Dominance and reproductive success in primates

A tale of two hierarchies: Hormonal and behavioral factors underlying sex differences in social dominance in cooperative breeding callitrichids

Callitrichid primates are recognized for high levels of sociality in small groups, their great behavioral flexibility, and single-female dominant hierarchies. Previous work has highlighted that dominant, breeding callitrichids engage in behavioral and hormonal reproductive suppression of related and unrelated subordinates by both producing more offspring, having higher levels of ovulatory hormones, and accessing more sociosexual opportunities. This suppression constitutes a nexus of changes in pituitary responsiveness, ovarian cyclicity, sexual behavior, affiliation, and aggression. In this review, I will highlight important features that characterize callitrichid social hierarchies across broad social contexts. Dominant females sometimes exert reproductive suppression on subordinate nonbreeding females, but this suppression varies across callitrichids based on social stability and changes in group composition, particularly related to the number, experience, and age of nonbreeding subordinates. Meanwhile, dominant males may induce suppression of reproduction in subordinate males, but these effects occur by different behavioral and endocrine mechanisms and to a much lesser extent than their female counterparts; While dominant female callitrichids usually show higher levels of aggression relative to their male counterparts, callitrichids show a general absence of intersexual dominance, likely as an effort of maintaining a cohesive breeding pair within a stable social group and social cooperation. Future efforts are needed to identify precise neuroendocrine mechanisms underlying the presence of sex differences in callitrichid behavior separate from peripheral reproductive function. This is especially important with regard to parental experience, social relationships, development and aging, with larger implications toward understanding sex differences in overall health and wellbeing.

Social correlates of androgen levels and dispersal age in juvenile male geladas

Androgens offer a window into the timing of important male life history events such as maturation. However, when males are the dispersing sex, piecing together normative androgen profiles across development is challenging because dispersing males are difficult to track. Here, we examined the conditions that may be associated with male androgen status (via fecal androgen metabolites, fAMs) and age at dispersal in wild male geladas (Theropithecus gelada). Gelada male life histories are highly variable - dispersal may occur before sexual maturation, dispersal itself can be immediate or drawn out, and, due to their multi-leveled society, social conditions affecting dispersal can vary for juveniles living in different reproductive units within the same band. Using longitudinal data from known natal males, we examined how androgen levels and age at dispersal were associated with: (1) access to maternal resources (i.e., maternal rank, birth of a younger sibling, experiencing maternal loss), and (2) access to male peers (i.e., number of similar-aged males in their unit). We found that androgens were significantly lower in males with high-ranking mothers (in males >2.5 years of age; infant androgens were unrelated) and that having more male peers in their social group and larger groups overall predicted an earlier age at dispersal. Moreover, dispersal in geladas was not preceded or followed by a surge in androgen levels. Taken together, results suggest that social environments can cause individual variation in androgens and dispersal age. Whether this variation leads to differences in male fitness in later life remains to be determined.

Coexistence between Humans and ‘Misunderstood’ Domestic Cats in the Anthropocene: Exploring Behavioural Plasticity as a Gatekeeper of Evolution

Welfare and management decisions for unowned free-ranging cats in urban environments should no longer be based on knowledge about behavioural ecology of solitary cats living and breeding in more natural ‘wild’ environments. We provide evidence that urban free-ranging domestic cats in the Anthropocene have responded to rapidly changing environments, such as abundance of food and higher population densities of conspecifics by adapting their behaviour (behavioural plasticity—the ability of a genotype (individual) to express different behaviours according to its environment) and social organisation to living in complex social groups, especially those living in colonies. Urban free-ranging cats are now more social, as demonstrated by different breeding patterns, lower infanticide, more frequent affiliative interactions in general, and different spatial groupings. We argue that this knowledge should be disseminated widely, and inform future research and strategies used to manage free-ranging cats across environments. Understanding behavioural plasticity and other recently evolved traits of domestic cats may lead to management strategies that maximise health and welfare of cats, wildlife, and humans—otherwise domestic cat behaviour may be ‘misunderstood’. Importantly, interdisciplinary research using expertise from biological and social sciences, and engaging human communities, should evaluate these management strategies to ensure they maintain optimal welfare of free-ranging domestic cats while preserving biodiversity and protecting wildcats.

My path to primatology: some stories from the field

In this paper, I summarize the major facets of my 50-year career as a primatologist. I briefly describe the aspects of my upbringing and early education that led me to the study of primate behavior, first in captive settings and, later, in the wild. My research on the Arashiyama West Japanese macaques and my interactions with Japanese primatologists was a formative stage in my career, and I present the background of this international project and how it led to my growing focus on female life history studies. After a couple of failed attempts to establish a long-term study of primates in their native habitats, I began the Santa Rosa Primate Project in Costa Rica in 1983, which focuses mainly on white-faced capuchins, and to some extent on howlers and spider monkeys. The Santa Rosa project has expanded over the past four decades and continues to this day, with the participation of a large team of colleagues, local field assistants and students. I present some of the major findings of our Santa Rosa monkey research in the areas of female reproduction, sexual conflict and conservation of primates in a regenerating tropical dry forest. I also briefly describe how and why I came to develop a sideline of research on gender and science.

Mating and Reproductive Success in Free-Ranging Stump-Tailed Macaques: Effectiveness of Male–Male Coalition Formation as a Reproductive Strategy

Male coalition-like formation, recently found in stump-tailed macaques (Macaca arctoides), occurs when several top-ranking males collaboratively guard females to prevent mating with other rival males and actively share secured mating opportunities with their allies. We lack a comprehensive understanding of the proximate mechanisms underlying such male coalitions, e.g., the effect of genetic relatedness. Such cooperative partner choice among males is particularly interesting in animals, such as M. arctoides, that live in matrilineal (female philopatry/male dispersal) society. Theoretically, in such a social system, females, and not males, are often related to each other by kinship. Thus, the occurrence of cooperative behavior between related females are generally common, but it would be rare between unrelated/non-kin males in such matrilineal society. Herein, for the first time, we report detailed copulatory behaviors, including mating and reproductive success, in relation to male coalition-like formation in free-ranging M. arctoides following direct behavioral observation and genetic analysis. We found that coalition-forming male–male alliances often occur among both individuals that are highly related and those that are less related. We identified two groups with different mating strategies, i.e., single-male monopoly and coalition-male monopoly groups. In both groups, nearly 80% of copulations were monopolized by a single male or by coalition males. However, the single-male monopoly strategy allows opportunistic/sneaky copulations by other males with a relatively high probability. Thus, the degree of reproductive success did not reflect mating success. In contrast, the males employing a coalition strategy successfully shared their mating and reproductive success, particularly in the largest group. Compared with single-male monopolized groups, the coalition-male monopoly groups copulated with a considerably more number of females, suggesting that coalition males can effectively guard against opportunistic/sneaky copulation by rival males. We also found that coalition-forming male–male alliances often occur regardless of the degree of kinship/relatedness, indicating the complexity and flexibility inherent in the male social bond of M. arctoides.

Developmental transitions in body color in chacma baboon infants: Implications to estimate age and developmental pace

OBJECTIVES

In many primates, one of the most noticeable morphological developmental traits is the transition from natal fur and skin color to adult coloration. Studying the chronology and average age at such color transitions can be an easy and noninvasive method to (a) estimate the age of infants whose dates of birth were not observed, and (b) detect interindividual differences in the pace of development for infants with known birth dates.

MATERIALS AND METHODS

Using a combination of photographs and field observations from 73 infant chacma baboons (Papio ursinus) of known ages, we (a) scored the skin color of six different body parts from pink to gray, as well as the color of the fur from black to gray; (b) validated our method of age estimation using photographic and field observations on an independent subset of 22 infants with known date of birth; and (c) investigated ecological, social, and individual determinants of age-related variation in skin and fur color.

RESULTS

Our results show that transitions in skin color can be used to age infant chacma baboons less than 7 months old with accuracy (median number of days between actual and estimated age = 10, range = 0-86). We also reveal that food availability during the mother's pregnancy, but not during lactation, affects infant color-for-age and therefore acts as a predictor of developmental pace.

DISCUSSION

This study highlights the potential of monitoring within- and between-infant variation in color to estimate age when age is unknown, and developmental pace when age is known.

Individuals in urban dwelling primate species face unequal benefits associated with living in an anthropogenic environment

In primates, living in an anthropogenic environment can significantly improve an individual's fitness, which is likely attributed to access to anthropogenic food resources. However, in non-professionally provisioned groups, few studies have examined whether individual attributes, such as dominance rank and sex, affect primates' ability to access anthropogenic food. Here, we investigated whether rank and sex explain individual differences in the proportion of anthropogenic food consumed by macaques. We observed 319 individuals living in nine urban groups across three macaque species. We used proportion of anthropogenic food in the diet as a proxy of access to those food resources. Males and high-ranking individuals in both sexes had significantly higher proportions of anthropogenic food in their diets than other individuals. We speculate that unequal access to anthropogenic food resources further increases within-group competition, and may limit fitness benefits in an anthropogenic environment to certain individuals.

On the sunny side of (new) life: Effect of sunshine duration on age at first reproduction in Japanese macaques (Macaca fuscata)

To produce offspring early in life is energetically demanding and depends greatly on environmental conditions. In female primates, age at first reproduction (AFR) has been associated with social parameters (e.g., population density and social rank), food availability and meteorological conditions (e.g., photoperiod, rainfall patterns, and temperature). Regarding the latter, less attention has been given to the influence of sunshine. In nonhuman primates, including the northern‐most distributed Japanese macaque (Macaca fuscata), sunbathing is an effective thermoregulatory strategy to maintain sufficient energy intake during harsh winter months. Furthermore, the energetic value of sunshine and its role in the synthesis of essential vitamins important for sexual development and overall fertility is well investigated using human and animal models. In the present study, we hypothesized that female's AFR is influenced by the amount of sunshine in a semi‐free‐ranging, provisioned a group of Japanese macaques. To test this, we gathered data on sunshine duration in the year females theoretically experienced the onset of puberty. This phase of the female life cycle is particularly prone to the effects of environmental conditions. In addition to the investigation of sunshine duration and other meteorological conditions (i.e., rainfall and temperature) we controlled for social parameters (i.e., group size and sex ratio) as potential covariates. We found a clear effect of sunshine duration on female AFR: Females who entered puberty in years with more sunshine reproduced for the first time at significantly younger ages than females who experienced less sunshine during this specific period of their development. Possible mechanisms for how the sunshine influences sexual maturation in Japanese macaques are discussed.

We investigated the influence of sunshine during the year of puberty onset on a female's age at first reproduction (AFR).

We used 20 years of birth data from a semi‐free‐ranging group of Japanese macaques and meteorological data provided online by local weather stations

We controlled for the influence of other meteorological conditions as well as for group size and socionomic sex ratio

Higher mean sunshine duration in the year of puberty onset significantly decreased females’ AFR.

Masculinity, femininity, and leadership: Taking a closer look at the alpha female

An extensive review and textual analysis of the academic and popular literature of the human alpha female was conducted to examine the social construction and expression of the alpha female identity in a small non-random sample of North American women (N = 398). This review revealed 2 predominant alpha female representations in the literature-one more masculine versus one more feminine-and 21 alpha female variables. In this sample of women, the "alpha female" was found to be a recognized socially constructed female identity. Univariate analysis revealed positive and highly significant differences in self-reported mean scores between alpha (N = 94) and non-alpha (N = 304) females for 10 variables including, masculine traits, leadership, strength, low introversion, self-esteem, life satisfaction, sexual experience, initiates sex, enjoys sex and playing a dominant role in sexual encounters, with alpha females scoring higher than non-alphas. The measure of masculine traits was identified as the only predictor of alpha female status as per the multiple regression model. Interestingly, both alpha and non-alpha women scored the same for the measure of feminine traits. Further, both groups scored higher for feminine traits than masculine traits. The results also revealed that neither social dominance nor sexual dominance were predictors of alpha female status which challenge academic and popularized representations of this identity. The results suggest that although the alpha female is often regarded as an exceptional and, at times, an exoticized form of femininity, like other femininities, her identity is marked by contradictions and tensions.

Balancing contest competition, scramble competition, and social tolerance at feeding sites in wild common marmosets (Callithrix jacchus)

Models of primate sociality focus on the costs and benefits of group living and how factors such as rank, feeding competition, alliance formation, and cooperative behavior shape within-group social relationships. We conducted a series of controlled field experiments designed to investigate how resource distribution (one or three of four reward platforms) and amount of food on a reward platform affected foraging strategies and individual feeding success in four groups of wild common marmosets (Callithrix jacchus) living in the Caatinga of northeastern Brazil. At our field site, common marmoset groups are characterized by a single breeding female who can produce twin litters twice per year, strong social cohesion, and cooperative infant care provided principally by several adult male helpers. We found that except for the dominant breeding female, rank (based on aggression) was not a strong predictor of feeding success. Although the breeding female in each group occupied the highest rank position and obtained the greatest daily feeding success, all other group members, including adults and juveniles experienced relatively equal feeding success across most experimental conditions. This was accomplished using a balance of behavioral strategies related to contest competition, scramble competition (associated with a finder's advantage), and social tolerance (sharing the same feeding platform). Based on these results, the social structure of common marmosets is best described as "single female dominance," with the breeding female maximizing food intake needed to offset the energetic costs associated with reproductive twinning and the ability to produce two litters per year. Cooperative infant caregiving, in which the number of helpers is positively correlated with offspring survivorship, requires a set of behavioral strategies that serve to reduce contest competition and promote prosocial behaviors at feeding sites.

Weak effects of common genetic variation in oxytocin and vasopressin receptor genes on rhesus macaque social behavior

The neuropeptides oxytocin (OT) and arginine vasopressin (AVP) influence pair bonding, attachment, and sociality, as well as anxiety and stress responses in humans and other mammals. The effects of these peptides are mediated by genetic variability in their associated receptors, OXTR and the AVPR gene family. However, the role of these genes in regulating social behaviors in non-human primates is not well understood. To address this question, we examined whether genetic variation in the OT receptor gene OXTR and the AVP receptor genes AVPR1A and AVPR1B influence naturally-occurring social behavior in free-ranging rhesus macaques-gregarious primates that share many features of their biology and social behavior with humans. We assessed rates of social behavior across 3,250 hr of observational behavioral data from 201 free-ranging rhesus macaques on Cayo Santiago island in Puerto Rico, and used genetic sequence data to identify 25 OXTR, AVPR1A, and AVPR1B single-nucleotide variants (SNVs) in the population. We used an animal model to estimate the effects of 12 SNVs (n = 3 OXTR; n = 5 AVPR1A; n = 4 AVPR1B) on rates of grooming, approaches, passive contact, contact aggression, and non-contact aggression, given and received. Though we found evidence for modest heritability of these behaviors, estimates of effect sizes of the selected SNVs were close to zero, indicating that common OXTR and AVPR variation contributed little to social behavior in these animals. Our results are consistent with recent findings in human genetics that the effects of individual common genetic variants on complex phenotypes are generally small.

The role of intragroup agonism in parent-offspring relationships and natal dispersal in monogamous owl monkeys (Aotus azarae) of Argentina

Agonistic behaviors are common in many group-living taxa and may serve a variety of functions, ranging from regulating conflicts over reproduction to defending food resources. However, high rates of agonism are not expected to occur among close relatives or individuals in established mating relationships, which are characteristics of monogamous groups. To contribute to our understanding of agonism within socially monogamous groups, we collected behavioral and demographic data from Azara's owl monkeys (Aotus azarae) in the Gran Chaco of Argentina over 14 years. We examined factors related to age, sex, kinship, and behavioral context to evaluate predictions of the hypotheses that agonism functions to regulate dispersal and that it mediates competition for food and/or mates. Intragroup agonism was relatively rare: the group rate was approximately one event every three and a half hours. Rates of agonism were generally similar for both sexes, but there were marked differences among age categories. Agonism performed by adults was more frequently directed at subadults than at younger offspring. In contrast, agonistic interactions involving infants were very rare. Among interactions between adults and subadults, adults were much more frequently the actors than the recipients, suggesting that agonism from adults may influence natal dispersal of subadults. Agonistic events were most frequent during foraging, but also occurred more frequently than expected during bouts of social behavior. Overall, our results suggest that agonism in owl monkeys serves as a mechanism for regulating dispersal, and also likely plays a role in mediating mating and feeding competition.

The role of individuals in the history of a free-ranging group of Japanese macaques: Some thoughts on the relevance for human behavioural research

Taking the stance that the study of nonhuman primates can be useful in understanding the mechanism of long-term changes in social organisation, this short paper seeks to shed light on the role the individual might play in changes in future human society. This is done by examining the causes of social order change in a group of free-ranging macaque monkeys. The paper suggests that it may be useful to examine the role of genetically transmitted characteristics in human “success” stories, particularly concerning adaptability in times of change, and that the examination of individual life-history data across generations, will be essential for clarifying the mechanism of changes in human social organisation.

Birth order, sibship size, and status in modern Canada

This paper investigates the possibility that birth order affects the degree to which individuals attain higher status. Humans give birth to a variable number of (usually) single offspring spaced one to many years apart, and continue to maintain contact with them for extended periods of time. The continued presence of older siblings, and arrival of younger ones, means that each child is reared in a different family environment. Research findings from the field of behavior genetics suggest that these differences have a significant impact on the development of individual differences between children in the same family. Although no two families are likely to be exactly the same, factors such as birth order remain constant across them, and may have similar influences. The present study examines the relationships between birth order, sibship size, and several variables thought to index future status attainment (status striving) in a random sample of Canadians. Firstborn children appear to be more status oriented than lastborns, and this effect is mediated by sibship size. While firstborn children are unaffected by the number of younger siblings they have, the status ambitions of youngest children decrease the more older siblings they have. Birth order effects on status attainment are not as strong as they are on status ambitions.

Social variables affecting mate preferences, copulation and reproductive outcome in a pack of free-ranging dogs

Mating and reproductive outcome is often determined by the simultaneous operation of different mechanisms like intra-sexual competition, mating preferences and sexual coercion. The present study investigated how social variables affected mating outcome in a pack of free-ranging dogs, a species supposed to have lost most features of the social system of wolves during domestication. We found that, although the pack comprised multiple breeding individuals, both male copulation success and female reproductive success were positively influenced by a linear combination of dominance rank, age and leadership. Our results also suggest that mate preferences affect mating outcome by reinforcing the success of most dominant individuals. In particular, during their oestrous period bitches clearly searched for the proximity of high-ranking males who displayed affiliative behaviour towards them, while they were more likely to reject the males who intimidated them. At the same time, male courting effort and male-male competition for receptive females appeared to be stronger in the presence of higher-ranking females, suggesting a male preference for dominant females. To our knowledge, these results provide the first clear evidence of social regulation of reproductive activities in domestic dogs, and suggest that some common organizing mechanisms may contribute to shape the social organization of both dogs and wolves.

Intermale sexual effect elicited by volatile urinary ether extract inMicrocebus murinus (Prosimian, Primates)

The effect of volatiles of diethyl ether-extracted dominant male mouse lemur urine on conspeciflc testosterone plasma concentrations was determined in an apparatus permitting olfactory isolation of the experimental subjects. Two experimental groups were exposed to air odorized with either ether or aqueous extracts of dominant male urine, while a third group received nonodorant air. Testosterone concentrations were measured before, during, and after the period of stimulation. The group submitted to the lipid fraction of dominant urine (N = 15) showed a significant decrease in testosterone concentrations. In contrast, no difference was observed between the group submitted to the aqueous fraction (N = 10) and the controls (N = 11). This physiological effect, which occurs in response to conditions of persistent stress, appears to be due to lipophilic components present in the urine of dominant males. The results are discussed in terms of the social structure of this primitive primate species.

Adolescent male chimpanzees at Ngogo, Kibale National Park, Uganda have decided dominance relationships

Dominance relationships are common among primates, and are usually the result of resource competition. Adult male chimpanzees have pronounced dominance relationships, and can be ranked relative to one another in dominance hierarchies. Adult male chimpanzees achieve, and maintain, high rank through a combination of physical size, strength and political ability. Dominance in male chimpanzees has very real benefits as high-ranking males have priority access to resources and potential mates, and father more offspring than do low-ranking conspecifics. Previous research has suggested that adolescent male chimpanzees do not have dominance relationships with each other. Here, I report the first evidence of decided dominance relationships and a linear dominance hierarchy in adolescent male chimpanzees from an unusually large community of wild chimpanzees at Ngogo, Kibale National Park, Uganda.

Cultural and reproductive success in industrial societies: Testing the relationship at the proximate and ultimate levels

In most social species, position in the male social hierarchy and reproductive success are positively correlated; in humans, however, this relationship is less clear, with studies of traditional societies yielding mixed results. In the most economically advanced human populations, the adaptiveness of status vanishes altogether; social status and fertility are uncorrelated. These findings have been interpreted to suggest that evolutionary principles may not be appropriate for the explanation of human behavior, especially in modern environments. The present study tests the adaptiveness of social status with actual mating and reproductive data in a representative sample of males from an industrial society. Reproductive success, even when assessed by a more reliable measure ofactualmale fertility than the one commonly used, fails to correlate with social status. In striking contrast, however, status is found to be highly correlated withpotentialfertility, as estimated from copulation frequency. Status thus accounts for as much as 62% of the variance in thisproximatecomponent of fitness. This pattern is remarkably similar to what is found in many traditional societies and would result in a substantial positive relationship between cultural and reproductive success in industrial populations were it not for the novel conditions imposed by contraception and monogamy. Various underlying mechanisms are suggested for these findings, illustrating the value of current behavioral and reproductive data in the study of adaptation. It is concluded that evolutionary explanations of human behavior remain entirely relevant in modern societies.

Female mate choice in mammals

Studies of mate choice in vertebrates have focused principally on birds, in which male ornaments are often highly developed, and have shown that females commonly select mates on the basis of particular phenotypic characteristics that may reflect their genetic quality. Studies of female mate choice in mammals are less highly developed and they have commonly focused on female mating preferences that are likely to be maintained by benefits to the female's own survival or breeding success. However, recent experimental studies of mate choice in mammals--especially rodents--provide increasing evidence of consistent female preferences that appear likely to generate benefits to the fitness of offspring. As yet, there is no compelling evidence that female mating preferences are less highly developed in female mammals than in female birds, although these preferences may more often be masked by the effects of male competition or of attempts by males to constrain female choice.

Dominance and submission: the ventrolateral prefrontal cortex and responses to status cues

Status hierarchies constitute a fundamental organizing principle of human society. However, little is known about the neural systems that process nonverbal cues that indicate status. Preliminary neuropsychological work has suggested a role for the ventrolateral and ventromedial prefrontal cortex (VLPFC/VMPFC) and the superior temporal cortex (STC). We used functional magnetic resonance imaging to delineate the nature of these roles. Analyses revealed signal changes in the right VLPFC in connection with two primary functions attributed to status cues. Status cues moderate behavior and the right VLPFC showed increased signal for high-status relative to neutral and low-status cues. The VLPFC also showed increased signal for high-status cues displayed by individuals of the opposite gender to the perceiver; this may be relevant to the role status cues play in moderating mate choice behavior. Connectivity results indicated significant positive connectivity between the VLPFC and both the VMPFC and the STC. We suggest that the VLPFC retrieves information from these regions when processing hierarchy cues to facilitate socially adaptive behavior.

Male dominance rank, mating and reproductive success in captive bonobos (Pan paniscus)

In the recent past, application of DNA genotyping techniques has enabled researchers to more accurately test relationships between dominance rank (DR), mating success (MS) and reproductive success (RS). Paternity studies often reveal that reproductive outcome does not always correlate with male DR and/or MS and thus open room for discussion and interpretation of alternative reproductive tactics of both sexes. In this study, we analysed male DR, MS and RS in a group of bonobos at Twycross Zoo (UK). Genetic relationships were determined using 8 tetrameric microsatellite loci. Despite clear and asymmetric dominance relationships, analysed using normalised David's scores based on a dyadic index of dominance among the group's 3 mature males, we found that the most dominant male did not sire the most offspring. In fact, both infants conceived during the observation period were found to be sired by the lower-ranking males. Although the alpha male had almost exclusive mating access to one of the females during the time she was showing a maximal anogenital swelling, her infant was sired by the lowest-ranking male who mostly mated with her when outside the maximal swelling period. This result suggests that either sperm competition operates and/or ovulation is decoupled from the phase of maximal anogenital swelling which could allow greater female choice.