While flashy colors come and go in the fashion world, bright beaks are always in style for male birds seeking to woo a mate. New research suggests that the red and orange-hued beaks of certain male birds are signs of a healthy immune system--a "must have" in any mating season.
A pair of studies appearing in the 04 April issue of the journal, Science, published by the American Association for the Advancement of Science, provide the first direct evidence that the colorful beaks on the males of two bird species gives honest information about individual immune systems. When females choose bright beaks, they select healthy males. A U.K. research group focused on beak color in male zebra finches, and a French team studied beak color in male blackbirds.
In the animal kingdom, males often sport the species' more flamboyant features, from peacock's tails to rooster's combs. While scientists have long guessed that these traits advertised the males' fitness two new studies confirm this hypothesis with direct measurements and connect male-bird's bright beak colors with a strong immune system.
"These two papers complement each other. They both use immune system measurements to move beyond assumptions and show that brighter beaks accurately highlight healthier birds," said Stephen Simpson, a Science editor working in Cambridge, U.K.
"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, UK. His research team found that the male finches with the reddest beaks received the most sexual interest from females and had the healthiest immune systems.
Carotenoids are the red and yellow pigments that color the beaks of the males of certain species of birds including blackbirds and zebra finches. Birds can not synthesize carotenoids and must obtain them through the diet. 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 Université de Bourgogne and CNRS in Dijon, France who provided some historical context for the new findings.
"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.
Carotenoids can be considered the "biological currency" of the two beak color studies. One study elevated the birds' carotenoid levels with deposits while the other reported carotenoid withdrawals.
The scientists from the United Kingdom supplemented the diet of one group of male zebra finches with carotenoid pigments and 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 enhanced immune defenses and reddened beak color. These males were also more attractive to prospective mates.
The French researchers approached carotenoids and immune function from a different direction. They started with a baseline carotenoid balance based on the blackbirds' beak color. When they taxed the blackbird immune systems, their beak color dulled due to carotenoid declines.
"In Blackbirds, dynamic reallocations of carotenoids from the beak to the immune system appear to convey a continual update on male health," said Frank Cézilly, co-author on the Faivre paper and professor at Université de Bourgogne in Dijon, France. "Reallocations 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 carotenoid-immunocompetence connection, said Faivre.
"The colored surface of the bill may rapidly signal a change in carotenoid allocation because it is continually renewed with carotenoids. 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," said Faivre.
Faivre speculated 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."
The coauthors on the Blount paper are Neil Metcalfe from University of Glasgow, Tim Birkhead from University of Sheffield, and Peter Surai from Scottish Agricultural College.
Funding for the Blount et al. research was provided in part by the Natural Environment Research Council (UK)
The coauthors on the Faivre paper are Arnaud Grégoire, Marina Préault, and Frank Cézilly from Université de Bourgogne, and Gabriele Sorci from CNRS and Université Pierre et Marie Curie.
Funding for the Faivre et al. research was provided in part by the Institut Français pour la Biodiversité, CNRS (GDR 2155 "Ecologie Comportementale" and ACI Jeunes Chercheurs to G. S.), ONCFS, and the Conseil Régional de Bourgogne.
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