The socio-ecology of sex ratio variation in primates: evolutionary deduction and empirical evidence

Abstract Fisher (1930, The genetical theory of natural selection. Clarendon Press, Oxford) has shown that evolution on the basis of individual selection must yield population sex ratios of 1.0 (50% males and 50% females), provided that the costs of producing a male or a female are equal, and that there is panmixis. Evolutionary deduction predicts that mechanisms for adaptive sex ratio manipulation could evolve when the above conditions do not hold. Empirical evidence for such adaptive manipulation exists especially for haplo-diploid insects. Here the mechanism by which the manipulation is achieved is evident: selective admission by the female of stored sperm to the maturing ova. For “our kind of animals”, the mammals, both the theoretical models and the empirical evidence are considered much more ambiguous. The primates offer an illustrative case. On the one hand there is the ‘male quality hypothesis’. It predicts that females will shift the sex ratio of their offspring depending on their social position and their condition, provided that status and condition infleunce the competitive potential of their sons. this should apply when the mating system is one of male contest competition, allowing dominant males to monpolize access to fertile females. In this case dominant females should shift the sex ratio towards more sons. There is another hypothesis, a modified version of the ‘local resource competition hypothesis’. It predicts that in female-bonded species, where males tend to emigrate and where natal female kin form within-group power blocks to raise their inclusive fitness, dominant females should shift towards producing more daughters and subordinate females towards more sons. However, empirical data are confilicting: biases towards either side have beenr eported for one and the same species. This has cast doubts whether adaptive sex ratio manipulation exists and all in these and other tax. Forone thing, so-called significant deviations from a 50-50 ratio could be the seletively reported, accidental extremes, from a population with a random 50-50 distribution. Recently Van Schaik and Hrdy (1991) have tried to integrate the earlier ‘male quality’ (MQ) and ‘local resource competition’ (LRC) hypotheses. Their model supposes that the strength of the local resource competition, experienced in a population, is the crucial factor. It predicts that dominant females will choose the ‘male quality’ option when LRC is low. They will choose to ‘strengthen the female power block’ option when LRC is high. The model indeed appears to reconcile the seemingly contradictory results that have been obtained from some cercopithecoid primates. Similarly, recent studies on the influence of social variables in human females, such as dominance status and being socially challenged, on the sex ratio of their offspring seemed contradictory. Here also the van Schaik and Hrdy model offiers a possiblity to accomodate these results. Even though the mechanism(s) by which psychological and conditional factos might affect the sex of mammalian offspring remain largely a matter of speculation, there is increasing evidence that adaptive sex ratio manipulation is a real phenomenon alsoin mammals, that can be accounted for in evolutionary terms.