BioEd Online

Mickey Mouse may have kept quiet during his early days on the silver screen, but his lab counterparts seem to have a penchant for song. That's the finding in an analysis of the ultrasonic sounds made by male mice wooing potential mates.

For years, animal-behaviour experts have known that mice make vocalizations that are too high in pitch to be picked up by the human ear. Young mice, for example, make 'isolation calls' when cold or distressed. And male mice emit ultrasonic sounds in the presence of a potential mate or in response to chemical sex cues, called pheromones, in the urine of female mice.

But until now, scientists had not examined these sounds for musical patterns. Thanks in part to a sophisticated computer program, Timothy Holy and Zhongsheng Guo of the Washington University School of Medicine in St Louis, Missouri, were able to tackle this challenge.

Holy began by writing software that shifts the pitch of the male mouse's sounds, making the sounds deeper so that they can be heard by humans. "No one had ever pitch-shifted the mouse vocalization," he says. "The first time I played it back it was pretty surprising: it sounded so much like birdsong."

Holy and Guo then exposed male mice to female mouse urine to elicit mating vocalizations, and recorded the sounds. They analysed the minute details of captured sounds, comparing the pitch from each millisecond with the one immediately preceding it. They looked for patterns in these pitch changes, as well as in the spacing of vocalizations over time.

See how they sung

The animals' high-pitched squeaking has song-like characteristics, the researchers discovered, with distinct pairs of notes arranged in repeating phrases. Holy likens the mouse songs to juvenile bird songs, which lack a complex fixed pattern of musical themes. The findings appear in the journal PLoS Biology¹.

The researchers hope to record songs in wild mice, too, and aim to understand whether these contain greater richness and complexity than those produced by lab mice.

They also aim to understand whether the mice learn these songs from one another or produce them automatically. If mice do teach one another tunes, they will join an exclusive club of animals: researchers have so far only documented this skill in humans, whales and birds.

A broader group of animals produces unlearned sounds. Some insects, such as cicadas, instinctively produce unlearned routines of clicking sounds as a part of courtship. And many birds, in addition to their learned songs, will instinctively chirp to communicate alarm if a potential predator approaches their nest.

A mouse prerogative

Holy notes that the mice he studied each seemed to have a preference for singing certain songs, even though they were all genetically identical. "That's probably the best evidence we have that it's a learned behaviour," he explains. "But I would guess that the degree to which learning plays a role is more limited than in birds."

The complex mouse vocalizations are not necessarily linked to our own gift for music. Birdsong expert Daniel Margoliash of the University of Chicago, Illinois, notes that there is no compelling evidence for learned vocalizations even among our closest primate relatives, suggesting that we evolved our musical skills independently of other species.

But the mice's songs may have something to teach us about the origins of human speech, Holy suggests. He points to evidence that a gene called FOXP2 is essential for both, and adds that further studies could explore this connection.