New research suggests that as testosterone in male birds increases, so does the level of carotenoids, the chemicals that create the bright coloring on birds' feathers, beaks, and legs.
The brilliant reds, yellows, and oranges serve as indicators of sexual competitiveness, signaling to females that the bearer is healthy and a potentially good mate.
A bird in good shape should be colorful and also should sing more," said Blas, whose research appears this week in the journal Proceedings of the National Academy of Sciences. "Until now these lines of evidence have been researched independently of each other. What we did is connect these two lines of research."
Immune System Puzzle
The finding could solve another outstanding puzzle.
High levels of testosterone come with a price, as the hormone usually depresses the immune system, increasing birds' susceptibility to disease.
But recent studies have shown that birds manipulated in the lab to have high testosterone could still have robust immune systems.
The link between testosterone and carotenoids may be the answer, Blas says. Carotenoids help build vitamins and are strong antioxidants—chemicals that help animals detoxify harmful molecules called free radicals.
In short, carotenoids appear to compensate for the effect of testosterone by keeping the immune system strong.
Sick male birds have dull coloring. This is probably because the carotenoids are being used by the struggling bird's immune system in an effort to fight off disease, Blas says.
"When a chicken becomes sick, its yellow legs become paler," he said. "Why? Because it is using its carotenoids to fight illness."
But birds that are in good shape can have it all—elevated testosterone, a healthy immune system, and large deposits of color-carrying carotenoids in their legs, beaks, and feathers.
"It may be that only the really high-quality individuals can withstand the immunosuppressive effect of testosterone," said Lynn Siefferman, a biologist at Indiana University in Bloomington who studies bluebirds, feather color, and testosterone.
"The idea is that they will put health before reproduction" and not mate, she said.
A similar mechanism is probably at work in other vertebrates, Blas says.
Color Checkup
Blas has only just begun his inquiry into the relationship between testosterone and carotenoids.
In future research, he plans to study exactly how the carotenoids end up in the animals' blood and livers.
For example, he says, testosterone may cause stored deposits to be mobilized into the blood or may increase the absorption of carotenoids from food. Alternatively, the birds may actually consume more carotenoids under the influence of testosterone.
Geoffrey Hill, a biologist at Auburn University in Alabama, studies coloring in bird feathers. Each year many birds, such as finches, molt and regrow their feathers.
In preparation, "they definitely alter their diet to have a more carotenoid-rich diet," he said. Some birds even ingest so many carotenoids that their fat turns red.
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