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Bright Beaks Signal Health to Female Birds, Study Says

John Pickrell
for National Geographic
April 3, 2003
 
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.


Health-o-meter

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

At the same time, and unknown to Blount's team, an independent team of researchers, led by biologist Bruno Faivre at the University of Bourgogne in Dijon, France, was testing the link between bright beaks and immune function in male blackbirds (Turdus merula).

The French researchers approached the problem from a different angle, however, stimulating the immune system of healthy blackbirds in the laboratory. Unlike dowdy, brown females of the species, male blackbirds possess bright yellow-to-orange beaks and shiny black plumage.

As carotenoids can be mobilized by the body, as and when required, the researchers predicted that otherwise healthy blackbirds should lose some of the bright orange vibrancy of beak color when the chemicals need to be reallocated for defensive purposes.

The scientists found that within three weeks, bill color significantly decreased in those males experimentally immunized to stimulate an immune response. "We didn't think the answer could be so quick", said biologist Frank Cézilly co-author at Borgogne. "In blackbirds, dynamic reallocations of carotenoids from the beak to the immune system appear to convey a continual update on male health," he said.

In the wild, "Only those [male birds] with the fewest parasites and least disease, are likely to be able to allocate enough carotenoids to produce the best…displays, " said Blount. The benefits for a female selecting a healthy mate are likely to include "a reduced risk of picking up parasites and diseases and better chance of having a mate that will go the distance in rearing offspring," said Blount.

Females could possibly additionally benefit by begetting offspring that share the good genes responsible for their father's health, though this remains to be tested.

Though many of the pieces of the puzzle had been examined in prior work, "Everything has been put together very neatly" in these studies, said Lozano. "Both support the idea that carotenoids have a role in sexual selection [by females]," he said.

Intriguingly, said Blount, some work has suggested that carotenoids and related antioxidant vitamins could be responsible for delaying some of the physical effects of aging in people, such as wrinkly skin. It would be interesting to investigate whether people with higher levels of such antioxidants retain a more youthful appearance later in life, and are therefore more attractive mates, he said.

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