Sexual Selection - Example: Intersexual Selection

Example: Intersexual Selection

The conditions determining which sex becomes the more limited resource in intersexual selection can be best understood by way of Bateman's principle which states that the sex which invests the most in producing offspring becomes a limiting resource over which the other sex will compete. This can be most easily illustrated by the contrast in nutritional investment into a zygote between egg and sperm, and the limited reproductive capacity of females compared to males. Thus, 'sexual selection' typically refers to the process of choice (the limiting factor, which is typically females) over members of the opposite sex (the non-limited factor, typically males).

The peacock provides a particularly well known example of intersexual selection, where ornate males compete to be chosen by females. The result is a stunning feathered display, which is large and unwieldy enough to pose a significant survival disadvantage. Biologists have suggested that the layers of the ornate plumage of males provide a means of demonstrating body symmetry, such that peahens are "trying" to discover the health of the male or the quality of his genes. Diseases, injuries, and genetic disorders may impair the body's symmetry. There is also evidence that reduced symmetry of males may result in fewer offspring, or allow males access only to females with lesser parenting skills.

Bird species often demonstrate intersexual selection, perhaps because - due to their lightweight body structures - fights between males may be ineffective or impractical. Therefore, male birds commonly use the following methods to try to seduce the females:

• Colour: Some species have ornate, diverse, and often colourful feathers.
• Song: Male birdsong provides an important way of protecting territory (intrasexual selection).
• Nest construction: In some species, males build nests that females subject to rigorous inspection, choosing the male that makes the most attractive nest.
• Dance: Males dance in front of females. Cranes provide a well known example.

In some bird species, both the male and the female contribute a great deal to offspring-care. In these cases, the male and female will be continuously assessing each other based on sexual characteristics. In the Blue-footed Booby, the females tend to choose males with brighter blue feet, because birds with brighter feet are younger, and thus have greater fertility and ability to provide paternal care. When researchers put make-up on the males' feet to make them look duller after the laying of the first eggs, their mates consequently laid smaller second eggs, which shows that female boobies continuously evaluate their mates' reproductive value. Males also vary their behavior based on the females' foot color. Males mated to females with brighter feet are more willing to incubate their eggs.