A gene for positive emotions is responsible for male mice emitting squeals of delight when they are mating with female mice, according to researchers at the University of Toronto, Canada.
Researcher Haoran Wang and colleagues found that 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 squeal when they're in good mood, Wang said.
It has long been known in the scientific community that adult mice emit ultrasonic vocalisations in the 25 to 120 kHz range when they come across females or their pheromones. However, it is not easy to find out which genes are crucial in controlling emotions. Since humans and mice have similar numbers of genes, mice are useful in studying human genetic behaviour.
The research team used the number and density of different male and female vocalizations to index the emotional responses of mice. They used stimuli such as urine, female odour, contact with female mice and amphetamines to make the males call out. Inaudible to humans, these high-frequency calls are measured using special microphones and computer programs.
The quantity and intensity of the sounds correspond to emotional states, the researchers said.
Males often made simple whistles or modulated calls before mounting the females, i.e. during courtship period. However, the vocalizations shifted to more complex "chirp-like" calls after mounting. As intensity increased during this phase, the sounds increased in number and became more complex.
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.
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," New Scientist quoted Wang, as saying.
Tim Holy at the Washington University School of Medicine in St Louis said: "This is the most detailed look at the quality of these vocalisations during mating behaviour in the literature."
"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," he added.
The study appears in the journal PLoS One.