It seems that rats emit ultrasonic screams of joy simply because they are in the company of another rat.
Using newly developed miniature microphones placed on the noses of mice, scientists have been able to determine which mouse is making high-frequency sounds at any given time. In an unexpected result, the recordings revealed that the mice were not making the squeaks as a way to communicate with each other or in response to anything done by their fellow mice, but simply to be together. They were screaming for joy, says Shai Netser at the university. Haifa in Israel.
“We think it’s not a language, but really just another way to pronounce happiness in general,” he says.
People can hear the “alarm” squeak that rats sometimes make when disturbed, but the frequency of the sounds of most rodents is far above the human hearing range, which in adults typically peaks at about 17 kilohertz.
Scientists have previously discovered that rats make very high-pitched, shrill screams (at about 50 kilohertz) when they are happy, and lower-pitched, longer shrieks (at about 22 kilohertz) when they are unhappy. This is somewhat similar to dogs barking in joy or growling in anger, says Netser.
The microphones in the cages have already been taken away Groups of rats produce high-pitched happy screams, but such set-ups do not accurately differentiate which rats are screaming under different circumstances. To overcome this, Netser and his colleagues invented a miniature microphone that they could attach to the noses of 13 mice through a surgically implanted tube. They then placed the rats in cages with other rats, so that they were either free to interact with each other or separated by wire mesh, and recorded the noises they made.
As expected, Netser says that when the rats were in physical contact with another rat, they made their happy screams louder. However, to their surprise, the researchers found that the rats did not make such screams in response to any other animal. Instead, the squeaks were coming randomly, as if the rats were expressing positive feelings about being in the company of another rat.
The mini-microphone also discovered a new mouse sound: a very soft, low-pitched, squeaky squeak that is within the human hearing range, at about 6 kilohertz. “But they don’t produce it when they’re close to us,” says Netser. “They only produce it near each other.” He says that the exact meaning of this newly discovered sound is yet to be investigated.
Netser says the findings also overturn previous assumptions that in groups of males and females, male rats do most of the squawking, because microphones often capture the squawks of both sexes.
Future studies with mini-microphones could help scientists study social behavior 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 each other in their sensory world,” says Netser. “