Population Regulation in Mammals: An Evolutionary Perspective

Abstract

1. I present a conceptual model that predicts the proximate and ultimate factors that determine whether a mammal species has the potential to be regulated by intrinsic (behavioural) or extrinsic factors. The model is based on three behavioural phenomena that purportedly regulate mammal populations: female territoriality, dispersal, and reproductive suppression. 2. The model predicts that intrinsic regulation can occur only in those species in which females are territorial, offspring-rearing space (or alpha status) is limited, and young females exhibit reproductive suppression. Female territoriality should occur in species that have altricial young and serve as a counter-strategy to infanticide from conspecific females. Female mammals that have precocial young or mobile altricial young that are not vulnerable to infanticide should not commit infanticide and should not be territorial. Thus, developmental state at birth would be an ultimate factor that determines whether offspring-rearing space potentially can be limited. 3. Dispersal should be density-independent in nonterritorial species and inversely density-dependent in territorial species and thus has limited potential to regulate population density in any species. 4. Behavioural reproductive suppression of young females is proposed as an adaptive mechanism to avoid inbreeding or to conserve reproductive effort in response to the threat of infanticide. 5. Intrinsic regulation should be most likely to occur in monogamous territorial species in which daughters grow up in the presence of male relatives (such as in canids and some primates). Polygynous species, in which females are territorial such as most rodents, have the potential for self-regulation; however, exposure to unrelated males and the fact that young females can breed on their mothers' territories usually preclude benavidural reproductive suppression. 6. Intrinsic regulation should not occur in species with precocial young, nonterritorial species, or in species in which daughters do not associate with male relatives, such as the ungulates, marine mammals, bats and marsupials. 7. The model predicts that female territoriality, the threat of infanticide, and the presence of male relatives in the natal home range are the proximate mechanisms for intrinsic population regulation in mammals. These factors apparently occur only under a limited set of conditions; therefore, most mammal populations are probably controlled by extrinsic factors. The model is presented with a series of a priori predictions from which hypotheses can be formulated and tested.