When a male mouse is about to have sex with a female, he lets out ultrasonic squeals of delight.

The vocalisations are controlled by a gene that is also involved in the production of the brain chemicals that mediate emotions, in mice as well as men. This leads psychologists to conclude that mice squeak when they’re in good mood.

It’s been known for almost half a century that adult mice emit ultrasonic vocalisations in the 25 to 120 kHz range when they encounter females or their pheromones. But no one knew what genes controlled the vocalisations.

So Haoran Wang at the University of Toronto, Canada, and his colleagues performed five separate experiments designed to elicit the calls in both wild type and knockout mice.


Squeaky clean fun

The researchers used stimuli such as urine, female odour, contact with female mice and amphetamines to make the males call out. They recorded the vocalisations, which cannot be heard by humans, then processed them to make them audible to human ears. The sounds are structured like birdsong, say the researchers.

The quantity and intensity of the sounds correspond to emotional states, they say. During courtship, for instance, the males made simple whistles and calls. After mounting the females, when things got more intense, the vocalisations became more chirp-like.

Listen to a recording of the male mice squeaking

When Wang’s team looked at mice lacking various receptors, such as the M2 and M5 muscarinic receptors, which are involved in emotional expression, the males no longer produced the vocalisations.

Bonus odour

Female mice didn’t make the same positive squeaks during sex. But when a female was reunited with a female chum after several hours of separation she did make positive chirps that were “very similar” to those made by bonking males.

“Males and females are emotionally different,” says Wang, in an understatement that speaks volumes.

“This is the most detailed look at the quality of these vocalisations during mating behaviour in the literature,” says Tim Holy at the Washington University School of Medicine in St Louis.

“There’s been a general question for some period of time about whether these sexual odours are intrinsically rewarding. Their data argue that it’s not an irrelevant question.”

Journal ref: PLoS One, DOI: 10.1371/journal.pone.0001893

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