A beauty tip from starlings: Get some household help.
In African species where several adults tend the chicks of one mother, females tend to grow as big and flashy as the males, says Dustin Rubenstein of Columbia University. In nests without extra helpers, dad tends to outshine mom in finery.
What’s likely pushing these female starlings to iridescent glory in cooperative breeding species is an intense rivalry to be the nesting group’s egg-layer, according to Rubenstein and Irby Lovette of Cornell Laboratory of Ornithology in Ithaca, N.Y.
Having to compete with other female starlings for any advantage — in food or status or outright male attention — cranks up the evolutionary pressures on females, Rubenstein and Lovette argue in the Dec. 10 Nature. Among cooperative breeders, females face evolutionary forces such as the sexual selection that drives competitive males to peacock extravagance.
The study represents the first time researchers have analyzed how female ornamentation relates to cooperative breeding, says behavioral ethologist Trond Amundsen of the Norwegian University of Science and Technology in Trondheim, who was not involved in the study.
“Why females are beautiful in the animal world is a question that has been neglected until the last decade or two,” Amundsen says. In plenty of the well-studied bird species, for example, females look drab compared with males. Darwin and most evolutionary biologists after him focused on male colors, tails, crests and other ornaments when musing about how such frippery evolves without obviously contributing to survival.
Rubenstein and Lovette looked at female ornamentation in African starling species, which come in a range of fancy feathers in iridescent purples and greens and blues. The researchers worked out a family tree of 45 starling species and mapped out breeding and physical traits by examining 1,614 museum specimens as well as published descriptions of these species.
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The researchers found that more than 80 percent of the cooperative breeders had females that looked like males, but only about 30 percent of the noncooperative breeders did. The variations among the species suggest that the females are catching up with the males’ ornamentation instead of males drabbing down to match females, the researchers say.
In theory, this link between social living and intense selection on females could show up in other animals, Rubenstein suggests.
“The longstanding dogma has been that males are subject to more intense sexual selection,” says Eileen Lacey of the University of California, Berkeley. But some research on cooperative-breeding mammals has found evidence of strong competition among females to reproduce. Now, there’s a growing realization that, depending on the species, rivalries can vary in intensity, she says, “and may even be more intense on females than males.”