PS 19-188 - Social organization and density-dependent feedback in rhesus macaques.

Background/Question/Methods

Density dependence is a fundamental process influencing the dynamics of populations. In particular for social animals, competition and social organization are hypothesized as major drivers of the strength and direction of density dependent feedback on survival and reproduction. Yet, the population-level consequences of group life remain unexplored. Here, we aimed to describe the potential effects of density on social group demographic performance. We used demographic data from Cayo Santiago rhesus macaques, a socially complex primate population known to exhibit negative density dependence in mean fertility. During a relatively high population density year (n = 1,385), we tracked the fate of 682 females from seven social groups and parameterized group-specific matrix population models using developmental (infants; yearlings; juveniles) and reproductive stages (non-breeders; failed breeders; successful breeders). For each matrix model, we determined the mean population growth rate (λ, the long-term population fitness), the stable stage distribution (w), and the reproductive value vector (v) by computing the dominant eigenvalue and corresponding right and left eigenvectors, respectively. Finally, we explored associations between λ and the annual proportion of reproductive stages, and social group density using linear regression models.

Results/Conclusions

Group size ranged from 88 to 327 individuals; however, we found no evidence of density effects on social group demographic performance. The seven groups exhibited sizes of 88, 144, 146, 178, 201, 297, and 327 individuals with corresponding λs of 1.08, 1.11, 1.16, 1.03, 1.09, 1.12, and 1.13, respectively. In contrast to the annual proportion of non-breeders and successful breeders, the proportion of failed breeders – those giving birth to a nonviable infant – was slightly driven by group density showing its lowest proportion at intermediate densities (y=-0.002x+3.377e-06x²+0.238; R² = 0.67; p=0.049). Whether this indicates optimal social support to rise infants at intermediate densities remains to be explored. Disregarding group density, non-breeders were the most represented stage across all groups (range: 0.46-0.70). Future research will expand this analysis to encompass years of different population densities to ascertain if the previously reported density-dependence in mean fertility at the population-level translates to social groups.