Like lovers in an opera, male and female plain-tailed wrens, small songbirds that live in the bamboo forests of Ecuador, sing elaborate, synchronous duets. It's such a coordinated vocal tour de force that a pair of wrens sounds like a single bird. Now researchers report they've figured out how the birds do it—a finding that also applies to closely timed, cooperative behaviors in other species, such as humans dancing the tango, rowing a boat together, or having good sex.
Previously, scientists thought that each wren's portion of the song was coded in its brain's circuitry; when one bird began to sing, an internal pattern generator in the brain of the other bird would kick in to control the timing of the song. But Eric Fortune, a biologist at Johns Hopkins University in Baltimore, Maryland, and his colleagues discovered that the male and female wrens' brains actually have a memory of the whole song. "We thought that each bird's best memory would be for its own part" in the duet, Fortune says. Instead, "their strongest memory" was for the duet, which is "something that neither bird can do alone." The birds are thus similar to humans working on a joint task, he says. "We do a better job when we know what we're trying to accomplish together."
To find out how the brains of the male and female wrens encode these antiphonal duets, as such rapidly alternating songs are called, Fortune's group set up a neurophysiology lab at the Yanayacu Biological Station on the slopes of Ecuador's Antisana volcano. They recorded and analyzed more than 1000 of the wrens' songs, both while the birds performed duets and while males and females sang alone. Here, too, the team made new discoveries, finding that the males' songs vary more than do those of the females and that the males make more mistakes than the females when singing in the duets. It seems, too, that the females take the lead in these duets. "Sometimes, the male drops out, but she'll just continue, and then he'll join in again," says Fortune, who suspects that the duets are a way for a female to challenge and test a male.
The scientists also captured three pairs of wrens for experiments. After recording the birds' duets in the wild, they anesthetized the wrens, putting them into a sleeplike state. They then inserted tiny wires the diameter of a human hair into the portion of the wrens' brains that governs song production. The researchers then recorded the electrical impulses on the individual neurons as the birds listened to playbacks of their songs. A wren's neurons reacted to its own song as well as to its partner's, the team reports online today in Science. But by far, the neurons' strongest response was to the birds' combined effort, the rapid alternation of male and female syllables in the actual duet.
"That isn't what we expected to find," Fortune says. "It means the two birds know what they're doing together" and that they are listening closely to their partner's song and adjusting their song to it.
"This is the first demonstration that the brain of these birds is specialized for processing the rapidly alternating sounds typical of antiphonal duetting," says Eliot Brenowitz, a neuroscientist at the University of Washington, Seattle. Teresa Nick, a neuroscientist at the University of Minnesota, Twin Cities, says "Fortune's group has found that the wrens' song isn't simply a fixed-action pattern," a behavior generated automatically when one bird hears the other sing, "but a precisely timed dialog between mates, and that changes [in the songs] depend on partner participation." And that means, Nick says, that current models for how songbirds produce their songs "will have to be reworked," taking into account social cues the birds are exchanging along with their notes.