Sexual Dimorphism in Mammals: Avian Models and Unanswered Questions

Abstract

Current models for the evolution of polygyny and sexual dimorphism are largely derived from data on passerine birds. These models are less appropriate for taxa such as mammals, in which males emphasize mating strategies, than for those such as passerines, in which males emphasize progeny rearing strategies. The Orians-Verner model is inadequate as a general explanation of the evolution of polygyny in mammals because many species do not meet one or more of its assumptions: that the need for male parental care is the main factor opposing the evolution of polygyny; that females choose to mate with particular males; and that the female raises her young on the resources contained in the territory of the male with which she mates. A two-factor model incorporating the concept of sexual bimaturism, developed by Wiley for grouse, is more appropriate for many mammals but still too simple. In mammals, large male parental investment is a good predictor of both monogamy and reduced sexual dimorphism, but small male investment is a poor predictor of extreme polygyny and increased sexual dimorphism. Thus, large male parental investment is only one of the important factors which oppose the evolution of polygyny. An adequate mammalian model will have to include another set of factors which oppose the evolution of polygyny by increasing the spacing or mobility of females. It will also have to explain why sexual dimorphism has evolved more frequently in large mammals than in small ones. Sexual selection cannot account for all the variation in degree of sexual dimorphism found in mammals. An emerging paradigm based on the consideration of bioenergetic constraints and the factors affecting female size promises new insight.