Rats are far more vocal than our blunt hearing will ever perceive: ‘The most interesting aspect of rat vocal communication is that it mainly occurs in the ultrasonic realm’, says Tobias Riede from Midwestern University, USA. Expressing fear and discomfort with shrieks around 22 kHz, and squeals of pleasure around 50 kHz, rats have an extensive repertoire of ultrasonic cries for most occasions. Riede is fascinated by how the animals assemble combinations of squeaks and peeps to learn more about vocal communication. So, when Christine Hernandez discovered that young inexperienced males seem to combine the two differently pitched ultrasonic cries when they first encounter a female, Riede was intrigued. Could the unexpected hybrid call have something in common with human speech? ‘Speech is based on our ability to concatenate syllables into words and words into sentences … during a single extended breath’, says Riede. Were the young males controlling their breathing to produce both components of the combined call during one breath, instead of inserting a swift minibreath between the two as they transition seamlessly between the differently pitched ultrasonic calls? Puzzled, Hernandez played Cupid, introducing young inexperienced male rats to females, while recording the novices’ cries and breathing patterns to learn more about how rats articulate their high-pitched cries.
‘Recording lung pressure and electromyographic [muscle] activity of laryngeal muscles in an awake and spontaneously behaving animal was a major challenge’, says Riede, recalling how difficult it was to implant the electrodes in the minute vocal muscles to record muscle activity while a young male serenaded females during his first encounter. Investigating the frequency structure of the unusual hybrid cries, Reide's colleague Mark Sabin was convinced that the lower pitched portion of the hybrid serenade was identical to the 22 kHz calls that the animals produce when they are fearful. Meanwhile, the 50 kHz trill, which forms an overture or finale to the curious call, matched the mating calls produced by more experienced males. However, it was only when Hernandez and Sabin analysed the lung pressure traces alongside the recordings of laryngeal muscle activity that they were sure that the two-stage call was emitted during a single breath. And when they checked the air pressure traces in the seconds leading up to the excited male's utterances, they could see evidence of the signature deep breaths also taken by humans as they prepare to speak.
‘At least during the initial encounter with a female, young and sexually mature male rats concatenate the two calls. They not only produce the calls in close succession but also adjust their breathing in order to produce the two calls in a single breath’, says Riede. As the animals are able to coordinate the two vastly different movements when merging the calls, Riede observes, ‘Non-human mammals and humans seem to share a number of features of how they control vocal movements’. He also suggests that we may be able to learn more about the neurophysiology underpinning breath control in human speech by studying how amorous young rat males coordinate their serenades during their first ‘teenage’ crush.