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Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, size, ornamentation and behaviour.
The peacock, on the right, is courting the peahen, on the left.
Male (bottom) and female mallards. The male mallard has an unmistakable green head.
Female (left) and male Argiope appensa, displaying typical sexual dimorphism differences in spiders where the male is dramatically smaller.
Orgyia recens above male, below female
Ornamentation / coloration
A common type of dimorphism is ornamentation. A frequent component of such dimorphic ornamentation is sexual dichromatism, meaning that the sexes of a given species differ in coloration, such as is conspicuously the case in many species of birds and reptiles.
Exaggerated dimorphic traits are used predominantly in the competition over mates.Ornaments may be costly to produce or maintain, which has complex evolutionary implications but the costs and implications differ depending on the nature of the ornamentation (such as the colour mechanism involved).
The peafowl constitute conspicuous illustrations of the principle. The ornate plumage of peacocks, as used in the courting display, attracts peahens. At first sight one might mistake a peacocks and peahens for completely different species because of the vibrant colours and the sheer size of the male’s plumage; the peahen being of a subdued brown coloration. The plumage of the peacock increases its vulnerability to predators because it is a hindrance in flight, and it renders the bird conspicuous in general. Similar examples are manifold, such as in Birds of Paradise and Argus pheasants.
Another example of sexual dichromatism is that of the nestling blue tits. Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of green lepidopteran larvae, which contain large amounts of the carotenoids lutein and zeaxanthin. This diet also affects the sexually dimorphic colours in the human-invisible UV spectrum. Hence, the male birds, although appearing yellow to humans actually have a violet-tinted plumage that is seen by females. This plumage is thought to be an indicator of male parental abilities. Perhaps this is a good indicator for females because it shows that they are good at obtaining a food supply which the carotenoid is obtained from. There is a positive correlation between the chromas of the tail and breast feathers and body condition. Carotenoids play an important role in immune function for many animals, so carotenoid dependent signals might indicate health.
In many instances, females show preference for exaggerated male secondary sexual characteristics when choosing a mate. Females tend to show direction preferences for more elaborate males. Females have been shown to discriminate against males which are dull in color regardless of the species’ vision. There have also been species such as estrildid finch where premating isolation was seen due to lack of vibrant colors by the males. This female preference for ornamentation may affect the evolution of discriminatory mating preferences. This is known as the ornamentation hypothesis.
Species with larger females than males
In some species such as insects, spiders, many fish, reptiles, birds of prey and certain mammals such as the spotted hyena, and blue whale, the female is larger than the male. As an example, in some species females are sedentary and sparsely distributed, and so males must search for them. Vollrath and Parker argue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males. Cases where the male is larger than the female have been studied as well, and require alternate explanations.
One example of sexual size dimorphism is the bat Myotis nigricans. In this species, females are substantially larger than males. They differ in body weight, skull measurement, and forearm length. The difference in size is believed to be caused by natural selection for a large female size due to a fecundity advantage. The interaction between the sexes and energetic needs such as time and energy required to produce viable offspring make it favorable for females to be larger in this species. Females bear the energetic cost of producing eggs which is much greater than that of the male who only bears the cost of making sperm. The fecundity advantage hypothesis states, that a big mother is able to produce more offspring and give those offspring more favorable conditions to ensure their survival. This is true for most ectotherms. Another reason why females are believed to be larger is due to the fact that they provide parental care for a substantial amount of time while the offspring matures. The time of gestation and lactation is fairly long in the M. nigricans, where females suckle their offspring until nearly adult size. They would not be able to fly and catch prey if they did not compensate for the additional mass of the offspring during this time. In addition to the hypothesis that explains an advantage of large female size, it is hypothesized that smaller male size is an adaptation for males to increase maneuverability and agility. This selection for agility in flying is a helpful adaptation which allows males to better compete with females for food and other resources.
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 and vice versa which clings to her (note that although males can survive away from females, they generally are not free-living).
Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss Dicranum and the liverwort Sphaerocarpos. There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, or to chemical signalling from females.
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.