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 spiderss, the female is larger than the male. Other sex-specific differences include color (most birds, including male and female cardinalss), 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.).
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".
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. To give an accurate picture of male and female size differences one would need to show how many individuals there are in each size category. There is a considerable overlap.
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.
- sexual size dimorphism index
- binomial dimorphism index