Population trend alters the effects of maternal dominance rank on lifetime reproductive success in yellow baboons (Papio cynocephalus)

We evaluated the association between dominance rank and lifetime reproductive success of 75 free-ranging female baboons in Mikumi National Park, Tanzania. Data were evaluated over a 22-year period that included a period of troop increase (1975-1987) associated with two troop splits in 1978 and 1979, followed by a precipitous population crash (1987-1996) where the troops successively fused back together in 1989 and 1994. Lifetime reproductive success was significantly greater for high- versus low-ranking females when examined across the entire study period. High-ranking females had a longer reproductive life span (7.4 vs 3.6 years after first birth), reached menarche earlier (4.6 vs 5.2 years), lived longer (12.0 vs 8.8 years), and had more offspring of both sexes (2.25 vs 1.33 for male offspring; 3.25 vs 0.94 for female offspring), with four times the number of offspring of each sex surviving to 4 years of age compared to low ranking females. Greater offspring production was associated with shorter interbirth intervals of dominant versus subordinate females (545 vs 723 days), partly owing to lower miscarriage rates (0.05 vs 0.2) and a shorter duration of lactation (244 vs 330 days). Rank effects were then partitioned by mothers experiencing the majority of their reproductive life prior to, versus during, the population decline. The majority of rank effects on measures of lifetime reproductive success were virtually eliminated for mothers reproducing during the troop decline, indicating that the considerable impacts of social status on lifetime reproductive success can be markedly altered by intrinsically and extrinsically mediated demographic events.