For at least one of North America's most common birds, mating songs are more than just empty amorous enticements, according to a new study from Johns Hopkins. Scientists have found that male starlings' singing ability is strong evidence of the health of their immune systems and thus their suitability as breeding partners.
The new finding may explain why female starlings take singing talent into account when choosing their mates and is an important first step toward proving a decade-old theory that suggests evolution has found a way to stop male birds from engaging in false sexual advertising.
"The theory, which is known as the Immunocompetence Handicap Hypothesis, or ICHH, proposes that males of lesser reproductive quality are prevented from cheating--producing a signal that falsely indicates high reproductive quality--by some cost associated with producing that signal," explains Deborah Duffy, lead author on the new paper.
Duffy, a recent Ph.D. graduate of Hopkins and now a postdoctoral fellow at Indiana University, is co-author with Greg Ball, professor of psychological and brain sciences in the Krieger School of Arts and Sciences, of a paper published in the April 22 issue of The Proceedings of the Royal Society of London.
According to Ball, "Understanding the function of mate choice is essential to understanding evolution. Females clearly have an important role in mate selection in the wild, and this selection process has a big effect on what genes will appear later in the population."
Ball finds the link between mate selection and the immune system particularly intriguing.
"Understanding how this variation in immunocompetence is regulated and maintained could be very valuable in our quest to understand the factors that control the immune system," he says.
When they proposed the Immunocompetence Handicap Hypothesis in 1992, researchers Ivar Folstad and Andrew John Karter of the University of Tromso in Norway theorized an immune system-mate selection link that keeps male birds "honest" about their reproductive fitness. They suggested that the physiological factors such as hormones that help male birds look like ideal mates might actually at the same time be suppressing their immune systems, making it too risky for any but the healthiest male birds to overindulge in production of sexual finery such as plumage or song.
"Choosy females benefit by mating with the males with the strongest immune systems because presumably the offspring inherit some of that high immunocompetence," Duffy explains.
For some birds, color and brightness of the male's feathers advertise healthiness and attract mates. But scientists haven't resolved clearly yet the hormonal factors linked to production of bright plumage. In contrast, several studies have shown that testosterone levels can influence song.
For their experiment, Duffy and Ball assessed the singing ability of 16 adult male European starlings in an outdoor aviary. They compared those results to two tests of the capabilities of the birds' immune systems and found that the birds who were the best singers, based on the number of times they sang per hour and the length of their songs, also tended to be the birds whose immune systems were in the best shape.
Duffy plans to continue study of the factors that influence mate selection in birds and the flexibility of this selection process. Duffy and Ball's research was supported by the National Science Foundation and the National Institutes of Health.