THE SOCIO-ECOLOGY OF SEX-RATIO VARIATION IN PRIMATES - EVOLUTIONARY DEDUCTION AND EMPIRICAL-EVIDENCE

Fisher (1930, The genetical theory of natural selection. Clarendon Pre ss, Oxford) has shown that evolution on the basis of individual select ion 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 mecha nisms for adaptive sex ratio manipulation could evolve when the above conditions do not hold. Empirical evidence for such adaptive manipulat ion 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 anim als'', the mammals, both the theoretical models and the empirical evid ence are considered much more ambiguous. The primates offer an illustr ative case. On the one hand there is the 'male quality hypothesis'. It predicts that females will shift the sex ratio of their offspring dep ending on their social position and their condition, provided that sta tus and condition influence the competitive potential of their sons. t his should apply when the mating system is one of male contest competi tion, 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 re source competition hypothesis'. It predicts that in female-bonded spec ies, where males tend to emigrate and where natal female kin form with in-group power blocks to raise their inclusive fitness, dominant femal es should shift towards producing more daughters and subordinate femal es towards more sons. However, empirical data are conflicting: biases towards either side have been reported for one and the same species. T his has cast doubts whether adaptive sex ratio manipulation exists and all in these and other tax. For one thing, so-called significant devi ations 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, e xperienced in a population, is the crucial factor. It predicts that do minant females will choose the 'male quality' option when LRC is low. They will choose to 'strengthen the female power block' option when LR C is high. The model indeed appears to reconcile the seemingly contrad ictory results that have been obtained from some cercopithecoid primat es. 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 t he van Schaik and Hrdy model offers a possibility to accommodate these results. Even though the mechanism(s) by which psychological and cond itional factos might affect the sex of mammalian offspring remain larg ely a matter of speculation, there is increasing evidence that adaptiv e sex ratio manipulation is a real phenomenon also in mammals, that ca n be accounted for in evolutionary terms. (C) 1997 Elsevier Science B. V.