Contribution of adult sex ratio to trauma and reproductive output in large breeding groups of rhesus macaques (Macaca mulatta)

Abstract

Maintaining stable breeding groups of rhesus macaques (Macaca mulatta) can be challenging due to the complex social dynamics and despotic nature of the species. Trauma from aggression is a common problem in rhesus colonies and can cause social disruption, strain veterinary and animal management resources, and potentially affect reproduction. Previous research has shown that increasing the number of non-natal adult males in a breeding group can improve group stability, reduce trauma, and increase reproduction. Here, we used mixed-effects regression models to examine the effects of sex ratio and other factors on trauma and reproduction at the Yerkes National Primate Research Center using a historical dataset made up of four large rhesus groups over an eleven-year period (2003-2013). As expected, sex ratio was a significant predictor for both trauma and reproduction. However, group age since formation was a stronger predictor of trauma frequency and the amount of space available was a slightly better predictor of reproduction than sex ratio or trauma. These results indicate that improving sex ratios can be a viable management strategy to reduce trauma and improve reproduction, particularly when it is difficult to manipulate the group compositions and/or their housing situations. Reducing trauma is a primary goal for rhesus breeding colonies, as it directly impacts the monkeys' health and psychological well-being. Such improvements are necessary for the ethical treatment and care of the animals themselves, but also to reduce financial burdens and maintain a healthy colony for research purposes.

Keywords: animal welfare; reproduction; rhesus macaque; sex ratio; social stability; trauma.

Figure 1.

Average number of adult males (5+ years) and adult females (3+ years) across the four study groups of rhesus macaques at Yerkes from 2003–2013. Error bars represent standard error of the mean. Overall, the relationship between group size and group age was r = 0.27.

Figure 2.

Yearly (a) trauma frequency and (b) trauma rate over the years since original group formation (each group is represented by a different symbol and shade). Overall, the relationship between trauma frequency and group age was r = 0.53 (p <0.001) and between trauma rate and group age r = 0.50 (p < 0.001).

Figure 2.

Yearly (a) trauma frequency and (b) trauma rate over the years since original group formation (each group is represented by a different symbol and shade). Overall, the relationship between trauma frequency and group age was r = 0.53 (p <0.001) and between trauma rate and group age r = 0.50 (p < 0.001).

Figure 3.

Yearly reproductive success over the years since original group formation (each group is represented by a different symbol and shade). Overall, the relationship between reproductive success and group age was r = −0.40 (p = 0.009).

Figure 4.

Yearly trauma frequency across sex ratios (groups represented by different symbols and shades; note that each group had 10–11 data-points across the studied time-frame). Overall, the relationship between trauma frequency and sex ratio was r = 0.38 (p = 0.013).

Figure 5.

Yearly reproductive success across sex ratios (groups represented by different symbols and shades; note that each group had 10–11 data-points across the studied time-frame). The overall relationship between reproductive success and sex ratio was r = −0.27 (p = 0.081).