Birth rate and survival in relation to group size in the lion-tailed macaque,Macaca silenus

Abstract

Birth rate and immature survival rate in group living primates have been predicted to vary as a function of group size. These predictions were tested with data from a wild population of lion-tailed macaques in the Anamalai Wildlife Sanctuary, Tamil Nadu (India). Group size and composition, births, and mortality were monitored from eight groups for one to six years. The rate of growth of groups, birth rate, and survival rate were examined in relation to group size. The rate of growth of a group was a decreasing function of group size. Birth rate was also a decreasing function of group size and the number of adult females in the group. The sample size for survival rate of adults was too small to test for their dependence on group size. Immature survival was independent of group size, but the analysis was based on a very small sample. It is possible that the nature of dependence of net growth rate, birth rate, and survival rate on group size could be different outside the range of group sizes on which the analysis was based. The stronger relationship between birth rate and group size in the lion-tailed macaque, in relation to other primates, might be the result of its low birth rate as well as the relatively low temporal variation in resources in the rain forest. Limited data available from other sites indicate that the observed relationship between birth rate and group size might be spatially stable.