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Thursday, April 10, 2003

NATURAL SELECTIONS

Immune system linked to mating habits


David Beckham might wear a sarong and Takuya Kimura of SMAP may sometimes wear lipstick, but in humans, most males are dull compared to the females. In other animals, of course, the opposite is true: it is the males that are showy, brightly colored, flashy.

Biologists have known why this is since Darwin explained the process of sexual selection, the corollary to natural selection that describes how males compete for females and females choose among males. This means that males will often have "weapons," such as horns for fighting, but it also means that they will be as attractive, colorful and pretty as possible, so that females will be more likely to choose them.

What biologists have not known, not for sure, is why females prefer the prettiest, brightest males. Researchers suspected that brighter males were somehow better -- but in what way? Two papers published in Science last week suggest an answer.

One is from a British group who studied beak color in male zebra finches; the other from a French team working on beak color in male blackbirds. The bird species and the teams' approaches are different, but the conclusions are the same: Birds with brighter beaks have better immune systems. At least there is something that Britain and France can still agree on.

"Lots of people since Darwin have pondered how male display is linked to physical condition," said Jonathan Blount from the University of Glasgow in Scotland. His research team found that the male zebra finches with the reddest beaks received the most sexual interest from females and had the healthiest immune systems.

The red and yellow color of the beaks of zebra finches and blackbirds is derived from a group of pigments called carotenoids. Birds can not synthesize carotenoids and must obtain them through food. Carotenoids stimulate the production of antibodies and absorb some of the damaging free radicals that arise during the immune response, according to Bruno Faivre from the Universite de Bourgogne and CNRS in Dijon, France.

It is thought that carotenoids lower the risk of some diseases, which is one reason we are urged to eat plenty of vegetables: Carrots are full of carotenoids, as are other vegetables. But it wasn't clear that there was a direct link between sexual display and the immune system.

"Scientists have hypothesized that a male bird's sexual ornaments signal his individual ability to cope with parasite infections and diseases. However, no study, to our knowledge, has directly shown the trade-off between immune activation and the expression level of sexual ornaments," said Faivre.

The scientists from the U.K. manipulated the diet of 10 pairs of zebra finch brothers. One of each pair drank water with carotenoid supplements, the other drank just plain water. The scientists measured the effects of their elevated carotenoid levels on immune function, beak color and sexual interest from female finches.

They found that elevating the level of carotenoids in the bloodstreams of zebra finches boosted immune defenses and reddened beak color. These males were also more attractive to prospective mates, the key mechanism of sexual selection.

The French researchers approached carotenoids and immune function from the opposite direction. Instead of boosting beak color, they tried to decrease it. For signals to work they must be "honest" -- i.e. they must tell the truth. So a signal like the color of a beak must reliably describe the state of the bird's immune system.

To test the idea, the scientists measured the beak color of about 50 birds, and then taxed the immune systems in 35 of them by injecting the birds with sheep red blood cells (a standard immunological test).

Sure enough, beak color in the immune-stressed birds dulled due to declining levels of carotenoid.

"In blackbirds, dynamic reallocations of carotenoids from the beak to the immune system appear to convey a continual update on male health," said zoologist Frank Cezilly, co-author on the Faivre paper and professor at the Universite de Bourgogne. "Re-allocations of carotenoids were observed in three weeks time. We didn't think the answer could be so quick."

Blount's group also found that the bright red beaked males' immune systems out-performed the control group.

While carotenoid-based beak color is not the only factor female zebra finches evaluate when selecting a mate. "It is now clear that carotenoid levels are linked to sexual attractiveness and immune function," said Blount.

The next step, according to Blount, is to tease out the relative importance of the different factors that may determine the range in carotenoid-dependent signals. These include immune function, foraging efficiencies, susceptibility to and cost incurred by parasites, and the amount of energy an individual requires to convert dietary carotenoids into a usable form.

Though beak color is probably not the only factor involved in blackbird sexual signaling either, blackbirds are a relevant biological model for investigating the connection between carotenoids and the immune system, said Faivre.

"The colored surface of the bill may rapidly signal a change in carotenoid allocation because it is continually renewed with carotenoids," Faivre said. "This kind of information is not available from feather color, because pigments in sex-signaling plumage are only allocated to feathers once or twice a year during molt events."

Faivre suggested that carotenoid-based signals may have evolved because they are honest signals of individual quality.

"Once males and females started to react adaptively to carotenoid-based signals, their expression was further favored by natural selection."

Rowan Hooper welcomes comments at rowan.hooper@tcd.ie


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