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Color-Coordinated Courtship

20 March 2009 (All day)
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Sarah Pryke

Mismatch. When these two finches mate, they'll produce an overabundance of sons.

Finch couples take note: It pays to color coordinate. New research shows that a female Gouldian finch chooses to hatch an even number of boys and girls if she has the same head color as her mate. If there's a mismatch, mom produces more males and isn't as attentive to her offspring. This is the first example of such strong gender selection based solely on a mate's looks, experts say.

Gouldian finches (Erythrura gouldiae) come in two color varieties, red-headed and black-headed. When two different varieties mate, genetic incompatibility causes many of their offspring--especially the females--to die before they reach sexual maturity. With such a dire outcome, biologists Sarah Pryke and Simon Griffith of Macquarie University in Sydney, Australia, wondered whether mother finches had any power to adjust the ratio of sons to daughters. That ability might allow mothers to avoid wasting energy on doomed females.

First, Pryke and Griffith took 100 black-headed females and 100 red-headed females and mated each of them twice: once to a male of the same head color, and once to a male of the other head color. Mismatched pairs produced broods that were 82% male, but same-colored parents begot offspring with a balanced sex ratio. Was it just genetics at play, or did mom have some sort of control?

To find out, the researchers got out their paint brushes. They colored a few red-headed males black, and vice versa. The results were the same: When a red-headed female finch mated with a black-headed male, even if he was a natural red-head, she overproduced sons, with a brood that was 72% male on average, the team reports today in Science. When she mated with a bird of the same head color--even if that color was artificial--the ratio was even. What's more, mom produced more eggs and put more effort into feeding and caring for her young when the colors matched than when they didn't. But the paint couldn't overcome the genetic incompatibility: More daughters still eventually died when the parents were a true mismatch than when mom and dad's natural head color was the same.

The findings "illustrate a level of maternal control [over offspring gender] ... that I don't think has ever been demonstrated," says Scott Ramsay, an ornithologist at Wilfrid Laurier University in Ontario, Canada. The study "should remove any lingering doubts that birds are capable of flexibly adjusting the sex ratio of their offspring," adds biologist Ido Pen of University of Groningen in the Netherlands.

Still, there's one lingering question about the finch mother's behavior, says Pryke: "We still don't understand how the she's doing it."

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