Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. Examples include size, color, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers or tusks.
Dimorphism comes from two Greek words, di meaning two and morph meaning form. Thus, sexual dimorphism basically refers to two forms of sex, male and female.
In many species, including most mammals, the male is larger than the female. In others, such as most insects, spiders, birds, reptiles and amphibians, many fish, and certain mammals such as the spotted hyena, 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 thresholds for certain behaviors (aggression, infant care, etc).
Among vertebrates, sexual dimorphism is particularly apparent in ducks, and most gamefowl perhaps most dramatically including peafowl. Male pheasants are notably larger than females and possess bright plumage; females are usually a drab brown irrespective of the particular species. In some birds (most of which are waders such as the phalaropes and painted snipes), females have brighter colors than males. 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 considerably so. 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 sexual dimorphisms have obvious utility beyond mate attraction, such as the Blue Wildebeest (and many other biungulates). 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 the genus Osedax of polychaete worms, which lives on whale falls. The females feed on the bones of the dead whale, but 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 which encounter them to develop into tiny, semi-parasitic males which are swallowed and live inside the female's genital sac. The argonauts also have males which are tiny compared to the female. 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.
Some species of anglerfish also display extreme sexual dimorphism. Females are typical anglerfish, while males are tiny rudimentary creatures with no digestive systems. The males 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 (though males can survive away from females, they generally are not free-living).
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.
The handicap principle is the proposed evolutionary force that gives males of some species traits that at first glance seem to place the organism at a disadvantage.
For example, the bright colouration of male game birds makes them highly visible targets for predators, while the drab females are better camouflaged. Other examples are bird of paradise and lyrebird, whose males have such large plumes that their flight is inhibited. Strong smells, loud cries and singing can also attract predators.
The answer that has been suggested to this apparent paradox is that, at a biological level, the reproductive success of an organism is often more important than duration of life. This is particularly apparent in the case of game birds: a male Common Pheasant in the wild often lives no more than 10 months, with females living twice as long. However, a male pheasant's ability to reproduce depends not on how long he lives but whether females will select him to be their mate.
A brightly coloured and heavily plumed male presumably demonstrates to the female that he is fit in evolutionary terms – he has been able to survive in spite of impediments and must therefore be a good choice to father her chicks (especially her daughters, who will have his "fit" genes, but will not be hampered by male plumage). This explanation was first proposed by Amotz Zahavi.
Development of such characters could not at first be explained in terms of simple natural selection. In 1871 Darwin advanced the theory of sexual selection, which related sexual dimorphism with sexual selection.
Comparison of sexual dimorphism in birds and their mating habits shows that the time spent in search for mates, staking territories and mating competes with the demands of taking care of young. For birds and in general, it can be stated that the stronger the dimorphism in a species, the more likely is it to be polygamous and the less is the task of caring for offspring shared among the sexes. This theory is developed by R. L. Trivers' in the parental investment theory. It applies to all ecology.
Top: Stylised illustration of humans on the Pioneer plaque, showing both male (left) and female (right).Above: Comparison between a male (left) and a female pelvis (right).
Sexual dimorphism in humans is the subject of much controversy, especially relating to mental ability and psychological gender. (For a discussion, see biology of gender, sex and intelligence, gender, and transgender.) Obvious differences between men and women include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physiological and behavioural effects. Such undisputed sexual dimorphism include gonadal differentiation, internal genital differentiation, external genital differentiation, breast differentiation, muscle mass differentiation, and hair differentiation.
The basal metabolic rate is about 6 percent higher in adolescent boys than girls and increases to about 10 percent higher after puberty. Women tend to convert more food into fat, while men convert more into muscle and expendable circulating energy reserves. At age eighteen, men (on average) have about 50 percent more muscle mass than women in the upper body, 10 to 15 percent more in the lower. Men, on average, have denser, stronger bones, tendons, and ligaments. This allows for heavier work.
Men dissipate heat faster than women through their sweat glands. Women have a greater insulation and energy reserves stored in subcutaneous fat, withstanding cold better. Sex differences in endurance events are less significant than for sprinting events.
Men typically have larger tracheae and branching bronchi, with about 30 percent greater lung volume per body mass. They have larger hearts, 10 percent higher red blood cell count, higher hemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and higher peripheral pain tolerance.
Women typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than men. Hence they develop fewer infectious diseases and succumb for shorter periods. Ethologists argue that women, interacting with other women and multiple offspring in social groups, have experienced such traits as a selective advantage.
Some biologists theorise that a species' degree of sexual dimorphism is inversely related to the degree of paternal investment in parenting. Species with the highest sexual dimorphism, such as the pheasant, tend to be those species in which the care and raising of offspring is done only by the mother, with no involvement of the father (low degree of paternal investment).
Although there are many biologically-determined, sexually-dimorphic behaviours in other species, these have few, if any, implications for human society. However, analysis of sexually dimorphic human behavior naturally provokes controversy. One less controversial, but still hypothetical, area with considerable discussion in academic literature concerns potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies (David M Buss, 2007).
"The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals."
"Males should prefer attributes in potential mates associated with reproductive value or fertility, depending on whether males in human evolutionary history have tended to seek long-term or short-term mating partners (Buss 1987; Symons 1979; Williams 1975). Specifically, if males in our evolutionary past have tended to seek short-term mating partners, selection should have favoured male preferences for females in their early 20s who show cues positively correlated with fertility. If males in our evolutionary past tended to seek long-term mating partners, selection should have favoured preferences for females in their mid-teens who show cues indicative of reproductive value. Evolutionary theorists differ on which of these they judge to be most likely."
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