Researchers may have discovered how the bright plumage of male birds evolved as a sign of health and vigor to potential mates. Testosterone, which weakens the immune system, increases the circulation of immunity-enhancing pigments called carotenoids, they find. Healthy birds let the pigment collect in the skin to flaunt their vitality.

The bright coloration of some birds is a classic example of an animal advertising its high quality to potential mates. Carotenoids are the pigment in red, orange and yellow skin (and carrots), but they are also powerful antioxidants that boost the immune system. Only healthy male birds can afford to maintain a costly display of color by diverting resources away from the immune system, the theory goes. The male must therefore have good genes, and that's why a flashy male attracts mates.

In the mating game testosterone plays a similar role to carotenoids. The hormone makes male birds strut and croon but weakens their immune systems, and researchers knew that variations in testosterone levels between birds and seasons tend to match up with variations in the brightness of colors.

"There should be a connection between these two signaling systems," says ecologist Julio Blas of the University of Saskatchewan in Saskatoon, Canada. Blas and his colleagues in Spain and Canada reasoned that high testosterone should increase the amount of carotenoids in the blood. If a male is healthy, they hypothesized, he would not need the surplus pigment to bolster his immune system, so his skin and beak will become saturated with color instead.

To test the idea the group implanted capsules of testosterone under the skin of 13 red-legged partridges. These doped birds had 20 percent more carotenoids in their blood after the treatment, apparently because they absorbed more of the compounds from their food, whereas untreated birds showed no change. Higher blood levels of carotenoids translated to brighter coloration in general, according to digital images of the birds. Next the researchers injected a chemical into the doped birds to see which ones had the more active immune responses.

Tellingly, the birds with stronger immune systems saw bigger gains in brightness for the same amount of carotenoids, according to the group's report published online November 20 in Proceedings of the National Academy of Sciences USA.

"We think this relationship between carotenoids and testosterone may have evolved first to counteract the negative effects of testosterone," Blas says. The role of the antioxidants in mating would have come later, he explains. He adds that the group hopes to see if the link holds in other bird species and other vertebrates that color themselves with carotenoids, including fish, amphibians, reptiles and mammals.