Curiously, we also see sexual dimorphisms in many "socially monogamous" species—those in which males and females pair up and rear young together. Since males don't seem to be competing for females, why have they evolved bright colors and ornaments? This seeming contradiction actually provides further support for sexual selection theory. It turns out that in these cases, appearances are deceiving. The species are socially monogamous but not actually monogamous.
One of these species is the splendid fairy wren of Australia, studied by my Chicago colleague Stephen Pruett-Jones. At first glance, this species looks like the paragon of monogamy. Males and females usually spend their entire adult lives socially bonded to each other, and they codefend their territory and share parental care. Yet they show striking sexual dimorphism in plumage: males are a gorgeous iridescent blue and black, while females are a dull grayish-brown. Why? Because adultery is rife. When it comes time to mate, females mate with other males more often than they do with their "social mate." (This is shown by DNA paternity analysis.) Males play the same game, actively seeking and soliciting "extra-pair" matings, but they still vary far more than females in their reproductive success. Sexual selection associated with these adulterous couplings almost certainly produced the evolution of color differences between the sexes. This wren is not unique in its behavior. Although 90 percent of all bird species are socially monogamous, in fully three-quarters of these species males and females mate with individuals other than their social partner.
Sexual selection theory makes testable predictions. If only one sex has bright plumage, antlers, performs vigorous mating displays, or builds elaborate structures to lure females, you can bet that it is members of that sex who compete to mate with members of the other. And species showing less sexual dimorphism in behavior or appearance should be more monogamous: if males and females pair up and don't stray from their mates, there is no sexual competition and therefore no sexual selection. Indeed, biologists see strong correlations between mating systems and sexual dimorphism. Extreme dimorphisms in size, color or behavior are found in those species, like the birds of paradise or elephant seals, in which males compete for females, and only a few males get most of the matings. Species in which males and females look similar, e.g., geese, penguins, pigeons, and parrots, tend to be truly monogamous, exemplars of animal fidelity. This correlation is another triumph for evolutionary theory, for it is predicted only by the idea of sexual selection and not by any creationist alternative. Why should there be a correlation between color and mating system unless evolution is true? Indeed, it is creationists rather than evolutionists who should become sick at the sight of a peacock's feather.35
So far we've talked about sexual selection as if the promiscuous sex is always male and the picky sex female. But sometimes, albeit rarely, it's the other way around. And when these behaviors switch between the sexes, so does the direction of dimorphism. We see this reversal in those most appealing of fish, seahorses, and their close relatives the pipefish. In some of these species the males rather than the females become pregnant! How can that happen? Although the female does produce eggs, after a male fertilizes them he places them in a specialized brood pouch on his belly or tail, and carries them about until they hatch. Males carry only one brood at a time, and their "gestation" period lasts longer than it takes a female to produce a fresh batch of eggs. Males, then, actually invest more in child-rearing than do females. Also, because there are more females carrying unfertilized eggs than males to accept them, females must compete for the rare "nonpregnant" males. Here, the male-female difference in reproductive strategy is reversed. And just as you might expect under sexual-selection theory, it is the females who are decorated with bright colors and body ornaments, while males are relatively drab.
The same goes for the phalaropes, three species of graceful shorebirds that breed in Europe and North America. These are among the few examples of apolyandrous ("one female and many male") mating system. (This rare mating system can also be found among a few human populations, including Tibetans.) Male phalaropes are entirely responsible for child care, building the nests and feeding the brood while the female moves on to mate with other males. The male's investment in offspring, then, is greater than the female's, and females compete for males who will take care of their young. And, sure enough, in all three species females are colored much brighter than males.
Seahorses, pipefish, and phalaropes are the exceptions that prove the rule. Their "reverse" decoration is exactly what one expects if the evolutionary explanation of sexual dimorphism is true, but doesn't make sense if these species were specially created.
Was this article helpful?