Crast, J., Seelig, D., Meeker, T. L. et al. 2018. Rhesus monkey (Macaca mulatta) groups with more non-natal adult males per adult female have lower rates of aggression-induced trauma and greater reproductive success. American Journal of Primatology 80(S1), 45 (40th Meeting of the American Society of Primatologists Scientific Program, Abstract #140).

Limiting contact aggression and social stress is a principal animal welfare challenge in managing large groups of captive nonhuman primates. This challenge is especially pronounced in rhesus monkeys (macaca mulatta), a species characterized by a relatively high frequency of aggression in captivity. Recent behavioral studies have found that adult non‐natal male macaques play a pro‐social role in mitigating aggression between females through impartial conflict interventions (i.e. “policing“). Relatively low adult male:female ratios (M:F) may negatively impact effective policing; however, there is limited research linking M:F to long‐term clinical and reproductive outcomes. The current retrospective study examined demographic and clinical data from four large rhesus breeding groups over an 11‐year span to examine M:F in relation to trauma rate and reproductive success. We hypothesized that groups with fewer females per male (range 1:34 to 1:8 males to females) would have lower rates of injury and increased reproductive output, perhaps due to the group stability benefits provided by adequate male policing. Controlling for group size and limiting our sample to years in which there were no matrilineal overthrows, we found a positive Pearson Product‐moment correlation between M:F and reproductive success (r=.61,p=.003) and an inverse correlation between M:F and trauma (r = −.44, p=.045). These results underscore the importance of developing strategies to manage social groups such that M:Fs are optimal for group welfare and stability.