Dominance and reproductive success in primates

Theoretical models of the relationship between competition and differential reproduction in primates share the premise that agonistic dominance hierarchies determine differential access to those limited resources which are essential to reproduction. In particular, the priority-of-access-to-estrous-female model is based on the postulate that high rank in males enhances reproductive success. Tests for a correlation between rank and reproductive success in males have produced mixed results. Problems in measuring male reproductive success and in conceptualizing dominance systems are implicated in the diversity of conclusions reached. Other attributes which affect reproductive success have been proposed, and alternative models of mating systems, based on the concepts of reproductive strategies, social skills, and life histories have also been developed. Studies of differential reproduction in female primates are fewer, but have the advantage over male studies of directly measuring infant production and offspring survival. Research on the relationship between rank and reproductive success in females has shown that under conditions of low resource availability, subordinate females sometimes suffer reduced fecundity and infant survival as a result of restricted access to food and water. Under conditions of social stress, low-ranking females may suffer from disruption of their reproductive cycles, resulting in lowered fecundity. Dominant females may also incur reproductive costs as well as benefits for their social position. Better insights into the relationship between dominance and reproduction are potentially offered by improved genetic paternity measures, new models of social and demographic processes, and the recent availability of life history data from field studies.     

Male and Female Reproductive Success in <Emphasis Type="Italic">Macaca sylvanus</Emphasis> in Gibraltar: No Evidence for Rank Dependence

The relationship between social rank and reproductive success is one of the key questions for understanding differences in primate social group structures. We determined the paternity of 18 infants in a social group of Barbary macaques (Macaca sylvanus) born over a period of 6 yr in the provisioned, free-ranging colony in Gibraltar. We successfully used 13 pairs of primers of variable microsatellite loci to amplify DNA from blood and hair samples and applied the computer programs CERVUS 2.0 and KINSHIP 1.3 to assign paternity to 13 candidate males. We collected data for 19 females that had given birth to 66 infants over a period of 7 yr. We used paternity analyses and female birth records to test the hypothesis that social rank is correlated with reproductive success. Results showed that numbers of paternities and maternities were equally distributed among all reproducing individuals in the social group regardless of rank. Subadult males reproduced as often as adult males. High-ranking females did not start to reproduce earlier than low-ranking females. Interestingly, there was a tendency toward a positive correlation between the ranks of mothers and the ranks of the corresponding fathers. It might be concluded either that a correlation between social rank and reproductive success is generally absent in Barbary macaques or that artificially favorable environmental conditions in Gibraltar preclude any correlation between social rank and reproductive success.     

The effects of dominance rank and group size on female lifetime reproductive success in wild long-tailed macaques,Macaca fascicularis

Demographic changes were recorded throughout a 12-year period for three social groups ofMacaca fascicularis in a natural population at Ketambe (Sumatra, Indonesia). We examined the prediction that females' lifetime reproductive success depended on dominance rank and group size. Average birth rate was 0.53 (184 infants born during 349 female years). For mature females (aged 8–20 yr) birth rate reflected physical condition, being higher in years with high food availability and lower in the year following the production of a surviving infant. High-ranking females were significantly more likely than low-ranking ones to give birth again when they did have a surviving offspring born the year before (0.50 vs 0.26), especially in years with relatively low food availability (0.37 vs 0.10). Controlled comparisons of groups at different sizes indicate a decline in birth rate with rroup size only once a group has exceeded a certain size. The dominance effect on birth rate tended to be strongest in large groups. Survival of infants was rank-dependent, but the survival of juveniles was not. There was a trend for offspring survival to be lower in large groups than in mid-sized or small groups. However, rank and group size interacted, in that rank effects on offspring survival were strongest in large groups. High-ranking females were less likely to die themselves during their top-reproductive years, and thus on average had longer reproductive careers. We estimated female lifetime reproductive success based on calculated age-specific birth rates and survival rates. The effects of rank and group size (contest and scramble) on birth rate, offspring survival, age of first reproduction for daughters, and length of reproductive career, while not each consistently statistically significant, added up to substantial effects on estimated lifetime reproductive success. The group size effects explain why large groups tend to split permanently. Since females are philopatric in this species, and daughters achieve dominance rank positions similar to their mother, a close correlation is suggested between the lifetime reproductive success of mothers and daughters. For sons, too, maternal dominance affected their reproductive success: high-born males were more likely to become top-dominant (in another group). These data support the idea that natural selection has favored the evolution of a nepotistic rank system in this species, even if the annual benefits of dominance are small.     

Social Dominance among Male Meadow Voles is Inversely Related to Reproductive Success

Intrasexual selection can occur through direct aggressive interactions between males for access to females. We tested the relationship between social dominance and male reproductive success among meadow voles, Microtus pennsylvanicus. Dominance ranks of wild‐caught males were determined using neutral arena trials, with the winner of two of three trials considered dominant. These males were then released into field enclosures and allowed to visit females housed in nestboxes for 8 wk, and males’ home range sizes were determined using weekly grid trapping. Male reproductive success was determined using molecular paternity analysis (six microsatellite primers) for all pups born during the field experiment. Males with higher dominance ranks had larger home ranges. However, male dominance rank was not predictive of the number of total visits to females’ nestboxes or the number of visits to each male's most frequently visited nestbox. Males that made more visits to nestboxes sired more litters. Males that had higher dominance ranks sired fewer litters. These results suggest that there is a reproductive disadvantage to having higher dominance rank among male meadow voles.     

Determinants of fecundity and reproductive success in captive vervet monkeys

Between 1975 and 1983, adult female vervet monkeys (Cercopithecus aethiops sabaeus) over 3.5 years of age, living in two undisturbed social groups in a captive colony in Sepulveda, California, have averaged 1.0 births per female year with a mean interbirth interval of 10.7 months. Increased fecundity did not result in decreased survival rates of offspring in this population. Fecundity was influenced by the mother's age and dominance rank. The primary factor in the age-fecundity relationship was the age at first birth, which varied from three to five years. High-ranking females contributed the most to the high rate of fecundity, with significantly shorter interbirth intervals, more births per female year, and more surviving infants compared to low-ranking females.     

Factors affecting fecal glucocorticoid levels in semi-free-ranging female mandrills (Mandrillus sphinx)

Subordinate female cercopithecine primates often experience decreased reproductive success in comparison with high-ranking females, with a later age at sexual maturity and first reproduction and/or longer interbirth intervals. One explanation that has traditionally been advanced to explain this is high levels of chronic social stress in subordinates, resulting from agonistic and aggressive interactions and leading to higher basal levels of glucocorticoids. We assessed the relationships among fecal cortisol levels and reproductive condition, dominance rank, degree of social support, and fertility in female mandrills (Mandrillus sphinx) living in a semi-free-ranging colony in Franceville, Gabon. Lower-ranking females in this colony have a reproductive disadvantage relative to higher-ranking females, and we were interested in determining whether this relationship between dominance rank and reproductive success is mediated through stress hormones. We analyzed 340 fecal samples from 19 females, collected over a 14-month period. We found that pregnant females experienced higher fecal cortisol levels than cycling or lactating females. This is similar to results for other primate species and is likely owing to increased metabolic demands and interactions between the hypothalamus-pituitary-adrenal axis, estrogen, and placental production of corticotrophin releasing hormones during pregnancy. There was no influence of dominance rank on fecal cortisol levels, suggesting that subordinate females do not suffer chronic stress. This may be because female mandrills have a stable social hierarchy, with low levels of aggression and high social support. However, we found no relationship between matriline size, as a measure of social support, and fecal cortisol levels. Subordinates may be able to avoid aggression from dominants in the large enclosure or may react only transiently to specific aggressive events, rather than continuously expecting them. Finally, we found no relationship between fecal cortisol levels and fertility. There was no difference in fecal cortisol levels between conceptive and nonconceptive cycles, and no significant relationship between fecal cortisol level and either the length of postpartum amenorrhea or the number of cycles before conception. This suggests that the influence of dominance rank on female reproductive success in this population is not mediated through chronic stress in subordinate females, and that alternative explanations of the relationship between social rank and reproduction should be sought.     

Life history and the competitive environment: trajectories of growth,maturation, and reproductive output among chacma baboons

The social environment is a key feature influencing primate life histories. Chacma baboons (Papio hamadryas ursinus) are a female-bonded species with a strict linear dominance hierarchy. In this species, the allocation of energy to competing demands of growth and reproduction is hypothesized to vary as a function of competitive ability, which in turn increases with social rank. Since growth rate is a major component of life history models, measures of age-specific growth were used to analyze variation in life history traits across social ranks. Weights of 42 immature baboons were obtained without sedation or baiting from a troop of well-habituated chacma baboons in the Okavango Delta, Botswana. Using demographic and weight data from this wild population, five main findings emerged: 1) Weight for age and growth rate of infant and juvenile females are positively associated with maternal rank. 2) Male growth is not influenced by maternal rank. 3) Female growth shows smaller variation across feeding conditions than male growth. 4) Low-ranking adult females continue investment in offspring through prolonged lactation until they reach a weight comparable to that of high-ranking infants. 5) The benefit of rank to reproductive success shown in this study is 0.83 additional offspring. Reproductive span determined predominantly by age at maturation contributes 27-38% to the difference in expected number of offspring by rank, vs. 62-73% due to reproductive rate. These findings have major implications for understanding the role of social environment in phenotypic plasticity of life history traits, and in the evolution of primate life histories.     

Correlates and Consequences of Dominance in a Social Rodent

In harem‐polygynous societies, body condition is often correlated with dominance rank. However, the consequences of dominance are less clear. High‐ranking males do not inevitably have the highest reproductive success, especially in systems where females mate with multiple males. In such societies, we expect male reproductive success to be more highly skewed than female reproductive success, but reproductive skew in females can still arise from rankings established within matrilineal societies. Dominance can also impact life‐history decisions by influencing dispersal patterns in yearlings. To better understand the function of dominance in harem‐polygynous societies, we studied the causes and consequences of dominance in yellow‐bellied marmots (Marmota flaviventris), a social rodent with skewed male reproductive success and female reproductive suppression. Specifically, we examined body condition as a predictor and the probability of breeding, number of offspring, and dispersal as outcomes of dominance. Additionally, we looked at variation in dominance between males and females and adults and yearlings, because marmots can engage in distinct interactions depending on the type of individuals involved. We found that marmots in better body condition have higher dominance rank than those in poorer condition. In addition, adults are dominant over yearlings. Within yearlings, dominance does not influence dispersal, but those in better body condition are less likely to disperse. Within all adults, individuals in better condition produce more offspring per year. Within adult males, more dominant males have greater reproductive success. Despite previous evidence of reproductive suppression in females, we found no effects of dominance rank on female reproductive success in the current study. The function of dominance in female marmots remains enigmatic.     

Is There Adaptive Value to Reproductive Termination in Japanese Macaques? A Test of Maternal Investment Hypotheses

Evolutionary biologists often argue that menopause evolved in the human female as the result of selection for a postreproductive phase of life, during which increased maternal investment in existing progeny could lead to enhanced survivorship of descendents. Adaptive theories relating menopause to enhanced maternal investment are known as the mother (first-generation) and grandmother (second-generation-offspring) hypotheses. Although menopause—universal midlife termination of reproduction—has not been documented in primates other than humans, some researchers have argued that postreproductive alloprimates also have a positive impact on the survivorship of first and second generation progeny. We tested the maternal investment hypotheses in Japanese macaques by comparing the survivorship of offspring, final infants, and great-offspring of females that terminated reproduction before death with females that continued to reproduce until death. SURVIVAL analyses revealed no significant difference in the survivorship of descendents of postreproductive and reproductive females, though final infants of postreproductive females were 13% more likely to survive than final infants of females that reproduced until death were. We also explored possible differences between these two groups of females, other than survivorship of progeny. We found no difference in dominance rank, matrilineal affiliation, body weight, infant sex ratio, age at first birth, fecundity rate or lifetime reproductive success. However, postreproductive females are significantly longer-lived than reproductive females and as a result experienced more years of reproduction and produced more infants in total. Apart from final infants, offspring survival is marginally lower in postreproductive females. Since offspring survival is not significantly enhanced in postreproductive females, the greater number of infants produced did not translate into greater lifetime reproductive success. Our findings fail to support the maternal investment hypotheses and instead suggest that reproductive termination in this population of Japanese macaques is most closely associated with enhanced longevity and its repercussions.     

Reproductive success in relation to dominance rank in the absence of prime-age males in Barbary macaques

In some primate species dominance rank of males is correlated with reproductive success, whereas in other species this relationship is inconsistent. Barbary macaques (Macaca sylvanus) live in a promiscuous mating system in which males are ranked in a dominance hierarchy that influences their access to females. High-ranking males usually monopolize fertile females during their estrous period and show increased mating activities. Subadult males generally rank below adult males. For Barbary macaque females in the Gibraltar colony, there was no correlation between dominance status and reproductive success. Paternity data for 31 offspring collected over four consecutive breeding seasons were used to test whether male social rank was associated with reproductive success and whether reproductive success was mainly confined to a small number of males. Genetic variation was assessed using 14 microsatellite markers for a dataset of 127 individuals sampled in all five social groups of the Gibraltar colony. Paternity analysis was conducted for offspring in one social group only, where all in-group males were sampled. Eighty-three percent of the offspring could be assigned to an in-group candidate father; none of the extra-group males appeared to have sired an infant. Male dominance rank was not found to contribute to the observed variation in male reproductive output. Fifty-nine percent of the offspring was sired by two low-ranking males, whereas the two top-ranking males sired one-fifth. A highly significant correlation was found for male age and dominance rank. Reproductive success of subadult males might be explained by the gap in the age distribution of male group members. These missing prime males are usually regarded as serious competitors for older males. Subadult males may have gained easier access to females in their absence. In addition, the presence of inbreeding avoidance mechanisms, which might also have overpowered possible rank effects, cannot be excluded.     

Correlates of Male Consortship Rate in Free-Ranging Rhesus Macaques (Macaca mulatta)

Male–male competition for access to receptive females can take the form of nonrecurring fights and/or a sustained contest over mating opportunities. Male physical condition has been linked to dominance rank and reproductive success in species characterized by intrasexual fights for dominance and access to females. In group-living species characterized by endurance rivalry, however, factors contributing to male reproductive success are less well understood. In such species, particularly seasonally breeding ones with low sexual dimorphism and seniority-based rank, age and social factors other than rank may prove important. In the absence of genetic data, male mate guarding or consortship can serve as an indicator of male reproductive success. To evaluate the contribution of age and intragroup sociality to male consortship rate, I collected behavioral data during one nonmating and one mating season in two social groups of free-ranging rhesus macaques that experience no predation or food scarcity. Higher-ranking males, younger males, or males that exhibited lower rate of intrasexual aggression had higher consortship rates. Male–female dyads that groomed outside consortship did not form consortships as often as dyads that did not engage in nonconsort grooming. This is the first study to identify the significance of male–male aggression and male–female affiliation to male consortship rate in a species characterized by endurance rivalry, high male rank stability, and strong female mate choice. Social behaviors and male age may be particularly important in determining male reproductive success in populations experiencing high food availability and a lack of predation, which are typical of an increasing number of vertebrates in areas densely populated by humans.     

Demography and reproductive parameters of a free-ranging group of Japanese macaques (Macaca fuscata) at Katsuyama

Demographic and reproductive data were analyzed for a period of 28 years in the females of a free-ranging group of Japanese macaques at Katsuyama, Okayama Prefecture, Japan. The overall mean, age-specific fecundity rates were 5.43% for 4-year-olds and 41.86% for 5-year-olds, increasing to a peak of 66.67% for 13-year-olds. Fecundity remained relatively high (52.31–54.24%) in 16–19-year-olds, but decreased sharply (45.45–17.86%) in 20–23-year-olds, and became very low in 24–26-year-olds. Females aged 27 years or more did not produce infants. The average age at first birth was 5.41 years. Births peaked in mid-May. The timing of the first births each year remained essentially unchanged during the study period, whereas the timing of the median and last births shifted towards the later part of the season. The mean interbirth interval for all females was 1.56 years. The value was 1.54 years for multiparous females and 1.29 years for females following infant loss. These intervals were significantly shorter than those for primiparous females, and females with surviving infants. The overall mean infant mortality within the first year of life was 10.2%. The value was 8.6% for 10–14-year-olds, and 7.5% for 15–19-year-olds. The timing of birth differed among the four female matrilineal dominance rank-classes. The female fecundity rates increased as a function of matrilineal dominance rank. It is suggested that all demographic and reproductive data should be analyzed in detail with respect to the group's history.     

Predictors of reproductive success in female white-faced capuchins (Cebus capucinus)

Early investigations into variable reproductive success in nonhuman primates tended to focus on the benefits conferred by high dominance rank. However, the effect of high rank on individual reproductive success has been found to vary both intra- and interspecifically, requiring researchers to expand their investigations to include additional factors. Here we examine the age and rank of the mother, sex of the infant, group size, number of close kin, replacement of group males, and resource availability as possible predictors of female reproductive success in white-faced capuchins (Cebus capucinus) in the Santa Rosa sector of the Area de Conservación Guanacaste, Costa Rica. We examine the length of interbirth intervals (IBI) and infant survivorship as measures of individual reproductive success for the 31 adult females that resided in our three study groups between 1986 and 2007. The greatest predictor of IBI length was whether or not the first infant in the interval survived (number of matrilineal kin and resource availability were also significant predictors); while infant survivorship was most significantly predicted by the occurrence of a turnover in group males in the year following the birth of an infant (infant sex was also a significant factor). Based on these findings, we conclude that male and female reproductive strategies are at odds in this species, with male strategies strongly influencing female reproductive success.     

Male rank,reproductive behavior,and reproductive success in free-ranging rhesus macaques

Paternity assessment through DNA fingerprinting by synthetic oligonucleotide probes was applied to one birth cohort in a social group of free-ranging rhesus macaques (Macaca mulatta) on Cayo Santiago. The 11 group males and 9 males from other groups were observed mating with the females. Paternity was determined for 11 of the 15 infants. Male dominance rank was not associated with reproductive success. High-ranking resident males (N=5) sired 27% of the infants born during a one-year study. Four of the 11 infants of known paternity were sired by males of other social groups. The four infants of unknown paternity were sired either by males not observed mating with the females or the low-ranking male who was not fingerprinted. Male dominance rank was not associated with reproductive activity during conception cycles. These results suggest that the effect of rank on male reproductive success is not a predictable correlation, but a conditional probability.     

Dominance and its Behavioral Measures in a Captive Group of Bonobos (Pan paniscus)

We investigated the existence of a social dominance hierarchy in the captive group of six adult bonobos at the Planckendael Zoo. We quantified the pattern of dyadic exchange of a number of behaviors to examine to what extent each behavior fits a linear rank order model. Following de Waal (1989), we distinguish three types of dominance: agonistic dominance, competitive ability and formal dominance. Fleeing upon aggression is a good measure of agonistic dominance. The agonistic dominance hierarchy in the study group shows significant and strong linearity. The rank order was: 1. female (22 yr), 2. female (15 yr)., 3. male (23 yr.), 4. female (15 yr.), 5. male (9 yr.), 6. male (10 yr.). As in the wild, the females occupy high ranks. There is prominent but nonexclusive female agonistic dominance. Teeth-baring does not fulfil the criteria of a formal submission signal. Peering is a request for tolerance of proximity. Since its direction within dyads is consistent with that of fleeing interactions, it is a useful additional measure to determine agonistic ranks in bonobos. In competitive situations, the females acquire more food than other group members do. The rank obtained from access to food resources differs from the agonistic rank due to female intrasexual social tolerance, expressed in food sharing. We typify the dominance styles in the group as female intrasexual tolerance and male challenging of rank differences. The agonistic rank order correlates significantly with age and has a strong predictive value for other social behaviors.     

Dominance and reproductive rates in captive female olive baboons, Papio anubis

The reproductive cycles of 23 captive olive baboons were studied over two successive parturitions. Interbirth intervals of 450 days were reduced by 60% in comparison to wild baboons, and consisted of 145 days of postpartum amenorrhea, 3.5 cycles, and a gestation of 185 days. Dominance rank was found to be one significant factor affecting female fertility. Low-ranking females had longer total intervals between successive births and, in particular, they experienced a longer delay to conception once they had resumed sexual cycles. Mothers of infants who were heavy for age resumed cycling more quickly and had fewer cycles before a subsequent conception. Mothers best able to sustain rapid early infant growth were those of high dominance rank and of high body mass; these females had more rapid reproductive rates. As female energy intake was unrelated to dominance, we suggest that social stresses are important suppressors of the hormonal and lactational competence of subordinate females.     

Fitness-related benefits of dominance in primates

Dominance hierarchies are thought to provide various fitness-related benefits to dominant individuals (e.g., preferential access to food or mating partners). Remarkably, however, different studies on this topic have produced contradictory results, with some showing strong positive association between rank and fitness (i.e., dominants gain benefits over subordinates), others weak associations, and some others even revealing negative associations. Here, we investigate dominance-related benefits across primate species while controlling for phylogenetic effects. We extracted data from 94 published studies, representing 25 primate species (2 lemur species, 4 New World monkeys, 16 Old World monkeys, and 3 apes), to assess how dominance affects life-history and behavior. We used standard and phylogenetic meta-analyses to analyze the benefits of dominance in primates. Dominant females had higher infant survival to first year, although we found no significant effect of dominance on female feeding success. Results for female fecundity differed between the two meta-analytical approaches, with no effect of dominance on female fecundity after controlling for phylogeny. Dominant males had a higher fecundity and mating success than subordinate males. Finally, the benefits of dominance for female fecundity were stronger in species with a longer lifespan. Our study supports the view that dominance hierarchies are a key aspect of primate societies as they indeed provide a number of fitness-related benefits to individuals.     

Dominance index: A simple measure of relative dominance status in primates

A simple measure of relative dominance status (cardinal rank) is described which we have termed the dominance index. Like more familiar techniques for assessing rank order, it is based on the direction of aggressive and submissive behaviors between all possible paired combinations of animals in a social group. Using data from five groups of female rhesus monkeys, it reliably produced the same ordinal ranks as fight interaction matrices. There was also good agreement with the cardinal ranks produced by two additional measures of dominance and with those produced by observer ratings. The dominance index can be calculated when fights have not actually occurred and is largely independent of the frequency of agonistic interactions. It has, therefore, wide application and can estimate dominance during brief sampling periods (one hour) and also in stable groups when agonistic interactions are low. Its application is described in experiments in which the male in a group of females was changed and the hormonal status of the females was altered. Estrogen increased female dominance status relative to other females.     

The role of female dominance hierarchies in the mating behaviour of mosquitofish

While studies of sexual selection focus primarily on female choice and male-male competition, males should also exert mate choice in order to maximize their reproductive success. We examined male mate choice in mosquitofish, Gambusia holbrooki, with respect to female size and female dominance. We found that the number of mating attempts made by a male was predicted by the dominance rank of females in a group, with dominant females attracting more mating attempts than subordinates. The number of mating attempts made by males was independent of the female size. The observed bias in the number of mating attempts towards dominant females may be driven either by straightforward male mate choice, since dominance and female fecundity are often closely related, or via the dominant females mediating male mating behaviour by restricting their access to subordinate females.     

Age,reproductive value,and dominance-related behaviour in vervet monkey females: cross-generational influences on social relationships and reproduction

Behavioural interactions among captive socially-living adult female vervet monkeys were analysed to determine whether (1) the motivation to maintain and increase, dominance rank declines with age as individual reproductive value declines, or (2) the cross-generational inheritance of dominance rank causes adult females to continue to involve themselves in dominance-related behaviours for the benefit of their daughters, even though their own reproductive value, is declining. The results indicate that adult females do not reduce their involvement in aggressive and dominance-related behaviours as they age. There was strong support for the hypothesis that older adult females maintain affiliative and supportive ties with their adult daughters, and that the daughters benefit from this relationship both in terms of social relationships and reproductive success. Young adult females without mothers were the recipients of more aggression, were less likely to be defeded when they were attacked and were less likely to challenge the rank of others compared to females with mothers living in the group. Young adult females with mothers produced significantly more surviving offspring and had a lower level of infant mortality than comparable adult females without mothers.