Sex Ratio, Conflict Dynamics, and Wounding in Rhesus Macaques (Macaca mulatta)

Abstract

Rhesus macaques, like many other primates, live in stable, multi-male multi-female groups in which adult females typically outnumber adult males. The number of males in multi-male/multi-female groups is most commonly discussed in terms of mate competition, where the sex ratio is a function of an adult male's ability to monopolize a group of females. However, the relationship between sex ratio and group stability is unclear because the presence of many males may either reduce stability by increasing mate competition or may improve stability if adult males are key conflict managers. We investigated the relationship between sex ratio, male intervention behavior, and trauma in seven groups of captive rhesus macaques. Our results show that high-ranking adult males intervene twice as frequently as adult females (P<0.0001) and are about twice as successful (P<0.0001). Furthermore, the type of adult males present in the group affects the relationship between intervention behavior and rate of traumas: males must be unrelated to the highest-ranking matrilines. Groups with a lower ratio of females per male unrelated to the alpha and beta matrilines had better intervention success (P<0.04) and fewer traumas requiring veterinary care (P=0.003). We conclude that conflict management behavior by adult males, particularly those unrelated to the highest-ranking matrilines, is the mechanism by which sex ratio influences the frequency of traumas, and thus group stability. We therefore suggest that monitoring and managing the sex ratio of captive primate groups is one of many measurements to predict group stability.

Fig. 1. Intervention frequency by sex

The expected frequency of interventions per individual (N = 323) for male and female interveners, calculated from model coefficients from the multi-level Poisson model of intervention frequency. Intervention frequency was measured as the total count of interventions per individual over the study period.

Fig. 2. Intervention success by sex and rank

The expected intervention success per individual (N = 323) for male and female interveners across all ranks, calculated from model coefficients from the negative binomial regression model of intervention success. Intervention success was calculated as the number of successful interventions multiplied by the proportion of interventions that were successful, which accounts for both rate and extent of success.

Fig. 3. Intervention success for adult males based upon relatedness to alpha and beta matrilines

The expected intervention success per adult male (N = 88) across all ranks for males unrelated to the alpha and beta matrilines and those related to them, calculated from model coefficients from the Poisson model of male intervention success. Intervention success was calculated as the number of successful interventions multiplied by the proportion of interventions that were successful, which accounts for both rate and extent of success.

Fig. 4. Annual trauma rate by sex ratio

The annual rate of traumas per group is plotted by group sex ratio for 13 captive rhesus groups at the CNPRC across two years (N = 26 group years). Sex ratio was calculated as the number of adult females per adult male, and annual trauma rate was calculated as the total frequency of injuries per group per year (for both 2008 and 2009) divided by the mean group size for that year. The least-squares regression line is included.

Fig. 5. Sex ratio in wild versus captive rhesus groups

The ratio of adult females per adult males for 13 captive rhesus groups at the CNPRC (N = 26 group years) and 16 wild rhesus groups (Makwana, 1978; Sahi and Sharma, 2004; Southwick et al., 1965; Southwick et al., 1961; Teas et al., 1980). Boxes represent the inter-quartile range, and the black bar is the median sex ratio. The upper (and lower) whiskers are drawn to the largest (or smallest) data point not lying above the 75^th percentile (or below the 25^th percentile) + 1.5*IQR.