9.16: Sexual dimorphism is an indicator of a population’s mating behavior.

Male elephant seals are three to four times the size of female elephant seals. In contrast, the males and females of most bird species are the same size. In these species, even expert bird-watchers often cannot distinguish between the sexes, except when the female bird is carrying eggs (FIGURE 9-27). When the sexes of a species do differ in size or appearance, this is called sexual dimorphism. Why do species have such dramatic differences in the degree to which males and females resemble each other?

Body size is an important clue to behavior. We have seen how male elephant seals have a winner-take-all tournament for control of the beach, and as the winner, a male has access to mating opportunities. Because the largest individual will have the most offspring, there is selection for larger and larger body size. There is no selection for larger body size among female elephant seals, because females of any size can mate. Hence the dramatic size difference between the sexes.

In addition to body size, coloration can also be a clue to behavior. Because females of some polygynous species choose the males with the brightest or flashiest coloration rather than the largest males, male-male competition sometimes results in differences in physical appearance between the sexes other than (or in addition to) size.

What happens when there is little male-male competition for mates? Among bird species, males can provide significant investment in offspring. Eggs have to be incubated, and because many chicks emerge in a very poorly developed condition, it takes two parents to raise them. So a pair tends to stay together for a season or longer. If each female can pair up with only one male, there really isn’t much competition for mates. And without male-male competition, there is little selection for increased size. As a consequence, little sexual size dimorphism occurs in most bird species.

In turn, we can predict a bit about the parental practices of a species just by looking at a picture of a male and a female and examining the ratio of body sizes. If the two are dramatically different, as in elephant seals, we can hypothesize that the smaller sex is doing most of the care of the offspring and that the species is more likely to be polygamous than monogamous.