Source: Credit: Reinhold, Sanguinetti-Scheck, Hartmann & Brecht.

In a new study illuminating playful behavior in animals, scientists taught rats to play a simplified, rat-versus-human version of hide-and-seek.

Rats picked up the game within a week or two and learned how to alternate between hiding and seeking roles. Rather than offering food, the researchers rewarded successful hiding and seeking behaviors with playful social interactions, such as tickling and petting.

The game worked like this: First, the rat would jump into the start box. In “seek” trials, the experimenter closed the lid of the box, signaling that the rat would be the seeker. Then the experimenter hid in one of three locations. Once found by the rat, the experimenter gave a few tickles before returning the rat to the start box. In “hide” trials, the experimenter left the start box open and crouched still next to it, signaling it was the rat’s turn to hide. The rat had 90 seconds to choose one of seven hiding spots. Then the experimenter searched for the rat, rewarded the rat with some playful , and returned it to the start box.

The researchers found the rats were strategic players, behaving differently when assigned seeker or hider.

“We were very impressed with the animals’ play performance,” says Michael Brecht of the Humboldt-Universität zu Berlin, senior author of the study. He says seeking strategies included systematic searches, use of visual cues, and use of to target past hiding locations. When hiding, rats became silent and preferred opaque cardboard boxes to transparent ones.

Brecht says that, except when hiding, the rats were very vocal during play. He and his colleagues noted different vocalizations during different game events. For example, the rats always made noises when jumping out of the start box to begin a game. Brecht says the rats also always vocalized when they found the hidden experimenter, although they were silent when they themselves are found.

“We wonder if this relates to winning and losing,” says Brecht. “In human hide-and-seek, when you are found, you lose. When the rats are found, the don’t make any sounds and they look a bit startled.”

Brecht and his colleagues also recorded the activity of single neurons in the rat medial prefrontal cortex—a brain area that in humans is involved in perspective-taking, social , and rule-following—during games of hide-and-seek. They found neuronal activity specific to different phases of the game, such as the closing of the start box (which served as a cue that the rat would be the seeker).

As far as the question of why the rats played hide-and-seek, Brecht and colleagues argue, the evidence suggests it was for the pleasure of the game.

They say the animals looked like they were having fun: moving with quick, directed movements; frantically searching; teasing the experimenter; and making freudensprung (“joy jumps”). The rats seemed eager to engage in the game. Additionally, the rats’ behavior appeared to be purposeful, as shown by their strategic acts in each role. And, when found, rats often ran away and re-hid, prolonging the game.

Brecht and his colleagues wonder if role-play games like hide-and-seek are evolutionarily old.

“Our thinking is that we tapped into something the rats can already do,” he says. “We think it might be a very old game that rats play naturally, though we have not proven that.”