Using tools doesn’t make humans, dolphins, and crows smart. Rather, it’s the stress and challenge of living with others—recognizing friend from foe, calculating who to deceive and who to befriend—that led these and other social creatures to evolve their cognitive skills. That’s the gist of the social intelligence hypothesis, an idea that’s been around since 1966. But does having to remember whose lice need picking actually improve other mental abilities, like figuring out how to open a locked box with a hunk of meat inside? A new study of four carnivores—two social and two solitary species—suggests that it does.
“They’ve taken an important issue and tested it in a simple but novel way,” says Richard Byrne, an evolutionary psychologist at The University of St. Andrews in the United Kingdom, who was not involved in the study. “The results are clear: The cognitive benefit from being a social carnivore does transfer” to a mental ability that has nothing to do with being social, he says.
Other researchers think the results aren’t as clear-cut. “It is important and a valuable stepping stone in our quest to understand how intelligence evolved, but like all studies, it is one piece of a larger puzzle,” says Sarah Benson-Amram, a zoologist at the University of Wyoming in Laramie, whose recent comparative study of 39 species of carnivores reached the opposite conclusion.
Scientists devised the social intelligence hypothesis to explain the evolution of the human brain. They’ve found that most social species (from chimpanzees to social wasps) have relatively large brains and are cognitively sophisticated, adept at experiments designed to test their smarts. But some researchers argue that another factor—a challenging environment—may also stimulate cognitive evolution. If so, then more solitary species could also be large-brained and smart thanks to the ecological difficulties they face.
“I thought that carnivores offered a good way to test these two hypotheses,” says Natalia Borrego, who was a behavioral ecologist at the University of Miami in Florida at the time the study was conducted and the study’s lead author. She notes that the species she and her University of Miami colleague, Michael Gaines, selected to study are related, but socially distinct. All are in the family Carnivora. The spotted hyenas (Crocuta crocuta) have hierarchical societies similar to those of primates; lions (Panthera leo) live in egalitarian prides with as many as 21 members; whereas tigers (P. tigris) and leopards (P. pardus) lead more solitary lives, except when females have young or when males and females meet to mate. The four species also pursue widely distributed prey in similar patchy and challenging habitats, and they need flexible hunting strategies. Yet lions and hyenas typically cooperate with their own kind to bring home the bacon, whereas leopards and tigers hunt alone.
To find out which, if any, of these carnivores were better at solving a problem they’d never previously encountered, Borrego devised a large, rectangular box of marine-grade polymer that could be opened only by pulling a rope away from the box at a 180° angle. The rope was attached to a spring latch. She baited the box with raw meat, and she drilled holes in the box’s sides so that the prize could be seen and smelled.
Between May 2012 and May 2015, Borrego placed the box inside the outdoor enclosures of the four species at wildlife sanctuaries, parks, and zoos in Florida and South Africa. Each animal, other than the hyenas, encountered the box alone and for three 10-minute trials. Because of constraints at the hyena facilities, one to four animals were tested at a time. To ensure the carnivores were motivated, none were fed for 24 hours before the experiment. They could use either their mouths or paws to open the box. “I wasn’t sure if they would even approach it,” Borrego says, because many animals regard novel items as dangerous.
She tested 48 individuals and found that the social animals—hyenas and lions—were the most successful. Eight out of nine hyenas, and 16 of the 21 lions correctly pulled the rope (as in the photo) and seized the meat; whereas only six of the 11 leopards and two of the seven tigers did so (see video, above). Lions were also the most exploratory species, circling, digging, biting, pawing, and pushing the box, the team will report next month in Animal Behaviour.
“This isn’t a task that requires social cognition,” Borrego says. “Yet, the social species were better at it, and that suggests there’s something about being social that bolsters cognition overall.”
Other researchers concur, but with caveats. “They did find a nice link between sociality and success” on this task, says Evan MacLean, a comparative psychologist at Duke University in Durham, North Carolina. But he wonders what type of cognition the ability to open a puzzle box actually demonstrates. “It may be reflective of trial and error learning, insight, or just of curiosity or interest in novel objects.”
The puzzle box is also not particularly “ecologically relevant,” to the carnivores, notes primatologist Frans de Waal at Emory University in Atlanta, who would like to see the animals tested on some type of predator-prey task. Still, it is “a good first step and a fresh approach to the intelligence of carnivores, a group we have neglected for too long.”