Figure 23

Traits that differ between males and females of a species -- such as tails, color, and songs -- are called ‘sexual dimorphisms,’ from the Greek for “Two forms” (figure 23 shows a few examples.) Over and over, biologists have found that sexually dimorphic traits in males seem to violate evolutionary theory, for they waste time and energy that reduce survival. Colorful male guppies are eaten more often than are the plainer females. The male black weatear, a Mediterranean bird, laboriously erects large cairns of stones in various locations, piling up fifty times his own weight in pebbles over a period of two weeks. Male sage grouse perform elaborate displays, strutting up and down the prairie, flapping their wings and making loud sounds from two large vocal sacks. These shenanigans can use up a tremendous amount of energy for a bird: one day’s display burns up the caloric equivalent of a banana split. The selection is responsible for these traits -- and it should be, given their complexity -- we need to explain how.