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Rats seem to emit ultrasonic squeaks of happiness just because they are in the company of another rat.

Using newly developed miniature microphones that were placed on rats’ noses, scientists have been able to determine which individual rat is making high-frequency sounds at any given moment. In an unexpected result, the recordings show that the rats weren’t squeaking as a way of communicating with each other or in response to anything their fellow rats did, but simply out of the joy of being together, says at the University of Haifa in Israel.

“We think that this isn’t a language, but actually another way to pronounce happiness in general,” he says.

People can hear the “alarm” squeaks that rats sometimes make when they are distressed, but most rodents’ calls have frequencies well beyond the human hearing range, which in adults usually maxes out at around 17 kilohertz.

Ů��С��Ƶs have previously discovered that rats make and make . This is somewhat akin to dogs barking with happy excitement or growling when they are angry, says Netser.

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Microphones set in cages have previously picked up high-pitched happy squeaks in groups of rats, but . To overcome this, Netser and his colleagues invented a miniature microphone that they could attach to the noses of 13 rats via a surgically implanted tube. They then placed the rats in cages with a second rat, either so that they were free to interact with each other or separated by a wire mesh, and recorded the noises they made.

As expected, the rats made more of their high-pitched happy squeaks when they were in physical contact with another rat, says Netser. To their surprise, however, the researchers found the rats didn’t make those squeaks in reaction to anything the other animal did. Instead, the squeaks seemed to come at random, as though the rats were just expressing positive emotions about being in the company of another rat.

The mini-microphones also discovered a new rat sound: a very soft, low-pitched, choppy squeak that is within human hearing range, at around 6 kHz. “But they just don’t produce it when they are near us,” says Netser. “They only produce it near each other.” The exact meaning of this newly discovered sound is yet to be investigated, he says.

The findings also overturn previous beliefs that in groups of males and females, male rats do most of the squeaking, as the microphones often picked up squeaking from both sexes, says Netser.

Future studies with the mini-microphones may help scientists study social behaviours among rats. “Once we really get the big picture, we can really understand their level of sophistication, because we can really understand what they’re saying to one another in their own sensory world,” says Netser.