Peekaboo: Rats enjoy playing hide-and-seek with researchers.

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Lab rats play hide-and-seek for the fun of it, new study shows

Every child knows that a proper game of hide-and-seek must follow a strict set of rules. Players can’t switch from being the “seeker” to the “hider” midway through the game, for example, and hiders have to stay put until they’re found. Now, scientists have discovered that lab rats can rapidly learn the rules to hide-and-seek and, so far as they can tell, love playing the game with people.

Neuroscientist Michael Brecht of the Humboldt University of Berlin got the idea for his experiment from YouTube. “There are all these YouTube videos from pet owners that say their animals love to do this,” he says. Although it’s well known that rats play lots of rough-and-tumble games, hide-and-seek is so much more elaborate that Brecht wondered whether they could really do it.

He and colleagues set up a 30-square-meter playroom equipped with cardboard shelters and an array of boxes made from opaque and transparent plastic. They made seven hiding places for the rats, and three hiding places for the study’s designated gamemaster, Humboldt University of Berlin neuroscientist Annika Stefanie Reinhold. After living in cages, it took a little while for the six adolescent male rats in the experiment to feel comfortable in the spacious room. But once they felt safe, they were ready to play.

Each game began with a rat inside a lidded box. When the rat was the “seeker,” Reinhold would close the box and hide, opening the lid with a remote control. After training, the rat knew that was the cue to leap out of the box and go looking for Reinhold. When it found her, Reinhold rewarded the rat by petting and tickling it; no food was offered. When the rat was the “hider,” Reinhold would leave the box open, and crouch beside it while the rat jumped out and scurried to one of its seven hiding places.

Within 2 weeks, five out of six adolescent male rats learned how to both seek and hide—and not switch between those roles when they were in the middle of a game, the team reports today in Science. In a second set of experiments, a different researcher trained four more rats to play the game.

“Many scientists think this is trivial, but these are very complex behaviors” because the rats assume different roles, follow rules, and even strategize about where to hide, Brecht says.

To study the neuronal underpinnings of the rats’ playful behavior, the team recorded electrical signals from about 180 neurons in a brain region involved in learning called the prefrontal cortex, using a portable device implanted in the rodents’ heads. Roughly one-third of the cells fired like crazy when Reinhold closed the lid on the box—the cue which told the rat whether it should be seeking or hiding—suggesting that region is particularly sensitive to learning the rules of a game, Brecht says.

Some of the rats’ behaviors even hinted at the ability to imagine another’s perspective, a higher level cognitive capacity called “theory of mind.” As Reinhold “searched” the room, for example, the rats would often dart to a place where she’d already looked, as though they were thinking she wouldn’t check it again. The rats also preferred hiding in an opaque box, rather than a transparent box, and stayed silent while they hid, despite making ultrasonic squeaks while seeking—all of which suggests the animals may be able to consider another’s viewpoint, Brecht says.

The team also wanted to know whether the rats were playing for the fun of it or for the reward of cuddles from the researcher.

Brecht says several clues point to the former. When the rats find the researchers, for example, they execute what are known as “joy jumps” or freudensprung. “This is something that a lot of mammals do when they are having fun,” including rabbits, lambs, and people, Brecht says. In addition, the rats often scurry off to a new hiding place after being found, extending the game and postponing the reward of being petted.

"They make a good case," says Ted Garland, an evolutionary physiologist at the University of California, Riverside, who was not involved in the work. Scientists have traditionally poo-pooed anthropomorphism, the attribution of human feelings and motives to animals. But findings that rats laugh when tickled and can respond with empathy to another’s pain have started to shift that perspective. Scientists have “come to the realization that pretty much everything that humans do, there are rudiments, at least, in other animals.”