10 Sep
Duke News Service
Duke University
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Male Birds'ability To Learn Song Affects Female Mating Response
Editors: Nowicki's paper may be accessed at
http://www.pubs.royalsoc.ac.uk/proc_bio/proc_bio.html. A photo of Nowicki is
available at
http://photo1.dukenews.duke.edu/pages/Duke_News_Service/Nowicki.jpg
DURHAM, N.C. -- Researchers have found that how well a male songbird
learns his song affects the female's mating response - the first evidence that
female birds use song-learning ability as an indicator of male quality. The
study goes beyond previous such studies, which have only demonstrated that very
poor or absent male songs affect female mating response.
According to the scientists, the finding offers broader insight into the
role that traits learned by males play in sexual success.
In an article in the September 22, 2002, Proceedings of the Royal Society
of London: Biological Sciences (now online), biologists led by Duke University
Professor of Biology Stephen Nowicki reported studies in which they tested the
mating response of female song sparrows to songs of captive-raised males.
Importantly, the scientists had analyzed the males' songs in detail to
determine the degree of accuracy with which the males copied songs they
attempted to learn. They found that the females preferred those songs that came
closest to wild-type songs they heard when young and presumably learned as
models. The scientists' research was sponsored by the National Science
Foundation.
According to Nowicki, he and his colleagues in the field have long
theorized that female songbirds pay attention to male song as an indicator of
fitness.
"We've developed experimental evidence that there is a link between early
stress, male brain development and song-learning," he said.
"But until now, experimental and field observations showing that females
were interested in song only contrasted the presence or absence of song, or
relatively gross features of song, like the size of the repertoire. This is the
first study to explicitly demonstrate that females care about song-learning
quality," he said.
To test the effects of fine differences in song quality on female
response, the researchers trained captive-reared male song sparrows to sing by
exposing them to the recorded songs of wild birds. To induce variation in
stress among the birds, some were placed on a restricted diet during
development. Using spectrographic analysis, the researchers rated the
captive-reared birds on two measures of song quality
-- how much of the wild-bird song they copied versus how much they
invented, a practice common among song sparrows. Those birds who did invent
more song elements also tended not to copy well those elements they did copy.
-- how close the males had come to actually matching just the wild-bird
song elements they were attempting to copy
To determine the effects of song quality, the researchers exposed
wild-caught adult females -- presumably experienced in listening to male
songs -- to the captive-reared males' songs. The scientists measured female
response to the songs by the amount they performed characteristic and
distinctive female mating presentation display -- which includes a shivering of
the wings, the lifting of the tail and a characteristic call.
As a control, the scientists exposed the wild-caught females to what the
scientists had judged as well-learned male songs, as well as the digitally
recorded wild songs. The female birds responded equally to both.
However, when the scientists exposed the females to the captive-reared
males' songs, they found the females responded more strongly to male songs that
had been better learned by both of the scientists' measures.
"The females showed a strong preference for songs that had been copied
well, as opposed to songs that had been copied poorly," said Nowicki. "And by
our measures, the males got points taken off for originality. That seems to
make sense because we would argue that males that deviate from original song
haven't learned the song as well." In addition to insight into bird song, said
Nowicki, such studies can give basic insight into the evolution of animal
signals in general.
"We know sexual selection is a very powerful evolutionary force that has
led to phenomena such as the evolution of extravagant displays and the
evolution of size differences between sexes. I believe that this work
demonstrates that sexual selection might not be acting directly on the obvious
trait that is expressed, but on the mechanisms that underlie the expression of
that trait. In the case of bird song, a male's song reflects the birds'
developmental history, and song expression is only the trait that the female
can gain access to for information about -- in this case -- brain mechanisms."
Also, said Nowicki, the discovery that females assess song quality
emphasizes the importance of studying the neurobiology of song expression and
placing it in an evolutionary context.
While the current studies show clearly that females prefer well-learned
songs, among the next research steps, said Nowicki, will be to determine how
females learn to judge song quality.
"There is only very thin evidence that females learn song, so it's a
major scientific question whether females are learning something about the
population that they're living in, and using that as a way of assessing males,"
he said. Such female studies also will reveal whether the female's ability to
distinguish good songs from bad reflects the birds' fitness and influences
evolution, said Nowicki.
http://www.dukenews.duke.edu/research/nowicki.html
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