The song of the sexes, avian style
She asks if she’s overweight, and you wait half-a-second before responding, “Of course not, dear! I’ve just been noticing how slim you look these days.”
Any well-schooled husband knows the pitfalls of faltering in this “marital duet.”
Video courtesy Science/AAAS
And now, we find a similar phenomenon among a singing duet by plain-tailed wrens, natives of the cloud forest in Ecuador.
Pairs of these wrens engage in a high-speed duet that relies on perfect timing: She utters a call, and if he chimes in on cue, she sings her part, and the duet continues.
If he’s late or silent, she is slow to resume the song.
This is cooperative behavior, but close examination also reveals a new mental phenomenon, says Eric Fortune, an associate professor of psychological and brain sciences at Johns Hopkins University. Fortune, first author of a study of the wrens that appears today, says his research “indicates that the full mental representation of the song exists in both birds, even though each one contributes only half of the song.”
The study looked at the interaction between the hearing and motor circuits in the brain via a concept called “mirror neurons.” Discovered in 1983 by Dan Margoliash of the University of Chicago, mirror neurons were “a key discovery that has profoundly shaped our thinking,” Fortune says. “He showed that an area of the brain used to control song responded only when the bird heard a playback of its own song, but not of any other bird’s song.”
These nerve cells, since seen in people, other primates and birds, are now called mirror neurons. In simple terms, mirror neurons allow a bird that hears its own song to “imagine” singing that song.
In the new study, however, the mirror response occurs when an individual in a pair hears both birds singing — a sound that each bird cannot produce by itself.
In 2006, scientists identified the plain-tailed wren’s song as a two-part composition that required cues from both partners. “When we heard about these wrens, where one-half of the song is produced by the female, and the other half by the male, we thought, ‘This is amazing. Here’s a song this bird has learned completely in the sensory part of the brain, but it has only half of the motor program.’”
How could this work?
To unravel the sensory-motor linkage, Fortune, with Gregory Ball of Johns Hopkins and Melissa Coleman of Claremont McKenna College, recorded pairs of plain-tailed wrens, manipulated the songs in various ways, and then played them back.
They found that the birds not only sang in pairs, but sometimes also sang solo, making the same calls it would otherwise contribute to the duet, but with altered timing. They found that when a male flubbed his lines, the female might continue to sing, but with a measurable delay. “She’s waiting for him, then gives up and sings anyway,” Fortune says.
The birds were basing their behavior on what they heard — not very surprising. But the fascinating part emerged from the fact that they were engaged in a truly cooperative, back-and-forth behavior that was deeply embedded in the mirror neurons.
Such cooperation, also evinced by dancers and musical ensembles, requires each party to know its own part, but the brain studies showed that they knew much more than that, says Fortune, who is also a visiting professor at Catholic University in Quito, Ecuador. “Both birds had very similar patterns of activity. The neurons responded most strongly to the combined song, not to their own part. The brain knows that they were trying to do this together.”
Got my eye (and ear) on you, mister!
Although Fortune says the songs are probably used to defend territory, he suspects she is also checking him out, gauging his evolutionary fitness, much as female birds rate a fellow’s feathers. “The female is testing the male’s ability to cooperate,” Fortune says. “She produces a long song, and the male has to work hard to insert his syllables at exactly the right time.”
These wrens, he says, “are wired to cooperate. There is a set of rules and the male’s job is to respond rapidly and accurately to the female’s challenge.”
It’s not just feathery guys that fail to respond on cue, and the evolutionary significance could extend far beyond birds. “This happens a lot in people,” Fortune speculates. “Why do women get annoyed when you forget their birthday? They are challenging your neural circuitry. It’s not like flexing your muscles; they are probing your brain. That’s a stronger cue for sexual selection.”
Bringing it back to birds, Fortune says, “It’s most surprising that these animals have a memory of their cooperative behavior in the brain, which includes the performance of another animal; this had not been shown before on a neurological basis. You can take their own half of the song, and play it back, and the motor neurons fire,” but the response is much more powerful when the bird hears the full, two-part song.
— David J. Tenenbaum
- Neural Mechanisms for the Coordination of Duet Singing in Wrens, Eric S. Fortune et al, 4 November 2011, Science ↩