Variability and stability in the rank relations of nonhuman primate females: Analysis by computer simulation.

This paper summarizes results of simulation analyses aimed at elucidating the way in which two important dimensions of female rank orders-nepotism and age-gradedness-are affected by variation in individual strategies of rank acquisition on the one hand and by variation in population-wide demographic parameters (eg, birth rate, death rate, etc) on the other. Female rank orders in simulated primate groups were characterized by substantial variability on a year-to-year basis in both nepotism and age-gradedness owing to stochastic processes alone. Although, in general, groups characterized by strongly nepotistic rank orders were not also strongly age-graded, quantitative measures of these two rank-order dimensions for individual groups showed little stability over intervals of time exceeding 3 years. Surprisingly, changes in birth and death rates did not strongly affect either nepotism or age-gradedness so long as average group size remained constant. However, in populations characterized by expanding or declining group size, variation in demographic parameters did indeed exert a powerful influence on the structure of female rank orders. Both nepotism and age-gradedness were also strongly affected, and in quite different ways, by changes in the rules of rank acquisition by females. In sum, the present analyses suggest that differences between species in the degree of nepotism and age-gradedness characteristic of their female rank orders does not in and of itself constitute evidence that the females of those species use markedly different strategies of rank acquisition.