Effects of natal male alliances on aggression and power dynamics in rhesus macaques

Abstract

In the wild, male rhesus macaques disperse at sexual maturity. In captivity, however, males cannot disperse from their natal groups. Thus, the presence of natal males in captive rhesus social groups is unnatural and has the potential to negatively influence group dynamics and stability. A primary difference between natal males and non-natal (immigrant) males is that natal males have the opportunity to form long-term alliances with their maternal kin as well as nonkin. We investigated the factors associated with natal males' kin alliances and the impact of these alliances on measures of natal male behavior, group dynamics, and group stability. We found that natal males more frequently formed alliances with maternal kin when they were from high-ranking matrilines, had more siblings, and were younger. More frequent kin alliances were associated with more frequent use of intense aggression, higher individual rank, and higher degree of integration within the male displacement network. Thus, it seems that natal males use their alliances to be more active and influential in the social group, which may affect group stability. It appears that juvenile natal males from high-ranking matrilines, in particular, have the largest impact on group stability. Younger natal males from high-ranking matrilines formed alliances with kin more frequently and used intense aggression more frequently than older or lower ranking males. Furthermore, groups with a higher proportion of juvenile males from high-ranking matrilines also had higher rates of wounding. We suggest that the presence of natal males in rhesus groups may act in opposition to group stability.

Fig. 1

Mean alliances between natal males and their maternal kin plotted for (a) all males from matrilines of a given rank and (b) males having different frequencies of alliances with nonkin. The highest-ranking matriline is set to rank = 1. Vertical bars represent standard errors.

Fig. 2

The predicted frequency of kin alliances per week, calculated from the best fit model, are plotted against natal males’ matriline ranks for males whose mothers were present in the group and males whose mothers were absent from the group. In plot (a), male age is held constant at 3 years old and in plot (b) 7 years old.

Fig. 3

Mean male rank category, where high-rank = 1, mid-rank = 2 and low-rank =3, is plotted for: (a) all males from matrilines of a given rank and (b) males having different frequencies of kin alliances. The highest-ranking matriline is set to rank = 1. Vertical bars represent standard errors.

Fig. 4

Mean male rank category, where high-rank = 1, mid-rank = 2 and low-rank =3, is plotted for all males from a given matriline rank for three age categories: males aged 2–3 years, 4–5 years, and 6–9 years.

Fig. 5

The predicted Bonacich power in the male displacement network, calculated from the best fit model, is plotted against (a) males’ kin alliances for four different age categories (males aged 4, 6, 8, or 10 years) and (b) males’ nonkin alliances for each male rank category.

Fig. 6

Male displacement network plots for six of the seven study groups. Group 16 was not included because only five months of data were collected on this group, compared to eight months of observation for the other six groups.

Fig. 7

The effect of males’ kin alliances on (a) mean fights started per week and (b) mean frequency of intense aggression used per week. Vertical bars represent standard errors.

Fig. 8

The predicted frequency of intense aggression initiated by males per week, calculated from the best fit model, is plotted against (a) males’ frequency of kin alliances for four categories of nonkin alliances and (b) males’ age for each male rank category.