Sexual dimorphism

Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. Examples include colour (specifically referred to as sexual dichromatism), size, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers, or tusks.

Examples

The peacock, on the right, is courting thepeahen, on the left.

Male (front) and female mallards. The male mallard has an unmistakable green head.

Female (left) and maleArgiope appensa, displaying typical sexual dimorphism differences in spiders where the male is dramatically smaller.

Orgyia recens above male, below female

In many species, including most mammals, the male is larger than the female.^[1] In others, such as most insects, spiders, many fish, birds of prey and certain mammals such as the spotted hyena and the blue whale, the female is larger than the male. Other sex-specific differences include differences in colouration (sexual dichromatism), presence vs. absence of certain body parts (such as horns, antlers, tusks or display feathers), size of the eyes (some insects), possession of stings (various kinds of Hymenoptera), and different frequencies of certain behaviors (aggression, infant care, etc.).

Among vertebrates, sexual dimorphism is particularly apparent in ducks and most gamefowl. This is perhaps most dramatic with species ofpeafowl. Male pheasants are notably larger than females and possess bright plumage, whereas females are usually brown irrespective of the particular species. In some birds, females have brighter colors than males; most of these cases are waders such as the phalaropes andpainted snipes. As this is the opposite of the usual sexual dichromatism, it is termed reverse sexual dimorphism. In many predatory birds, females are larger than males, often by a considerable margin. This seems to reduce competition between members of a pair, as they have different optimal prey sizes. Some cases of sexual dimorphism in birds are so striking that males and females of the same species were originally taken to be members of entirely different species, as in the case of the Eclectus Parrot (Eclectus roratus), where the male is predominantly green with an orange beak and the female scarlet and deep blue with a black beak.

The Huia (Heteralocha acutirostris), a New Zealand bird species (now extinct), was another striking example of sexual dimorphism. The male's bill was short, sharp and stout, while the female's was long, thin and crescent-shaped. This beak dimorphism allowed mated pairs of Huia to avoid competing for the same food source, with males chiseling into and breaking apart rotting logs, while females were adept at probing into fresher wood for grubs.

Certain cases of sexual dimorphism have obvious utility beyond mate attraction. An example of this is theBlue Wildebeest (and many other ungulates). The horns of the male are much larger, allowing the male to engage in combat more effectively as he competes with other bucks for mating privileges.

An extreme example of sexual dimorphism is found in the genus Osedax of polychaete worms, which lives onwhale falls. The females feed on the bones of the dead whale; the males live inside the females and do not develop past their larval stage, except to produce large amounts of sperm. In the echiuran Bonellia viridis, exposure to adult females causes larvae to develop into tiny, semi-parasitic males which are swallowed and live inside the female's genital sac. In the parasitic barnacles Sacculina, the males are tiny, free-ranging animals, whereas the females only exist as a web-like tissue inside their hosts. In the majority of scale insects, females are highly modified (eyeless and wingless, with non-functional appendages and reduced segmentation), attached permanently to their host plants, while males are rather ordinary though delicate insects, smaller and winged.

Female Triplewart seadevil, an anglerfish, with male attached near vent (arrow)

Some species of anglerfish also display extreme sexual dimorphism. Females are more typical in appearance to other fish, whereas the males are tiny rudimentary creatures with stunted digestive systems. A male must find a female and fuse with her: he then lives parasitically, becoming little more than a sperm-producing body. A similar situation is found in the Zeus water bug Phoreticovelia disparata where the female has a glandular area on her back that can serve to feed a male that clings to her (note that although males can survive away from females, they generally are not free-living).^[2]

[edit]Psychological and behavioral differentiation

Sex steroid-induced differentiation of adult reproductive and other behavior has been demonstrated experimentally in many animals. In some mammals, adult sex-dimorphic reproductive behavior (e.g., mounting or receptive lordosis) can be shifted to that of the other sex by supplementation or deprivation of androgens in fetal life or early infancy, even if adult levels are normal.