The effects of dominance rank and group size on female lifetime reproductive success in wild long-tailed macaques, Macaca fascicularis

Demographic changes were recorded throughout a 12-year period for three soc ial groups of Macaca fascicularis in a natural population at Ketambe (Sumat ra, Indonesia). We examined the prediction that females' lifetime reproduct ive success depended on dominance rank and group size. Average birth rate w as 0.53 (184 infants born during 349 female years). For mature females (age d 8-20 yr) birth rate reflected physical condition, being higher in years w ith high food availability and lower in the year following the production o f a surviving infant. High-ranking females were significantly more likely t han low-ranking ones to give birth again when they did have a surviving off spring born the year before (0.50 vs 0.26), especially in years with relati vely low food availability (0.37 vs 0.10). Controlled comparisons of groups at different sizes indicate a decline in birth rate with group size only o nce a group has exceeded a certain size. The dominance effect on birth rate tended to be strongest in large groups. Survival of infants was rank-dependent, but the survival of juveniles was n ot. There was a trend for offspring survival to be lower in large groups th an in mid-sized or small groups. However, rank and group size interacted, i n that rank effects on offspring survival were strongest in large groups. H igh-ranking females were less likely to die themselves during their top-rep roductive years, and thus on average had longer reproductive careers. We estimated female lifetime reproductive success based on calculated age-s pecific birth rates and survival rates. The effects of rank and group size (contest and scramble) on birth rate, offspring survival, age of first repr oduction for daughters, and length of reproductive career, while not each c onsistently statistically significant, added up to substantial effects on e stimated lifetime reproductive success. The group size effects explain why large groups tend to split permanently. Since females are philopatric in this species, and daughters achieve domina nce rank positions similar to their mother, a close correlation is suggeste d between the lifetime reproductive success of mothers and daughters. For s ons, too, maternal dominance affected their reproductive success: high-born males were more likely to become top-dominant (in another group). These da ta support the idea that natural selection has favored the evolution of a n epotistic rank system in this species, even if the annual benefits of domin ance are small.