DURHAM, N.C. -- A new study representing the largest study of animal intelligence to-date finds that animals with bigger brains and broader diets have better self-control.
Published this week in the journal Proceedings of the National Academy of Sciences, the study is part of a long history of research aimed at understanding the animal mind. Specifically, why are some species able to do things like make and use tools, read social cues, or even understand basic math, and others aren't?
Until now, most studies of animal intelligence have focused on only one or a few species at a time, explained co-author Evan MacLean of Duke University. But MacLean and other researchers working at the National Evolutionary Synthesis Center came up with an idea: if they could convince enough animal experts across the globe to conduct the same set of experiments, they could test ideas about how cognitive differences in the animal kingdom came to be in a much more rigorous way that wasn't possible before.
Working with nearly 600 animals representing three dozen species of birds and mammals in zoos and research facilities worldwide -- ranging from wolves in Austria and jays in the U.K. to spider monkeys in Mexico -- the researchers put the animals through two tests designed to measure "inhibitory control" -- a measure of brain function associated with the ability to control impulses and delay gratification. Most animals don't have to worry about buying one too many drinks or avoiding the all-you-can-eat buffet. But the ability to exhibit self-control could help animals wait for just the right moment to pounce on their prey, for example, or share food with relatives without first eating it all themselves.
In one test, researchers hid a piece of food in an opaque plastic cylinder made from a 2-liter soda bottle, and trained the animals to retrieve the treat by reaching around to one of the open ends. Then they switched out the opaque cylinder for a see-through one. To fish out the food, the animals had to resist the natural impulse to reach directly in front of them for the treat -- which would only cause them to bump their paws, beaks or trunks into the plastic -- and instead reach around through a side entrance as they learned before.
In another test, the researchers showed the animals where to find a treat hidden in one of three cups placed upside down on a table, then moved the treat to a different cup while the animals looked on. To get it right, the animals had to suppress the urge to keep looking under the first cup and search under the new cup instead.
Bonobos, gorillas and orangutans performed well on both tests, picking the correct cup or reaching for the right opening more than 90% of the time. But squirrel monkeys and a lesser-known primate called the Coquerel's sifaka did miserably, getting it right less than half the time.
Led by MacLean and Brian Hare and Charlie Nunn of Duke University, the researchers looked at a range of factors that could explain why some species excelled and others floundered.
Overall, the species with the highest scores on the self-control tests had bigger brains. Absolute brain size mattered, but relative brain size did not -- suggesting that species differences in self-control may be due to differences in how the brain is wired, rather than having big brains for their bodies per se.
In addition to brain size, the researchers also looked at whether lifestyle factors might play a role. One of the most widely accepted ideas is that animal intelligence evolved to deal with the demands of living in a group. The bigger the group, the better an animal needs to be at keeping track of social relationships and managing the delicate balance between competing and cooperating.
Surprisingly, group size didn't seem to matter for self-control, but within primates the researchers found support for another idea: Primates with superior self-control had more diverse diets, supporting the notion that animals owe their intelligence to the challenges of foraging for a wide array of foods, like fruit, that vary in ripeness and availability from one place and season to the next.
"Animals that rely on lots of different foods can't keep foraging in the same spots," MacLean said. They have to resist the tendency to return again and again to, say, the same fig tree or cache of nuts in favor of exploring new areas.
Previous studies have found links between various measures of cognitive ability and traits such as brain size or diet, but no other study has demonstrated a connection across such a wide range of species or on such a large scale.
Future studies will use their worldwide network of animal experts to try to understand how other aspects of the animal mind came to be.
"One particular aspect of cognition might be critical for one species but useless for another," MacLean said. "Intelligence isn't one-dimensional."
This study stemmed from a working group on comparative psychology that met from 2009 to 2011 at the National Evolutionary Synthesis Center in Durham, North Carolina. Learn more about their research at http://www.nescent.org/science/awards_summary.php?id=22 and https://www.nescent.org/wg_phylopsy/Main_Page.
CITATION: MacLean, E., et al. (2014). "The evolution of self-control." PNAS doi: 10.1073/pnas.1323533111
The National Evolutionary Synthesis Center (NESCent) is a nonprofit science center dedicated to cross-disciplinary research in evolution. Funded by the National Science Foundation, NESCent is jointly operated by Duke University, The University of North Carolina at Chapel Hill, and North Carolina State University. For more information about research and training opportunities at NESCent, visit http://www.nescent.org.
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