Sexual dimorphism
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.
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.
Source: wikipedia, under GFDL
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