Sexual Dimorphism in Mammals: Avian Models and Unanswered Questions

The degree of sexual dimorphism found in mammals ranges from species in which females are larger than males (Rails 1976a) to those in which males are much larger than females and possess striking secondary sexual characteristics which females lack. Although some work has been done on mammals, principally pinnipeds (Bartholomew 1970; Stirling 1975) and primates (Crook 1972), current theories as to the ultimate causes of this variation in sexual dimorphism were developed largely by workers most familiar with the natural history of passerine birds (Verner and Willson 1966; Orians 1969). Sexual selection is generally believed to be the principal cause of sexual dimorphism. Trivers (1972) extended earlier ideas and proposed that parental investment is the key factor influencing sexual selection. According to his model, the sex which makes a smaller parental investment in its offspring will compete for mates and be subject to sexual selection. If most sexual dimorphism is due to sexual selection, the degree of sexual dimorphism should in turn be correlated with the relative parental investments of the sexes. Ecological factors, such as the abundance and distribution of food, which influence the degree of sexual selection are pictured as operating mainly by influencing parental investment. Recent reviews apply this body of theory to all vertebrates (Wilson 1975; Brown 1975). This paper points out some reasons why it applies less well to mammals than to passerines, attempts to evaluate the importance of parental investment in governing the degree of sexual selection in mammals, and makes some alternative predictions as to the probable relative importance of various factors in influencing the evolution of mating systems and sexual dimorphism in mammals. It also discusses the difficulty of measuring male parental investment and contribution to zygotes.