increased within-group reproductive competition during this time and are in line with previous studies of other seasonally reproducing primates. For the birth season increase, we propose that the predictable risk of infanticide in this highly seasonal species affects male gonadal and adrenal endocrine activity. We evaluate alternative social and ecological factors influencing the production of both hormone classes and conclude based on our preliminary investigations that none of them can account for the observed pattern.     

Ecology,feeding competition and social structure in baboons

Predictions of the model of van Schaik (1989) of female-bonding in primates are tested by systematically comparing the ecology, level of within-group contest competition for food (WGC), and patterns of social behaviour found in two contrasting baboon populations. Significant differences were found in food distribution (percentage of the diet from clumped sources), feeding supplant rates and grooming patterns. In accord with the model, the tendencies of females to affiliate and form coalitions with one another, and to be philopatric, were strongest where ecological conditions promoted WGC. Group fission in the population with strong WGC was “horizontal” with respect to female dominance rank, and associated with female-female aggression during a period of elevated feeding competition. In contrast, where WGC was low, females’ grooming was focused on adult males rather than other females. Recent evidence suggests that group fission here is initiated by males, tends to result in the formation of one-male groups, and is not related to feeding competition but to male-male competition for mates. An ecological model of baboon social structure is presented which incorporates the effects of female-female competition, male-male competition, and predation pressure. The model potentially accounts for wide variability in group size, group structure and social relationships within the genus Papio. Socio-ecological convergence between common baboons and hamadryas baboons, however, may be limited in some respects by phylogenetic inertia. Received: 22 April 1994/Accepted after revision: 9 December 1995     

Habitat quality and heterogeneity influence distribution and behavior in African buffalo (Syncerus caffer)

Top-down effects of predators on prey behavior and population dynamics have been extensively studied. However, some populations of very large herbivores appear to be regulated primarily from the bottom up. Given the importance of food resources to these large herbivores, it is reasonable to expect that forage heterogeneity (variation in quality and quantity) affects individual and group behaviors as well as distribution on the landscape. Forage heterogeneity is often strongly driven by underlying soils, so substrate characteristics may indirectly drive herbivore behavior and distribution. Forage heterogeneity may further interact with predation risk to influence prey behavior and distribution. Here we examine differences in spatial distribution, home range size, and grouping behaviors of African buffalo as they relate to geologic substrate (granite and basalt) and variation in food quality and quantity. In this study, we use satellite imagery, forage quantity data, and three years of radio-tracking data to assess how forage quality, quantity, and heterogeneity affect the distribution and individual and herd behavior of African buffalo. We found that buffalo in an overall poorer foraging environment keyed-in on exceptionally high-quality areas, whereas those foraging in a more uniform, higher-quality area used areas of below-average quality. Buffalo foraging in the poorer-quality environment had smaller home range sizes, were in smaller groups, and tended to be farther from water sources than those foraging in the higher-quality environment. These differences may be due to buffalo creating or maintaining nutrient hotspots (small, high-quality foraging areas) in otherwise low-quality foraging areas, and the location of these hotspots may in part be determined by patterns of predation risk.     

Digit ratio (2D:4D) and dominance rank in female rhesus macaques (Macaca mulatta)

Female rhesus macaques exhibit matrilineal dominance structures, and high dominance rank confers fitness benefits across a lifetime and across generations. Rank effects are “inherited” through social processes that are well understood; however, biological mechanisms that might impact these processes are not well known. Recently, it has been shown that prenatal androgens appear to be implicated in supporting dominance rank hierarchies in some mammals. In humans, interindividual differences in the second (index) to fourth (ring) digit ratio (2D:4D) have been linked indirectly to variation in prenatal androgens, with low 2D:4D in both sexes associated with higher inferred prenatal androgen effects. 2D:4D has also been related to dominant social behavior and has been shown to co-vary with social systems across nonhuman primate species. Here, we investigate how 2D:4D co-varies with socially inherited dominance rank in female rhesus macaques. Low 2D:4D was associated with higher-ranking females, while higher 2D:4D was associated with lower-ranking females. Similar relationships were also shown between ranked families within matrilines. This is the first study to show such a relationship between 2D:4D and dominance rank in a nonhuman primate and suggests that prenatal androgen effects could be involved in the maintenance of dominance rank in female cercopithecine primates.