The biology of song and song learning comes mostly from research on songbirds, and shares important characteristics with human language. Song commonly figures in courtship rituals among birds, insects, and frogs, but aside from humans, such behavior in mammals had been restricted to whales and bats. And none of these organisms can be studied with genetic tools. In a new study published in the open access journal PLoS Biology, Timothy E. Holy and Zhongsheng Guo at the Washington University School of Medicine show that mice can sing too; and this could open whole new avenues of research into the genetic contributions to song and song learning.
Mouse social encounters prompt vocalizations, such as the inaudible ultrasonic calls of males presented with females or urine pheromones. Previous studies of these vocalizations focused on classifying them by when they happened, rather than on their acoustic patterns. In this study, Holy and Guo focused on the sounds themselves. The authors used cotton swabs coated with either female mouse urine, male mouse urine, or a combination of the two to elicit the male mouse’s ultrasonic sounds, and then recorded their vocal responses. Far from random patter, male ultrasonic calls contain complex passages with long sequences composed of diverse syllable types.
The authors manipulated the recordings to hear the ultrasonics. One approach used a slow playback (at one-sixteenth of the recorded speed) that distorted the temporal structure of the calls, which sounded like low, intermittent whistles. The other dropped the pitch to an audible level without interfering with the time sequence–the pitch-shifted recording sounds remarkably like birdsong. (To listen, go to DOI:10.1371/journal.pbio.0030386.sa004.) To bolster this subjective conclusion, the authors then undertook a quantitative analysis of the sounds.
The males produced rapid “chirp-like” syllables of varying duration, spaced at about ten syllables per second, with a burst of closely spaced syllables followed by periods of silence. In keeping with previous reports, some of the syllables showed sudden, significant changes in frequency (or pitch). The authors identified discrete clusters of pitch changes by analyzing a set of 750 syllables produced by one mouse in a single 210-second trial and determined that these pitch changes followed a stereotyped pattern instead of random occurrence. Trials with 45 different mice produced similar results, indicating that the pitch changes are a universal feature of mouse ultrasonic vocalizations.
Since the mice produced multiple syllable types arranged in regular, repeated time signatures, their vocalizations meet the definition of song. The authors also showed that individual males produced songs distinct from those of other males. “The richness and diversity of mouse song appear to approach that of many songbirds,” Holy and Guo write. And just like songbirds, the mice appear to be singing their own tune. Future studies can begin to unravel the physiological basis and mechanics of ultrasonic mouse song–and perhaps decipher the messages encoded in the notes and melody.
(To download samples of mouse song modified to make them audible to humans, go to http://mednews.wustl.edu/news/page/normal/6040.html.)
From Public Library of Science