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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.
The data come from our main study troop (VT; Nmean = 39; range: 31–50) at the De Hoop Nature Reserve in the Western Cape Province of South Africa (21) and were collected over an 11-year period (March 1997 through February 2008). An additional datum on one alpha male’s reproductive success comes from a second, more recently habituated, study troop (BT; N ≈ 50). As part of a daily troop census, we recorded the reproductive condition of females and the identity, if any, of their consort partners. We used the cessation of monthly sexual swelling as evidence of conception, and identified as the father the male who had been the consort partner over the 7-day period (D-7 to D-1) preceding the deflation of the sexual skin (D-0) on the female’s last reproductive cycle. Given that mating behavior predicts paternity even in those baboon taxa where consort turnover is high (39, 40), the 10-day duration of ultimate consortships in conceptive cycles (19, 20), together with the absence of consort disruption and surreptitious copulation, lend added confidence to this behavioral determination of paternity. Males were assigned rank on the basis of ad libitum observations of aggression and spatial displacement. Data on male activity budgets come from both troops, sampled over an 18-month period (March 2002 through October 2004). Data were collected using 20-min continuous focal sampling (41) that yielded 1,692 focal samples (564 h). We used these to calculate the amount of time(s) a male allocated to each of the three key activities (resting, moving, and foraging). These data, expressed as proportions of the total budget, were analyzed using general linear mixed models (GLMM), assuming binomial errors. Statistical analyses were conducted using either R statistical software (42) or JMP 7.1 (43). In addition to our empirical data, we use a model that describes the conflict between female primates and alpha males over the extent of polyandrous mating (44). The basic assumptions of this are as follows. The primate group is multimale, containing a dominant male, subordinate males, and only one female in estrous at a time. We further assume that infanticide by males is an adaptive strategy so that protection from possible sires against infanticide is crucial for infant survival. For simplicity, the effect of all subordinate males has been combined into a single strongest subordinate male. We also assume that there is one outsider male interested in taking over the group, who is just as strong as the strongest subordinate male in the group. Pradhan and van Schaik (44) proposed that female fitness can be maximized by maximizing infant survival to weaning, whereas male fitness is maximized only when the male’s paternity probability (q) × infant survival is maximized. Based on this idea, and using the parameters described below, we can write equations for impact on female and male fitness as: where g(q) = impact on infant survival as a function of male paternity probability (q); l = impact on infant survival due to maternal efforts; a = relative strength of the alpha male; c/a = probability of takeover in which c is a constant; ξ = probability of takeover from inside; and d = probability that the defeated alpha male is available to defend infants. Note that Eq. 1 differs slightly from the master Eq. 2 in Pradhan and van Schaik (44). This takes account of the fact that the probability with which the alpha male is available to protect unweaned infants, generally constant and high, is unusually low for chacma baboons, and so needs to be factored into the model. We have also assumed that the probability of takeover and death, and the percentage of takeovers from the outside, are both external parameters (i.e., not under the control of the alpha male) or are at values that are optimal for him for other reasons.
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