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

Hormonal responses to mating competition in male Tonkean macaques

Glucocorticoid and androgen hormones play a prominent role in male reproductive effort. Their production usually increases in non-human primates during mating competition, which may include rivalry for access to receptive females, struggles for high dominance rank, or social pressure on low-ranking individuals. It is generally assumed that glucocorticoids and androgens are associated with mating challenges rather than dominance status, but the involvement of multiple factors makes it difficult to disentangle the two. In this regard, Tonkean macaques provide a suitable model because they are characterized by relaxed dominance and year-round breeding, meaning that there is typically no more than one receptive female in a group, and thus first-ranking males can easily monopolize her. We studied two captive groups of Tonkean macaques over an 80-month period, recording the reproductive status of females, collecting urine from males and sampling behaviors in both sexes. Male urinary hormone concentrations could be affected by increased competition caused by the mating period, the number of males and the degree of female attractiveness. The highest increases in androgens were recorded in males performing female mate-guarding. Despite the importance of dominance status in determining which males can mate, we found no significant effect of male rank on glucocorticoids and only a marginal effect on androgens during mate-guarding. Both types of hormones were more directly involved in the mating effort of males than in their dominance status. Our results show that their function can be understood in light of the particular competitive needs generated by the species-specific social system.

Female behavioral strategies during consortship in Tibetan macaques (Macaca thibetana)

Consortship has been defined as a temporary association between an adult male and an estrous/receptive female. It has been considered as male mating strategies to improve male mating success and potential reproductive success. However, the female roles have been more or less neglected, and thus, less is known about female behavioral strategies during the consortship periods. In this study, during the two consecutive mating seasons, we collected behavioral data of free-ranging Tibetan macaques (Macaca thibetana) habituated in Mt. Huangshan, China, to investigate female behaviors when she was consorted by an adult male. The results showed that (a) females were more likely to approach and exhibit sexual solicitation to their consorting males during the consorted period, and females also exhibited less approach to their nonconsorting males; (b) females exhibited strong responses (either departed distantly or formed affiliative relationships with their consorting male partner) when their consorting males mated with rival females or showed sexual motivation toward rival females; (c) female preferences were positively correlated to the duration of consortships and the frequencies of ejaculation copulations, independent of the social ranks of their consorting male partners. Our results suggested that female strategies played much more important roles in forming and maintaining consortship than previously assumed. It provides new insight into understanding female adaptive strategies to male strategies by forming consortships in multimale-multifemale primate species when males could not identify female's fertile phase accurately.

A comparison of dominance rank metrics reveals multiple competitive landscapes in an animal society

Across group-living animals, linear dominance hierarchies lead to disparities in access to resources, health outcomes and reproductive performance. Studies of how dominance rank predicts these traits typically employ one of several dominance rank metrics without examining the assumptions each metric makes about its underlying competitive processes. Here, we compare the ability of two dominance rank metrics—simple ordinal rank and proportional or ‘standardized’ rank—to predict 20 traits in a wild baboon population in Amboseli, Kenya. We propose that simple ordinal rank best predicts traits when competition is density-dependent, whereas proportional rank best predicts traits when competition is density-independent. We found that for 75% of traits (15/20), one rank metric performed better than the other. Strikingly, all male traits were best predicted by simple ordinal rank, whereas female traits were evenly split between proportional and simple ordinal rank. Hence, male and female traits are shaped by different competitive processes: males are largely driven by density-dependent resource access (e.g. access to oestrous females), whereas females are shaped by both density-independent (e.g. distributed food resources) and density-dependent resource access. This method of comparing how different rank metrics predict traits can be used to distinguish between different competitive processes operating in animal societies.

Breaking the succession rule: the costs and benefits of an alpha-status take-over by an immigrant rhesus macaque on Cayo Santiago

Explaining intraspecific variation in reproductive tactics hinges on measuring associated costs and benefits. Yet, this is difficult if alternative (purportedly less optimal) tactics remain unobserved. We describe a rare alpha-position take-over by an immigrant male rhesus macaque in a population where males typically gain rank via succession. Unusually, male aggressiveness after the take-over correlated with rank and mating success. The new alpha achieved the highest mating and reproductive success. Nevertheless, he sired only 4 infants due to high extra-group paternity (59.3%). The costs of his immigration tactic were high: after the mating season ended, unable to deter coalitionary attacks by resident males, he was overthrown. The following year he had the highest relative annual weight loss and levels of immune activation among males in the group. Succession-based rank-acquisition in large, provisioned groups of macaques thus appears to be actively maintained by resident males, who impose high costs on challengers.