Reinhold, Sanguinetti-Scheck, Hartmann, and Brecht

Annika Reinhold says that she likes playing with animals (she has two cats) and “doing unconventional things that no one has done before.” When the chance came up to teach rats to play hide-and-seek, she was a natural candidate.

One might question the wisdom of training rats to hide, but there’s a good reason to do so. In neuroscience, animal research is traditionally about control and conditioning—training animals, in carefully regulated settings, to do specific tasks using food rewards. But those techniques aren’t very useful for studying the neuroscience of play, which is universal to humans, widespread among animals, and the antithesis of control and conditioning. Playing is about freedom and fun. How do you duplicate those qualities in a lab?

After watching YouTube videos of pets and their owners, Michael Brecht, a neuroscientist at the Humboldt University of Berlin, came up with the idea of using hide-and-seek. Reinhold, a master’s student in his lab, jumped at the chance. She knew that rats are social, intelligent, and playful, and will chase, roughhouse, and wrestle with one another, much like human children do. Perhaps they’d play with her. “I was optimistic enough to try it,” she says.

She began by getting six adolescent rats accustomed to a 300-square-foot room fitted with boxes and barriers behind which they (or Reinhold) could hide. She also habituated the animals to her by stroking them, chasing them with her hands, and tickling them.

After one month, Reinhold taught them to seek. She placed them in an open box and walked to a different part of the room. If they approached her, she rewarded them with tickles. At first, she stood in the open, but once the rats got the hang of approaching her, she incrementally raised the difficulty by partially hiding, then hiding fully while the rodents were watching, and finally hiding while the box was closed. Similarly, to teach them to hide, she rewarded them for leaving their box, then for scurrying to another area, then for finding and staying in a hiding spot. Crucially, she never remunerated the rats with classic rewards such as water or food. She treated them only to tickles and social contact.

All six rats learned to seek, and five also learned to hide. They clearly understood the rules of the game, and played strategically. When seeking, they searched systematically, beginning with past hiding locations. When hiding, they chose opaque boxes instead of transparent ones and kept quieter. They also seamlessly switched between the two roles, taking their cue from whether the starting box was closed (indicating “seek”) or open (indicating “hide”).

The rats learned the game in only a couple of weeks, which is “impressive in neuroscience,” says Juan Ignacio Sanguinetti-Scheck, who also took part in the study. “Animals can take months to learn tasks, even monkeys, but we’re generally trying to teach them to use joysticks or things they’d never do in normal life.” Hide-and-seek, by contrast, draws on behaviors such as concealing, finding peers, and switching roles, which aren’t just natural parts of rat life, but also frequent parts of rat play. In retrospect, it was the perfect game for uniting two different species. “It’s a clever and innovative approach,” says Gordon Burghardt of the University of Tennessee. “Many animals play with other species and engage in peekaboo, tug of war, or tag, but this example does seem unexpectedly complex.”

Why did the rats play along? It’s possible that they were going after tickles and other social rewards. After all, for two decades researchers have known that rats enjoy being tickled and react by producing ultrasonic chirps that can be compared to laughter. But Reinhold found that once discovered, the rats would often run away and re-hide, delaying their reward and prolonging the game itself. “It seemed really playful,” she says.

She and her colleagues believe that rather than pursuing rewards, the rats were playing for the sake of it. They played because they had fun. For a start, and this is an unusual but welcome line to see in a scientific paper, “the animals looked like they are having fun,” the team writes. When they reunited with Reinhold, they frantically jumped on the spot—a behavior delightfully known as freudensprung, or “joy jumps.” They also teased Reinhold by repeatedly getting close and running away.

The rats also behaved quite unlike rodents that take part in typical studies for typical rewards. When conditioned with food, rats are usually silent and can tirelessly perform hundreds of trials. Reinhold’s rodents were eager to play, highly vocal, extremely active, and tuckered out when each block of games was over. “It’s rare in neuroscience to see rats that are so engaged in a task,” Sanguinetti-Scheck says. “I’ve never seen a rat run so much in a lab.” It’s both heartening and a little tragic to see what rats are really like when they’re just allowed to be rats.

For many people, the idea of rats playing for fun would be wholly unsurprising, but there’s a long history of denying human emotions to animals for fear of anthropomorphism. Certainly, other species do sometimes lack mental skills that one would assume they had, and calling for objectivity is reasonable. But “it’s also important to not be completely obtuse and disregard the fact that we share a lot with animals,” Sanguinetti-Scheck says.

“Juvenile rats are one of the most playful mammals known,” says Lynda Sharpe of the Australian National University. “Although pet-rat owners won’t be surprised by these findings, the study is valuable because it demonstrates the cognitive complexity of play in this species.” Indeed, hide-and-seek might help tease out what mental feats rats are really capable of. In switching roles, for example, are they taking on the perspective of their human partners, showing what researchers call “theory of mind”?

The team has started analyzing the rats’ brains mid-game, by implanting wireless electrodes in their medial prefrontal cortex (mPFC)—a brain region associated with decision making, social interactions, and (in humans, at least) theory of mind. The electrodes have revealed that individual neurons in a rat’s mPFC will fire at very specific moments during a game. One neuron, for example, went off only when the rat was sealed inside the starting box at the beginning of seeking rounds. By looking at this cell’s activity, Sanguinetti-Scheck could count how many rounds Reinhold had played with the animal, without having to watch them. “The rat has agency throughout the game, so some trials can be very fast and some can be very long,” he says, “but regardless, [neurons in the mPFC] are responding to specific events.”

It’s not clear what that means, or what the region is specifically doing. But studies like this—brain recordings from free-running rats playing a game they’re actually engaged with—might help provide answers. “The rats are doing incredible behaviors that involve many things that neuroscience is preoccupied with, like decision making,” Sanguinetti-Scheck says. How does the animal decide where it wants to seek, or whether it wants to hide? Or what happens when it’s surprised? When it thinks it knows where the experimenter is, what happens in its brain when she isn’t there? “There are many things that we can now use hide-and-seek to start studying,” he says.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.