Sexual dimorphism
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Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species.
At its most basic, sexual dimorphism can be seen in primary sexual characters - that is, the different reproductive organs of male and female (though even this is by no means universal, many organisms being asexual or hermaphrodites). These differences are often referred to as sex-dichotomous differences, those possessed completely by one sex or the other. An example of a sex-dichotomous difference is a uterus, while phallic size is a sex-dimorphic difference.
The sexes of many species also differ in secondary sexual characters not directly related to reproduction, such as size, colouration, behaviour and so on. The difference between primary and secondary sexual characters is well illustrated by the example of a deer's antlers - the presence of antlers does not usually make any difference in whether a deer is physically capable of reproduction. However, it does make a difference in whether a deer will be able to find a mate.
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Examples of sexual dimorphism
In some species, including many mammals, the male is larger than the female. In others, such as some spiders and many insect species, the female is larger than the male; a larger size is advantageous for carrying or laying eggs. Other sex-specific differences include color (most birds), size or presence of parts of the body used in struggles for dominance, such as horns, antlers, and tusks; size of the eyes (bees); possession of stings (various kinds of bees), and different thresholds for certain behaviors (aggression, infant care, etc.).
Sexual dimorphism is particularly apparent in game birds such as the pheasant, with males possessing bright plumage while females are usually a drab brown. Some cases of sexual dimorphism in birds are so striking that males and females of a same species were originally taken to be members of entirely different species, as in the case of the eclectus parrot, where the male is predominantly green with an orange beak and the female mainly scarlet with a black beak.
Another striking example was the Huia, a recently-extinct New Zealand bird. The male's bill was short, sharp and stout while the female's was long, thin and crescent shaped. One purpose of the dimorphism allowed the male to chisel the entrance to beetle grub holes in living trees, thereby giving the female further reach to pull out the grub that they may then share. Huia had been little studied by Western naturalists before they were driven to extinction and other purposes of the dimorphism had yet to be ascertained.
One of the most extreme examples of sexual dimorphism is found in the small worms of the genus Osedax, which live on whale falls. The females feed on the whale bones. The males live inside the females and do not develop past their larval stage except to produce large amounts of sperm.
Sexual dimorphism in humans
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Sexual dimorphism in humans is the subject of much controversy, especially relating to mental ability. (For a discussion, see sex and intelligence.) Human male and female appearances are perceived as different, although Homo sapiens has a low level of sexual dimorphism compared with many other species. The similarity in the sizes of male and female human beings is a good example of how nature often does not make clear divisions. Standard growth curves give a fairly accurate picture of male and female size differences. The overlap is slightly less than 1 standard deviation. The CDC published new American curves in 2000 and it is available online here (http://www.cdc.gov/nchs/about/major/nhanes/growthcharts/clinical_charts.htm).
For example, the body masses of both male and female humans are approximately normally distributed. In the United States, the mean mass of an adult male is 78.5 kg (173.1 lb), while the adult female mean is 62.0 kg (136.7 lb). However the standard deviation of male body mass is 12.6 kg (27.8 lb), so 10% of adult males are actually lighter than the female average.
There is also sexual dimorphism in the amount and distribution of body hair, with males having more terminal hair, especially on the face, chest and back, and females having more vellus hair, which is less visible. This makes males generally appear more "hairy" than females, but again, the standard distributions of these patterns overlap and some females appear more "hairy" than some males. This may also be linked to neoteny in humans, as vellus hair is a juvenile characteristic.
Biological aspects of sexual dimorphism
The phenomenon of sexual dimorphism is a direct product of evolution by natural selection, in that the struggle for reproductive success drives many male and female organisms down different evolutionary paths. This can produce forms of dimorphism which, on the face of it, would actually seem to disadvantage organisms. For instance, the bright colouration of male game birds makes them highly visible targets for predators, while the drab females are far better equipped to camouflage themselves. Likewise, the antlers of deer and other forms of natural weaponry are very expensive to grow and carry in terms of the energy consumed by the animal in the process.
The answer to this apparent paradox is that, at a biological level, the reproductive success of an organism is often more important than its long-term survival. 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. His bright colouration demonstrates to the female that he is fit, healthy and a good choice to father her chicks.
In the case of herd animals such as deer, a male deer's reproductive success is directly proportional to the number of sexually receptive females with which he can mate. The males' antlers are an example of a sexually dimorphic weapon with which the males fight each other to establish breeding rights. Again, although they are expensive in terms of personal survival, they ensure that the largest and strongest males will be the most successful in reproducing and thereby ensure that those characteristics are passed on to the next generation.
Access to the opposite sex is not the only reason why sexual dimorphism exists. In insects in particular, females are often larger than the males. It is thought that the reason lies in the huge number of eggs that insects lay; a larger body size enables a female insect to lay more eggs. In some cases, sexual dimorphism enables males and females to exploit different food resources, thus increasing their collective ability to find food. Some species of woodpecker have differently-sized and shaped beaks, enabling the sexes to find insects in different layers of a tree's bark.
Sexual dimorphism is sometimes quantified by biologists through the dimorphism index, which is usually the ratio between the average adult male mass and average adult female mass. For some species mass is inconvenient to measure, so a similar parameter such as volume is used instead. This index is commonly written as the abbreviation "SSDI", for "sexual size dimorphism index". Species that are typically polygynous tend to have high SSDI ratios, while species that are typically polyandrous tend to have low ratios.
Sexual dimorphism is regarded as a classic example of genetic polymorphism, though its underlying mechanism varies in different organisms. It is often controlled by genes on the sexual chromosomes.
See also
de:Sexualdimorphismus es:Dimorfismo sexual fr:Dimorphisme sexuel pl:Dymorfizm