An African lion gazes up at a suspended wood box. Inside is a hunk of raw beef. To enjoy the snack, the lion needs to yank on a rope descending from the box, which is attached to a spring-loaded door latch. The scheme: to test the charismatic cat's cognitive abilities.

The social intelligence hypothesis posits that having to navigate a complex communal life, which involves challenges such as keeping track of who is a friend and who is an enemy, has pushed group-living animals to evolve the mental machinery required to solve and remember mental tasks such as the box puzzle. In other words, social complexity leads to cognitive complexity.

Researchers have long explored this idea by observing animals such as chimpanzees, dolphins and elephants, but biologist Natalia Borrego of South Africa's University of KwaZulu-Natal focuses on big cats. “You have a lot of closely related species with these diverse ecological challenges and different social systems,” she says.

Borrego and her team presented the rope challenge to 12 captive lions at Florida's Lion Country Safari. Eleven of them successfully solved it: seven on their own and four after watching another lion do it. Ten of the 11 recalled the solution five to seven months later. The results were recently published in the journal Animal Cognition.

“That they remember what they've learned isn't terribly surprising,” says Oakland University cognitive psychologist Jennifer Vonk, who studies cognition in bears. But she finds the social facilitation—the fact that some individuals figured it out after being paired with another lion—particularly exciting. “We don't always see those kinds of effects—even in primates,” she adds.

In a follow-up experiment using a similar conceptual puzzle, lions outperformed leopards and tigers (which are both solitary big cats)—more evidence for the social intelligence hypothesis. But Borrego acknowledges that habitat and diet could also be factors in cognitive evolution. “The evolution of cognitive complexity is itself complex,” she says.