Dominance Rank and Access To Estrous Females in Male Savanna Baboons

Abstract

Data on sexual consortships in a troop of chacma baboons are analyzed in relation to a priority-of-access model and to critical demographic and social variables. The data show a robust association between adult male rank and mating success, and a close approximation to priority-of-access type mating. Principal findings were: (1) Mating activity peaked during the four most likely days of ovulation, and was greater on conceptive than on nonconceptive cycles. (2) On each day of estrus, males of the alpha rank consorted more frequently than did males of other ranks for both classes of cycles. (3) Moreover, on each day of estrus, alpha males consorted at higher rates on conceptive cycles than they did on nonconceptive cycles. (4) Male dominance rank and consorting success were positively correlated, including a perfect rank correlation on conceptive cycles. (5) Mean consort duration was also positively correlated with male dominance rank. (6) The most striking aspect of male mating patterns was the predominance of alpha males. Male dominance rank and consort success were in qualitative but not perfect quantitative agreement with predictions of a priority-of-access model. Alpha males consorted on 86% of the days and 94% of the conceptive cycles they were expected to, and were selective of conceptive over nonconceptive cycles. (7) The size of the adult male cohort had no effect on the consort success of alpha males. (8) Alpha male consort success was reduced during intervals when two or more females were synchronously in estrus. Inconsistent results from other savanna baboon field studies have often led to the conclusion that dominance rank is not important as a determinant of male mating success. Analysis of the literature indicates that some of the reported variability among studies is explained by factors that are associated with differences in group demography. In addition, the importance of male dominance rank has often been treated superficially in discussions emphasizing alternative mating tactics, due in part to the uneven representation of published accounts emerging from individual study sites. I conclude that there is a general, underlying relationship between male dominance rank and access to mates in savanna baboons. Dominance rank is inarguably the primary determinant of mating success among high-ranking males, while the mating success of subordinate males is determined both by rank and the extent to which coalitions are expressed in the group. Rank/mating measures tend to be strongest in the chacma subspecies, among which male-male coalitions over mates are unknown. Male-male competition based in both solo and coalitional tactics appears to dominate the expression of individual partner preferences.