Male and female mammals obviously differ in various features that are directly linked to reproduction, as is the case with the sex organs of both sexes and the mammary glands of females (primary sexual characteristics). However, males and females can also differ in a variety of features that are not directly associated with reproduction (secondary sexual characteristics). Such secondary differences between males and females, like the human facial beard, are collectively labeled sexual dimorphism. The simplest form of sexual dimorphism distinguishing male and female mammals involves overall adult body size.
As a general rule in mammals, males tend to be bigger than females, but there are some cases in which females are bigger than males (reverse sexual dimorphism). Differences in body size between the sexes are often relatively mild, as is the case in humans, where adult males are about 20% heavier than females; but there are also some striking contrasts. In the most extreme case of dimorphism in body size found among mammals, namely in the elephant seal (Mirounga), adult males are about four times heavier than females (8,000 lb [3,629 kg] compared to 2,000 lb [907 kg]).
Sexual dimorphism in mammals can also affect other features, notably involving differences in external appearance (e.g. coat coloration), the size of the canine teeth and special appendages such as the antlers of deer (Cervidae).
Overall, there seems to be a general tendency for the degree of sexual dimorphism in size of the body or its appendages to increase with increasing body size (Rensch’s Rule), although the validity of this generalization has been questioned. An additional generalization that can be made is that sexual dimorphism in body size, canine size, and the size of such appendages as horns is generally lacking from species with a monogamous pattern of social organization. However, this does not apply to sexual dimorphism in coat coloration, as is shown by striking differences in external appearance between males and females in certain species of monogamous gibbons and lemurs.
The baseline expectation for mammals is that males and females will be similar in size and other features unless some special selective factor intervenes. However, there is no real reason why males and females should be similar in the size of the body and its external appearance or appendages. Given the major inequality in contribution to reproduction that characterizes all mammals, because gestation and lactation are exclusive to females, the baseline expectation should surely be that male and female strategies are quite likely to diverge. We should be more surprised by the numerous cases in which males and females are very similar in size and appearance than we are by sexual dimorphism. The standard explanation for sexual dimorphism in mammals is that selection acts on the male to increase the size of the body or its appendages because of competition for mating access to females (sexual selection). In elephant seals, for example, the big bull males fight one another to establish mating territories and maintain harems that may contain three dozen females. The large body size of males and their large canine teeth are therefore reasonably interpreted as features that increase male success in competition for females. A similar explanation is provided for the development of large antlers in male deer. Among primates, this interpretation is also applied to various species that show conspicuous sexual dimorphism. A prime example is the mandrill (Mandrillus sphinx), in which males are more than twice as heavy as females, vividly colored and equipped with very prominent canine teeth.
There has been a general tendency to overlook the potential part played by selection on females in the evolution of sexual dimorphism in mammals. In the first place, it is obvious that a single unitary explanation for sexual dimorphism, such as improved competitive ability of males, is inadequate because the different kinds of sexual dimorphism (e.g. body size, canine size, and coloration) can vary independently to a large degree. In primates, for example, it is possible to find marked dimorphism in coat coloration and mild variation in canine size without any matching difference in body size. Furthermore, most lemur species lack any kind of sexual dimorphism despite sometimes fierce competition among males for mating access to females. When males and females of a species differ in body size, it is commonly assumed that selection has operated to increase male body size, but it should be remembered that selection might also act to reduce (or increase) female body size. Because many reproductive features are scaled to body size (e.g. neonate size and age at sexual reproduction), one effect of reduction of female body size will be to decrease nutritional requirements for reproduction and increase the rate of reproductive turnover. In principle, sexual dimorphism between males and females in adult body size could be achieved by an increased rate of growth in males, by an extension of the growth period in males, or by some combination of these two possibilities. In practice, sexual dimorphism in mammalian body size is always associated with at least some delay in the attainment of sexual maturity in males relative to females, so there are consequences for reproductive dynamics in every case. Hence, it seems likely that sexual dimorphism reflects the effects of diverging selection pressures operating on both sexes.