Public release date: 18-Apr-2002
Contact: Michael Purdy
410-516-7160
Johns Hopkins University
http://www.jhu.edu/
Male starlings sing a song of reproductive fitness
For at least one of North America's most common birds, mating songs are more
than just empty amorous enticement, according to a new study from The Johns
Hopkins University. 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 towards
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 Johns Hopkins, is now a postdoctoral fellow
at Indiana University. She is co-author with Greg Ball, professor of
psychological and brain sciences in the Krieger School of Arts and Sciences at
Johns Hopkins, of a paper published in the April 22 issue of The Proceedings of
the Royal Society of London.
"Understanding the function of mate choice is essential to understanding
evolution," Ball says. "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 yet clearly resolved 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.
THE JOHNS HOPKINS UNIVERSITY
OFFICE OF NEWS AND INFORMATION
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Baltimore, Maryland 21218-3843
Phone: (410) 516-7160 / Fax (410) 516-5251
http://www.eurekalert.org/pub_releases/2002-04/jhu-mss041802.php
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