Sexual dimorphism

Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species.

For example, in some species, including many mammals, the male is larger than the female. In others, such as some spiders, the female is larger than the male. (In grammatically gender-sensititve languages, the grammatical gender of the animal species name besides humans indicates the larger gender of that species. Therefore, the animal species name is grammatically feminine if the female is larger than the male, and vice versa.) 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 (e.g., in the case of bees); possession of stings (various kinds of bees), and different thresholds for certain behaviors (aggression, infant care, etc.).

An extreme example of sexual dimorphism in birds is found in the eclectus parrot, where the male is predominantly green with an orange beak and the female mainly scarlet with a black beak. Some cases of sexual dimorphism in birds are so striking that males and females of a same species were originally taken to as members of entirely different species.

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 in humans is the subject of much controversy. 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: 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, while the adult female mean is 62.0 kg. However the standard deviation of male body mass is 12.6 kg, so 10% of adult males are actually lighter than the female average.

Sex dimorphic differences should be contrasted with 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.