Sexual Dimorphism

One common type of polymorphism is produced when there are differences between the sexes within a species. Since there are at most only two sexes in any species, this polymorphism is usually referred to as a dimorphism.

In many bird species there is a marked plumage dimorphism between the sexes. In most cases, it is the male that is more brightly colored. The bright plumage of the male functions to attract females, repel other males, and may function as a species-specific signal which ensures that females recognize males of her species. Correct species identification is especially critical where two or more closely related species breed sympatrically.

Usually in species showing a marked sexual dimorphism, it is the female that does most or all of the incubating of eggs. Her plumage is usually cryptic, thus rendering her inconspicuous on the nest.

The plumages of the males often cause them to be very conspicuous. You might expect that such coloration would be selected against since brightly-colored males are probably more visible to predators. However, remember that there are both advantages and disadvantages associated with most traits. The trait that is most successful will represent a balance between the various selective forces. In the instance of male plumage, males must both avoid predators and attract the correct females. The table below represents this situation.

very bright distinctive male plumage	dull male plumage
Several factors can operate to affect the need for bright or unique plumage: rarity of females the degree of competition among males the number of other species in the area with similar males	The greater the intensity of predation, the stronger the selective force towards inconspicuous plumage.

Examples of sexual dimorphism in local birds

female	male
Above: Indigo Bunting (Passerina cyanea), male is bright blue, female is cryptically colored.
Below: Northern Cardinal (Cardinalis cardinalis), male is bright red, female is cryptically colored.
female	male

Seasonal sexual dimorphism in the Bobolink (Dolichonyx oryzivorus)

Bobolinks are relatives of blackbirds native to Ithaca. They exhibit seasonal sexual dimoprhism. During the breeding season, male and female plumage differs considerably, but during the non-breeding season male and female plumage is nearly identical.

Download Printable (PDF) Version

Back to Main Menu / Previous Objective / Next Objective /