Sexual dimorphism

Sexual dimorphism exists when the differences between males and females of a given species are readily apparent.. That is, one can tell a male and female apart just by looking at them. The term literally means "two body types- one for each sex".

A good example of this is in ducks, where the duck (female) looks much different from the drake (male). Likewise, in cattle a bull (male) is easily distinguished from a cow (female) because of his horns.

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.

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