ITHACA, N.Y. -- In a recent series of studies, Cornell University neurobiologists are showing why females of some avian species choose suitors with the most elaborate courtship songs: Fancy singers have more elaborate brain structures (to learn singing and other life skills), brains that the females hope their offspring will inherit.
Reports linking sexual selection on the basis of song and the "heritability" of bigger brain structures in three different bird species were published this year by Cornell scientists in the Journal of Neurobiology , with European sedge warblers; Behavioral Neuroscience , cowbirds; and most recently in Proceedings of the Royal Society (Biological Sciences) with zebra finches.
While the Cornell scientists hesitate to extend their avian neuroscience discoveries to the evolution and amorous affairs of homo sapiens , the leader of the studies says it helps to think in human motivational terms.
"An elaborate bird song is like a Grand Cherokee in the driveway or an M.D. after the name -- a kind of shorthand for all the desirable qualities that a female wants in a mate and wants passed along to the children," said Timothy J. DeVoogd, professor of psychology and of neurobiology and behavior at Cornell. "Of course these birds are not scholars of evolutionary theory. They don't think Darwin's principle of sexual selection when they make up their minds about which male sings best. Nevertheless, their choices will have an immediate impact on the success of their parenting and, in a longer-term, evolutionary sense, on the success and survivability of their kin for generations to come."
When male birds render their courtship song, a certain amount of bragging is implied, according to DeVoogd. It's as if the males are saying: "I know where the food sources are located around here. I know how to find nesting areas that are hidden from predators. I can defend our territory against intruders. I would make the best mate for you and the best father for our children, so you should choose me!" The Cornell research showed that males with the most elaborate songs have bigger brains -- and, in particular, a larger region called the high vocal center, or HVC, that is essential for song learning and production -- to back up their braggadocio. Furthermore, females consistently choose males with the most elaborate songs, according to research studies. And the larger brain structures facilitate learning of complex songs and other skills. Most intriguing to neurobiologists is the finding that larger brain structures can be inherited.
The bird brain-song studies are helping to explain a phenomenon that puzzled Charles Darwin. While Darwin's natural selection clearly accounts for certain traits, like bigger antlers, that enhance survivability, he wondered about bigger and brighter tail feathers, for example, or complex songs that would seem to imperil animals by attracting predators. So Darwin proposed that a different process -- sexual selection -- is at work when potential mates are pairing up.
Still, evolutionary biologists had trouble explaining why secondary sexual characteristics, such as colorful plumage, are inherited. Recent technological developments, including better measurement techniques as well as cleverly designed experiments and statistical tools to determine heritability all helped show what the female really is getting when she falls for the fancy song.
Now the research poses a subsequent question: How do females get smart enough to choose the smartest males?
"Choosing the most elaborate song -- the one with 40 notes instead of 38 -- is not a trivial task," DeVoogd observed. "It takes us (scientists) three hours, with the most sophisticated computer sound-pattern analysis programs, and she does it in 10 to 15 minutes. The females must have some very special abilities to distinguish good songs, to recognize those fine gradations, and we'd like to know how she does it."
Other authors of the Royal Society article, "Variation in the volume of zebra finch song control nuclei is heritable: developmental and evolutionary implications" (Vol. 267, No. 1457, pp. 2099-2104), are David C. Airey, a Cornell graduate student at the time of the research and now a postdoctoral researcher at the University of Tennessee, Memphis; Hector Castillo-Juarez, a former graduate student; George Casella, professor of statistical sciences at Cornell; and E. John Pollak, professor of animal science at Cornell. The studies were funded by the National Institutes of Health.
Related World Wide Web sites:
o Royal Society Proceedings: http://www.pubs.royalsoc.ac.uk/proc_bio/proc_bio.html
o Cornell Department of Psychology: http://comp9.psych.cornell.edu/
o Cornell Neurobiology and Behavior: http://www.nbb.cornell.edu/neurobio/sofneurobio.html
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