California State University, Long Beach, United States
Abstract: Social organization is hypothesized as a major driver of density-dependent feedback in social mammals. Social mammals live in groups composed of members of different sex and age. Yet, we lack knowledge about the presence and role of differential density feedbacks at their multiple organizational levels. This is important because wild populations are currently threatened by significant habitat loss and climate change, which leads to changes in their social structure, social group density, and their total population density. Here, we quantified and contrasted the relationship between population and social group abundances on the reproduction and survival of a social primate by using 45 years of demographic data from Cayo Santiago rhesus macaques. We used generalized linear mixed effects models to test whether sex- and age-specific annual abundances at two organizational levels (group and population) were associated to the annual probability of survival for all individuals (n = 8,867) and the annual probability of reproduction for females (n=1,813). Annual population size ranged from 285 to 1490 monkeys, while the annual number of social groups ranged from 4 to 14 with sizes ranging from 15 to 402 monkeys.
Total adult female abundance and social group male abundance were significantly associated to the annual survival of males and females. However, these abundances had contrasting effects supporting both negative density-dependent and Allee effects. The probability of survival was nonlinearly associated to the number of adult females in the population showing maximum survival at intermediate densities (𝛽 = 0.004; SE = 0.002; 𝛽2 = -0.00001; SE = 0.000004). Conversely, the probability of survival increased as the total number of male members in a social group increased (𝛽 = 0.003; SE = 0.0005). Total female abundance and social group adult male abundance were significantly associated to the annual probability of reproduction. The probability of reproduction was lowest at intermediate female densities (𝛽 = -0.149; SE = 0.046; 𝛽2 = 0.071; SE = 0.027). In contrast, the probability of reproduction peaked at intermediate adult male densities within a social group (𝛽 = 0.132; SE = 0.029; 𝛽2 = -0.057; SE = 0.021). Our analysis demonstrates the presence of differential density feedback across organizational levels in a social mammal. Understanding more about such differential density effects, how they are maintained and balanced out, is key to advancing our current knowledge on the density-dependent population dynamics of social mammals.
