Sexual dimorphism

Sexual dimorphism is the physical differences that exist between males and females of a given species, other than the difference in sex organs. Such differences are generally readily apparent, such that males and females of the given species can be differentiated just by looking at them. The term literally means "two body types - one for each sex".

Sexual dimorphism may appear as differences in size and color and the existence of body parts in one sex but not the other, among many other differences.

One specific example of sexual dimorphism is in mallards, where the duck (female) looks much different from the drake (male). Likewise, in some breeds of cattle a bull (male) is easily distinguished from a cow (female) because of his horns.

About ten per cent of all flowering plants exhibit sexual dimorphism, with some plants only producing female flowers and others only males. Good examples are holly, date palm, and kiwifruit.

Sometimes, sexual dimorphism exists within breeds of the same species. This can be seen in some breeds of dog, but not in all. So, for example, in the breed standard of Great Danes, the minimum height required of a female Great Dane is lower than for a male, because male dogs are generally larger than females. The American Kennel Club standard says, “The male should appear more massive throughout than the bitch, with larger frame and heavier bone.’’ (1) Likewise, to a trained eye, male and female Irish Setters look different.

Evolutionary mechanisms for the development of sexual dimorphism

There are a number of evolutionary mechanisms that can explain particular sexual dimorphisms.

1. This statement appears in the Breed Standards of both the American Kennel Club and the Great Dane Club of America.