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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