Cornell Lab of Ornithology Ithaca, N.Y.
Red-backed fairy-wrens are an Australian species of small insectivorous birds. Males and females are socially monogamous—they pair together and stay in a family group. But they are also sexually promiscuous, so a lot of the offspring aren't sired by the male that is raising them at the nest.
When you have all this extra pair mating, there is a lot of competition among the males for females. Sexual competition of this kind can shape sexual signals like plumage, and so it can be an important driver of speciation events. This sort of sexual selection is thought to have been key in human evolution.
In the birds we study, there are two types of males: brightly colored red-and-black ones and drab brown ones. Hormones regulate which path they go on, and social interactions during the prebreeding season appear to regulate hormones. In a relatively high-ranking male, testosterone goes up, and he develops into a bright guy. But if he gets negative social feedback—like receiving little interest from females or aggression from other males—his testosterone goes down, and he develops into a drab guy. The red-and-black males sire a lot more young, and becoming brown makes the best of a bad situation. They are unlikely to be breeders, so they save whatever costs there are of having a bright breeding phenotype. But the plumage change is not permanent. They just wait for another year or two until they are in better condition and then breed as bright males.
Many earlier, simple models in evolution didn't take into account the idea of phenotypic plasticity, such as these temporary plumage changes. We are interested in understanding what the consequences of that plasticity are.
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