POSTED 26 SEP 2002
Photo courtesy ©2002 Michael & Diane Porter.
Why does the caged bird sing? It isn't for food. Or merely for pleasure, or for dreams of talent agents and lucrative record deals.
It is, of course, for love.
The goal is clear: sing the strongest, win the hearts of the best (and the most) females, have lots of strapping babies to carry on the family genes. But what really gets a fellow in the mood? And how does he know when to give it his all and when to let the vocal cords have a rest?
The answer is that birds' brains are less bird-brained than one might think. In fact, conditions have to be just right for a songbird to offer his best vocal stylings. When it's a go, hormones and neurons create a symphony of their own within the brain-- a sexual motivation system that tells a bird when to croon and rewards him for doing so with feel-good chemicals, even if he loses the girl.
Scientists have pretty well understood how birds sing for decades. But why they sing--the environmental and biological cues that provide a motive beyond the obvious-- is still "a fundamental and largely unanswered question," says Lauren Riters, a behavioral neuroscientist at the University of Wisconsin-Madison.
When Spring is in the Air...
"European starlings sing one of the longest and most complex songs, and they continue to learn new songs each year," says Riters. "If a bird was having a rough year, he wouldn't have time to practice singing....If he sings well, a female can assume he's healthy."
These characteristics make the species a good model for studying song behavior. Riters and her research team have studied European starlings for years, and are beginning to puzzle out what makes a bird feel like singing, and what happens in the brain between that moment and the time he twitters away.
First of all, fellas need a little ambiance. In fall, songs are short and perfunctory, but the long days of spring provide the right amount of light. And of course, why waste your breath when no one is around to hear you sing? Males sing longer and more elaborate melodies in the presence of females than when alone, Riters has found. And lastly, male starlings won't flaunt themselves without a comfortable pad to retreat to. Riters found that in a controlled setting-- whether or not a female was around-- males only sang more if a nesting box was present.
Photo © Dan Sudia, courtesy Florida Museum of Natural History
The Importance of Good Chemistry
Riters guessed that the medial preoptic area is involved in motivation, and that it interacts with other regions of the brain that produce the actual song. "Testosterone is acting in the medial preoptic area," she says. "It's acting in the song control system so they can sing and in the medial preoptic area so they will sing."
Incidentally, the analogous region of the human brain factors heavily in human sexual behavior. And testosterone levels rise when a man anticipates sex, Riters says. The hormone heads straight for the preoptic area of the human brain, she says. But it's impossible to say without a group of study participants willing to have brain surgery for the cause.
Riters isn't sure exactly what testosterone does for the birds, but when the medial preoptic area is lesioned out, the birds apparently have no motivation to sing. And when they don't sing, Riters noticed, their brains are lacking in other chemicals involved in motivation-- opioids.
Opioids are the classic feel-good molecules in the brain, released in birds and mammals alike as a response to pain or in reward systems. The medial preoptic area of a male European starling is flooded with opioids in the spring, but Riters can't say for certain which comes first-- the chemical or the song.
"A male who has a brain that's juiced up on opioids feels good, so he's singing. Or he's singing, so he feels good," she says.
Riters is confident that the medial preoptic area -- and the hormones that circulate in it -- create motivation to warble. And, she says, this area of the brain mingles with the regions of the brain that actually produce song. She has explored this theory using "tracer" molecules that follow the paths of brain cells to see how one brain area connects to another, watching as the medial preoptic area extends into the song control areas.
Photo by Marshall Ilef. Courtesy USGS
But much remains to be learned. Why, for example, do European starlings sing in the fall? Riters suspects it has something to do with maintaining the birds' notorious sky-packing flocks. And what regions in the female brain light up when she hears a song that she fancies?
They aren't easy questions, Riters says, but she doesn't intend to leave them for the birds.
-- Sarah Goforth