ST. LOUIS – A Washington University neurobiologist has shown that the ultrasonic chirps of male mice are songs, allowing mice to join whales, bats, insects and birds in the select club of animals that sing.

Female pheromones trigger the singing, leading study author Timothy Holy to suspect that the songs evolved to help male mice find mates.

The study was published online Tuesday in the journal Public Library of Science Biology. Biologists, who have strict definitions for the rhythms and melodic motifs required for animal song, found it persuasive.

“I never would have expected this from mice,” said University of Massachusetts behavioral ecologist Jeff Podos in an e-mail after listening to the mouse songs. “I agree that they are complex enough to be called songs. Very cool!”

Holy induced the crooning by dousing Q-tips with female urine – which contains scent hormones called pheromones. Male mice sniffed, tasted the Q-tip, and, about thirty seconds later, began to chirp.

Too high to hear

But the chirps are eight octaves above middle C on a piano – about two too high for humans to hear. So Holy makes the songs audible by shifting the pitch with software or by slowing down the playback, like spinning a 45 rpm record at 33 rpm.

During an interview last week, Holy, dressed in denim and flannel, jumped to his computer to play disc jockey. One song revealed a mournful warbling that Holy says sounds like a whale. Another clip seemed more bird-like, with its fluted trills and glissandos and grace notes.

The diversity of patterns and rhythms led Holy to rank mice, in ability, just below whales and birds, but above insects.

“They’re somewhat more improvisational than the birds. Maybe they’re less picky,’ he said.

Birds are picky, he said in that song-making is learned: Young birds learn songs from elders and practice them until perfect. Insects, on the other hand, recite simple songs by instinct. Whether mice learn or recite is as yet unknown.

Might be whistling

How the mice sing is also somewhat of a mystery. Birds sing by resonating flaps of throat tissue similar to the way we vibrate our vocal chords. But mice, Holy says, may be whistling.

As a neurobiologist, Holy wants to understand how mice are wired to sing. Scientists have already shown that a certain gene, expressed in bird brains during song learning, is also required for normal human speech and for ultrasonic mouse chirps.

Other biologists, like Podos, are interested in the evolutionary reasons for song. Animal singing is risky because it requires energy and alerts predators. For that reason, it evolved as an “honest signal” – one that can’t be faked – for males to advertise their fitness, Podos said.

Once females began choosing males based on the signal of singing, Podos said, “it snowballed into this crazy system.”

Podos said research has shown a link between a male bird’s vocal repertoire and its fitness.

Whether mouse song is also linked with fitness or mating ability is an open question, since Holy’s laboratory mice are uniformly coddled and mate no matter what. But bird song, Podos said, “is always about sex.”

What about human song? Said Holy:

“If you look at the proliferation of love songs on the radio, you might suspect there’s some connection.”

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