Although several models of reproductive skew have been proposed, “transactional” models, on the one hand, and “tug-of-war” or “incomplete control” models, on the other hand, are most commonly debated. The former hold that dominants control group size by yielding “incentives” (a share of total group productivity) to subordinates, while the latter advance the idea that the dominant's control over one or more subordinates is incomplete. While high skew has been identified in several mammalian societies, most research on this topic shows that social mammals are likely to display intermediate, low, or variable reproductive skew. In an attempt to explain this pattern of results, the present chapter shows that mammals have evolved to cope with heterogeneous environmental regimes (abiotic and biotic), yielding a eutherian adaptive complex comprised of endothermy, relative brain enlargement, and behavioral flexibility. It is argued that these adaptations to environmental unpredictability favor the evolution of alternative phenotypes as well as situation and condition-dependent responses decreasing the likelihood that dominants will be able to monopolize subordinates, including females, and that high skew will be observed. Additional research is required to highlight similarities and, most important, differences in the routes to sociality between insects, birds, and mammals (see Vehrencamp 1979).
A simple mathematical model is presented linking reproductive suppression by a dominant to his/her influence on a subordinate and consequent ability to monopolize the subordinate. This treatment shows that, depending upon conditions, both types of skew models are realistic. Reproductive skew is discussed in relation to allocation decisions and the different reproductive tactics of female and male mammals.