Many authors, dating back to Darwin (1871), have considered factors that might affect the degree of sexual dimorphism (reviewed by Clutton-Brock, Harvey & Rudder, 1977; Mace, 1979). Frequently, however, only one sex is considered, or it is unclear what limits the degree of sexual dimorphism. For example, intermale conflict is believed to select for increased male size (Darwin, 1871; Selander, 1972; Trivers, 1972). However, larger females achieve greater reproductive success (Chapter 4) and Rails (1976a) has argued that this may be why females are sometimes larger than males. On the other hand, Erlinge (1979) and Moors (1980) have argued that smaller females enjoy a bioenergetic advantage.

It is not yet clear what sets an upper limit to male size in species where males compete over females, and larger males enjoy a competitive advantage. One possibility is male-biased mortality. In many species where males are the larger sex, mortality is greater for males than for females (Potts, 1970; Selander, 1972; Clutton-Brock & Albon, 1982). Selander (1972) argued that the greater mortality of male great-tailed grackles, Quiscalus mexicanus, occurs partly because males are more vulnerable to predators, and partly because males are above the optimal size for efficient foraging. Another possibility, suggested by Belovsky (1978) and Searcy (1979), is that larger males have less energy available for reproduction.

The greatest degree of sexual dimorphism, in species where males are heavier than females, occurs in pinnipeds (Scheffer & Wilke, 1953; Bryden, 1969). Bartholomew (1970) suggested three factors which might set an upper limit on male size in pinnipeds. First, larger males are more aggressive and in the most dimorphic species oblivious of the pups, so that many pups are squashed to death.