Rats that play hide and seek.
Ready or not, here they come: Scientists who played hide-andseek with rats found that their furry subjects seemed to love the game — and they were remarkably good at it.
The unconventional experiment, described in the journal Science, sheds light on the sophisticated sense of play in these tiny rodents and the complex mechanisms at work in their brains. It also hints at the evolutionary usefulness of this type of play.
“I thought it was a major scientific contribution to the field,” said Jeffrey Burgdorf, a neuroscientist at Northwestern University who was not involved in the study.
In recent decades, scientists have begun to explore the neural, behavioral and evolutionary underpinnings of play.
Play is confusing because it’s done with no apparent purpose other than for its own sake, and yet all kinds of animals — from rats to elephants to humans — engage in it. In some ways, play appears to be an essential part of young mammalian development.
These behaviors probably help train the brain in some way, said Michael Brecht, a neurobiologist at the Humboldt University of Berlin.
“Many people think play and fun and all of these things are kind of trivial behaviors, but I think the opposite is the case,” said Brecht, the study’s senior author.
Researchers have documented simple types of play in all kinds of mammals. That includes laboratory rats, which have even been found to emit ultrasonic “giggles” when they’re tickled.
But Brecht said he and his colleagues wondered about accounts from pet owners who said their beloved rats could engage in a more complex game: hide-and-seek.
Compared with something like playful wrestling, hide-andseek is more complex for several reasons. It requires an understanding of the rules, a clear grasp of players’ distinct roles, and the ability to assume different roles on different rounds.
The researchers taught six adolescent male rats how to play a one-on-one version of hideand-seek. They outfitted a large room with cardboard barriers and small containers to serve as hiding places for humans and rats, respectively. The game started when the rat was placed in a small box in the middle of the room.
If the rat was the “seeker,” the scientist would hide and then remotely open the box. If the rat was the “hider,” the scientist would crouch by the box when the rat came out, prompting the little rodent to scurry for cover. All six rats learned how to be the seeker; five of them were able to handle hiding as well.
Typically, in experiments with lab rats, researchers offer food as a reward. But Brecht and his colleagues knew that rats can be trained to perform very complex sets of tasks just for a food reward, and they wanted a more natural response.
So when the scientist found a hiding rat or was discovered by a seeking rat, the animal was “rewarded” with petting, tickling or playful roughhousing before the game was reset for another round.
The rats turned out to be remarkably sophisticated players. If the scientists let them peek, the rats used visual cues to find them faster. The animals also checked hiding spots that their opponent used repeatedly. When the human was found, the rats made ultrasonic calls — which the scientists measured but couldn’t hear — that could be reminiscent of a seeker’s triumphant “Found you!”
The rats’ strategies completely changed when they were in the role of hider. They often switched up their hiding places and preferred to take shelter in opaque boxes over transparent ones. They didn’t make the same vocalizations when they were found, an indication that they were trying their best to remain hidden.
In fact, they’d often prolong the game by running away from the scientist and re-hiding, thus delaying the social interaction — a sign that the rats were playing for the fun of the game, not for any reward.
There were other signs that the rats enjoyed the activity, Brecht said. They frequently did “joy jumps,” or freudensprung, teased the scientist, and made lots of calls when the game ended and when it began.
In some ways, the rats trained the scientists how to play, too. The researchers found that whereas the rats loved to hide, they would run out to check on the scientist if he or she took too long in finding them. So the scientists had to shorten the time they took in finding the sequestered rodents.
“Certainly it went both ways,” Brecht said of the training.
While the rats played, the researchers recorded their brain activity from individual neurons in the medial prefrontal cortex, a region associated with rules and social proximity. They found that the rats’ neurons responded in very specific ways to different game events. For example, one neuron in the infralimbic cortex would spark only at the start of the seeking rounds, when the animal was cued to the role he was to play.
The fact that the rats so quickly picked up the rules, and could play with such sophistication, hints that hide-andseek might not be such a foreign concept to these animals, Brecht said. Indeed, he said the behavior is probably widespread in the animal kingdom, though exactly how many species might engage in it remains unsettled.
“Our whole thinking is that hide-and-seek might actually be a very old game,” Brecht said, “maybe more (like) 100 million years old than a few thousand years old. And that this is part of this repertoire. We were struck at how good they were at it.”
Burgdorf, who was not involved in the study, said the ability to track individual neurons, particularly in a relatively freer setting than typical reward-based laboratory experiments, was an impressive feat.
“This allows us to be able to study the basic mechanism of emotion at the level of a single neuron,” he said. “We couldn’t do this before.”
Peggy Mason, a neurobiologist at the University of Chicago who was not involved in the work, agreed that the neural readings were a “tour de force” but said that she was most impressed by the discovery that the rats didn’t just learn hideand-seek — they wanted to play it.
“They’re grooving on the game, and that’s pretty amazing,” Mason said. “To me, the behavioral results drive our thinking forward a lot.”