for National Geographic
Flamboyant male ornaments such as peacocks' tails, crabs' claws, roosters' combs, and stags' antlers have long been thought to advertise the quality of a potential partner to discerning females in the market for a mate.
Now, researchers in France and the United Kingdom report the first experimental evidence that brightly colored beaks, in two different bird species, are directly linked to the quality of the immune system.
"It's been known for a long time that females of many species choose to mate with the flashiest males," said Jonathan Blount evolutionary biologist at the University of Glasgow in Scotland. "Quite what they stand to gain from mating with these show-offs has been puzzling ecologists since the time of Charles Darwin," he said.
Now, Blount and his co-authors working with zebra finches in the U.K., and an independent research team working with blackbirds in France, think they may have found proof that bright red or orange beaks attract females because they advertise that their male owners are in top-notch physical condition. The findings are detailed in two studies published in the April 4 edition of the journal Science.
Previous studies had noted that food-derived plant chemicals, called carotenoids, are necessary to produce many of the bright red to yellow colors that animals display, commented freelance evolutionary biologist George A. Lozano, formerly of McGill University in Montreal, and now based in Ottawa. Examples include the red markings of salmon, stickleback, and guppies, in addition to many birds.
These chemicals are also behind many similar plant colors, such as that of the bright red leaves of some species, said Lozano. Animals are unable to synthesize carotenoids, so access to them is limited by the amount they can harvest from their diet.
Researchers have also found evidence that these same chemicals are important for a healthy immune system in animals, mopping up damaging metabolic by-products, and boosting the body's defenses, for example.
To test the link between carotenoid-dependent ornament flashiness and immune function, Blount's team gave carotenoid-supplemented water to ten zebra finches (Taeniopygia guttata) over an 8-week period. Over the same time frame, another group of finches, made up of a single brother of each of the birds in the carotenoid-supplemented group, was given untreated water.
The scientists found that the finch brothers provided with dietary supplements, developed significantly redder beaks by the fourth week of the study.
Furthermore, in additional tests, the researchers found, not only that nine out of ten females preferred to perch near carotenoid-treated, bright-beaked males, but also that these males exhibited more potent immune responses than their un-supplemented siblings.
Tale of Two Cities
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