[Show abstract][Hide abstract] ABSTRACT: Alpha male chacma baboons experience uncontested access to individual estrus females. Consequently, alpha male paternity certainty is high and underpins significant levels of infanticide by immigrant males that, in turn, has selected for male defense of infants. There is also, however, a high probability that alpha males will be absent during the period when their own offspring are vulnerable, suggesting selection for additional countermeasures. We use data from a long-term study to test the prediction that alpha male chacma baboons cede reproductive opportunities to subordinate males and that this leads to the presence of other fathers that can serve as a buffer against infanticidal attack. We found that subordinate males obtained significantly more conceptive opportunities than predicted by priority of access alone, and that this occurred because alpha males did not consort all receptive periods. There was no evidence that this was due to energetic constraint, large male cohorts, alpha male inexperience, or the competitive strength of queuing subordinates. The number of males who benefited from concession and the length of time that they were resident relative to those who did not benefit in this way greatly reduced the probability that infants of alpha males would face immigrant males without a surrogate father whose own offspring were vulnerable. The absence of such males was associated with observed infanticide as well as, unexpectedly, an increased likelihood of takeover when alpha males with vulnerable infants were present.
Proceedings of the National Academy of Sciences 02/2010; 107(5):2130-5. · 9.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Contrary to the expectations of biological market theory, in species where sexual coercion is effective males often exchange resources or services with females for the opportunity to mate. This suggests that an ability to control mating partners does not preclude the need for their cooperation. We argue that this is because, in many systems, female resistance to coercion can precipitate intermale competition and, as such, a male's mating strategy may often be better served by securing female compliance through affiliative rather than agonistic means. Based on this reasoning, we predicted that the need for males to exchange resources/services for mating access with females will be absent only when intermale power differentials are such that dominant individuals can secure uncontested access to receptive females. Accordingly, data from our long-term study site revealed no support for a biological markets model of intersexual mating exchange in chacma baboons, where the mating monopolies of alpha males are near absolute. Specifically, we found that males groomed females substantially less than they were groomed by them, their propensity to groom was poorly described by measures of female fertility, male rank, and the operational sex ratio, and their mating success was not associated with their grooming effort. We further predicted that an additional consequence of the degree of competitor suppression seen in chacma would be a reversal in the direction of intersexual trade of services for mating. We found that female grooming was positively associated with the probability that they would successfully initiate a copulation. Our study strongly suggests that it is variance in competitor suppression, not partner control, that mediates the potential for and direction of intersexual cooperative exchange. Copyright 2010, Oxford University Press.
[Show abstract][Hide abstract] ABSTRACT: The probability of ovulation in a number of primate species is associated with both visual and auditory cues. We use 18-month behavioral data from two chacma baboon troops to provide the first systematic assessment of the possibility that olfactory cues are also involved. Using variance in the rate of olfactory inspection by males as a proxy for changes in the intensity of female vaginal odor, we found that rates of inspection were broadly correlated with changes in female fertility. Males inspected cycling females significantly more than anovulatory, noncycling females and swelling females significantly more than nonswollen cycling females. Rates of inspection peaked around the time at which males first started guarding females and were sustained until the detumescence of the female's sexual skin. We conclude, therefore, that olfactory cues represent one component of a multimodal signal of ovulation in chacma baboons. The possible reasons for such a multimodal signal are discussed.
American Journal of Primatology 04/2009; 71(6):493-502. · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We argue that, in the absence of an infanticidal threat from resident males, female chacma baboons favour polyandry because it may predispose multiple males to protect their infants from infanticide by immigrant males. This assumes that the most likely sire, or principal protector, of an infant may often be absent during its period of vulnerability, thereby creating the need for additional protection. Accordingly, we found that, on average, 47% of principal protectors were absent for at least part of an infant's vulnerable period. We predicted that, to secure additional protection, females should attempt to augment the paternity estimates of as many males as possible and, therefore, should seek to influence overall patterns of mating. In line with this prediction, we found that females actively solicited copulations from all guarding males and did so irrespective of their rank or the underlying probability of ovulation. By these means, females were able to elevate significantly the mating success, and presumably, in turn, the estimated total paternity probabilities, of these males. The apparently indiscriminate nature of the female strategy highlights how, in species where female choice is limited, females may be forced to maximize the potential of all mating opportunities afforded them by intermale competition.
[Show abstract][Hide abstract] ABSTRACT: Historically, intertroop movement by males in female-philopatric species has been investigated without any consideration of the potential for variance in the competitive ability of males. This is despite the fact that in many species, particularly among the primates, males tend to move multiple times between groups and often vary substantially in their competitive ability. We investigated this issue using 7 years' data from a long-term study of chacma baboons, Papio hamadryas ursinus. We predicted that variance in competitive ability would promote differentiation in dispersal strategies, with competitively effective males favouring long-term measures of a troop's potential reproductive output and low-quality males favouring short-term measures. Using time series analysis, we found that overall patterns of movement were significantly associated with two independent measures of a troop's demography: absolute female number and excess male number. Furthermore, when the analysis was broken down by competitive ability, we found that only high-quality males responded to the absolute number of females and only low-quality males were sensitive to excess male number. Therefore, we conclude that variance in competitive ability has promoted the evolution of alternative dispersal strategies in male chacma baboons.