Infanticide and reproductive restraint in a polygynous social mammal

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.

Fig. 1.

Access to conceptive cycles by subordinate males (n = 23) in relation to the length of alpha male tenure. The solid line indicates the expected distribution (dashed lines: 95% CI) and the circles indicate the observed pattern of subordinate male conceptions. Large circles indicate conceptions by primiparous females (n = 6).

Fig. 2.

Survival curves for the three classes of male: nonfathers (−), subordinate fathers (^…), and alpha fathers (^___). The curves indicate the likelihood of male presence as a function of the time that has passed since their arrival. Circles indicate the month at which each immigrant, subordinate father first achieved a conceptive mating (median: dashed vertical line).

Fig. 3.

The impact on fitness for both the male and the female as a function of the dominant male’s paternity probability. The parameter values for chacma baboons are as follows: l = maternal impact on infant survival = 0.25; c = 0.924 and a = 1.1, giving c/a = takeover probability = 0.84; d = 0.5; 1 − ε, i.e., probability of takeover from outside is set at 1.0 because the takeovers are always from outside; k = 0.75, A = 1.0 (45).

Fig. 4.

This photograph, taken during a fight between an immigrant and the alpha male, show six mothers with vulnerable infants clustered around a subordinate male, who is the father of only two of them.