But animals are hard to work with. You need to design tests that objectively assess their mental skills without raising the spectre of anthropomorphism, and you need to carefully train them to perform those tests. These difficulties mean that researchers mostly resort to small experiments with just one species, often with their own bespoke tasks. This makes it very hard to compare species or pool the results of separate studies. If a lemur behaves differently to a monkey in separate experiments, is it because of some genuine biological difference, or some quirk of the respective studies?

These problems mean that the study of animal intelligence is rich but piecemeal. Each study adds a new piece to the jigsaw, but is everyone even solving the same puzzle?

Evan MacLean, Brian Hare, and Charles Nunn from Duke University have had enough. They led a international team of 58 scientists from 25 institutes in studying the evolution of one mental skill—self-control—in 567 animals from 36 species.

Chimpanzees, gorillas, baboons, marmosets, lemurs, squirrels, dogs, elephants, pigeons, parrots and more tried their hands (or trunks or beaks or snouts) at the same two tasks. “It was literally a mouse-to-elephant study,” says MacLean, “or at least a Mongolian-gerbil-to-elephant study.”

“I think it’s really showing the future of the field of cognition,” says Karin Isler from the Universtiy of Zurich. “Instead of just giving glimpses and suggestions, and sometimes contradicting evidence, one can find convincing explanations for the evolution of cognitive abilities.”

The team focused on self-control—the ability to stop doing that, put that down, eat that later. Animals exercise it when they stop themselves from mating in the presence of a dominant peer, when they forgo an existing source of food in favour of foraging somewhere new, or when they share resources with their fellows. In humans, a child’s degree of self-control correlates with their health, wealth, and mental state as adults. It’s important.

It’s also easy to measure. Swiss psychologist Jean Piaget did it in the 1950s when he repeatedly put a toy under a box in front of some infants, and then moved it to a second box. He found that babies under 10 months of age would keep on searching under Box A, despite what they had seen. They couldn’t resist their old habit to do something flexible and different; that ability only kicks in around our first birthday. MacLean, Hare and Nunn’s team gave this “A-not-B” test to their animals, using food rather than a toy.

They also tried a second task, where animals had to reach round the side of an opaque cylinder to get at food within. The team then swapped the opaque cylinder for a transparent one. Now, the animals had to hold back their natural instinct to reach directly for the food (which would have knocked the cylinder over), and go around as before.

The team tested all their animals on one or both tasks, and compared their performance to traits like brain size or group size. They found a few surprises. For example, the animals’ scores correlated with the absolute but not relative sizes of their brains. In other words, it didn’t matter whether the animals’ brains were big for their size, but whether they were big, full-stop.

“That’s funny because brain size and body size scale predictably. Big animals have big brains,” says MacLean. As such, many scientists believed that relative brain size mattered more. There’s even a measure called the encephalization quotient (EQ) that estimates intelligence by comparing an animal’s brain to that of a typical creature of the same size. And yet, for self-control at least, it’s absolute size that’s important. That was true whether they looked at all their 36 species, or just at the primates.

“That makes sense,” says Richard Byrne at the University of St Andrews. “If the brain is, to some extent, an on-board computer, it will be the number of components that determine its power [rather than] the size of the carrying case or body.”

The team also tested two leading explanations for the evolution of primate intelligence. One idea says that our smarts evolved so we could keep track of the relationships within our complex social groups. Indeed, you can make a decent guess about the size of community that a primate lives in by measuring the size of its skull. But the team found no link between group size and performance in their tasks. “That surprised us,” says MacLean. “It’s such a popular hypothesis but we found no evidence for it.”

Instead, the team found more support for a second idea: that primate intelligence was driven by the need to keep track of a wide range of food like fruit, which vary by place and season. They showed that the variety in the animals’ diets (but not the proportion of fruit) was indeed linked to self-control. Together, these two factors—absolute brain size and dietary breadth—explained around 82 percent of the variations in the primates’ scores.

“The nice thing about the tasks is that, because of their simplicity, they are very unlikely to depend a lot on species-specific aptitudes unrelated to cognition or to prior experience,” says Byrne. “I’d trust the results.”

But Robin Dunbar from the University of Oxford felt that the team’s conclusions are “misguided and naive” because their tasks weren’t a good measure of self-control, at least in any sense that matters in an animal’s social life. Instead they were “straight ecological or foraging tasks and nothing more, so it’s not awfully surprising that it correlates with diet,” he says.

Brain-scanning studies in humans and monkeys have also found links between the size of specific brain regions, size of social groups, and social skills. “It seems bizarre to be running an analysis against measures of total brain size,” says Dunbar.

Of course, this study just looked at one aspect of animal psychology, among many. The team found that the animals’ scores on the self-control tests did correlate with reports of other skills, like innovation, tool use, deception, and social learning. But MacLean suspects that if other teams focused on these skills, they would find different results. Group size may become more important if researchers focused on tasks that looked at social learning—the ability to imitate and learn from others. Alternatively, diet may again win out if scientists looked at memory skills.

This new study doesn’t settle the debates. It just points to a way forward. Each of the scientists in the team could easily have published their own papers using the collected data, but they decided to combine their efforts into one publication. “We thought it would be most powerful if it came out together,” says MacLean. “There’s never been a data set this size. We’re certainly hoping that it’s a game-changer in the way we do comparative psychology.”

And even Dunbar says, “It’s good to see comparative studies of this kind being done at last, and it’s very worthy that they have done the same task on many species.”