Why do humans and dolphins evolve large brains relative to the size of their bodies while blue whales and hippos have brains that are relatively puny?
While there has been much speculation regarding brain size and intelligence, a new paper in the Proceedings of the National Academy of Sciences confirms that species with brains that are large relative to their body are more intelligent.
“This is an important paper because it represents a novel and rigorous experimental test of the relationship between brain size and problem-solving ability, using mammalian carnivores as our test group,” said Kay Holekamp, Michigan State University Distinguished Professor of integrative biology, and senior author. “Our results are robust, showing that having a larger brain really does improve the animal's ability to solve a problem it has never encountered before.”
Brain size is often used as a proxy for cognitive ability. Whether brain size can predict cognitive ability in animals has frequently been questioned, however, mainly because of the lack of any experimental evidence, she added.
To tackle this lack of scientific data, Sarah Benson-Amram, University of Wyoming scientist and first author, and her collaborators traveled to nine U.S. zoos and presented 140 animals from 39 different mammalian carnivore species with a novel problem-solving task. The study included polar bears, arctic foxes, tigers, river otters, wolves, spotted hyenas and some rare, exotic species, such as binturongs, snow leopards and wolverines.
Each animal was given 30 minutes to extract food from a metal box, closed with a bolt latch. The box was scaled to the animal’s size and baited with each study animal’s preferred food – red pandas received bamboo and snow leopards got steak.
“Does a larger brain imply greater intelligence?” asked George Gilchrist, program director in the National Science Foundation’s Division of Environmental Biology, which funded the research along with NSF’s Divisions of Biological Infrastructure and Integrative Organismal Systems. “This is a key question for those studying brain evolution. These researchers devised a clever puzzle that could be presented to multiple species – and discovered a strong correlation between relatively large brain size and problem-solving ability.”
Overall, 35 percent of the animals successfully solved the problem. The bears had an almost 70 percent success rate, and meerkats and mongooses were the least successful, with no individuals from their species solving the problem.
“This study offers a rare look at problem solving in carnivores, and the results provide important support for the claim that brain size reflects an animal’s problem-solving abilities and enhance our understanding of why larger brains evolved in some species,” Benson-Amram said.
The study also showed that neither manual dexterity nor living in larger social groups improved problem-solving success.
“A hypothesis that has garnered much support in primate studies is ‘the social brain hypothesis,’ which proposes that larger brains evolved to deal with challenges in the social domain,” said Holekamp, who is part of NSF's BEACON Center for Evolution in Action. “This hypothesis suggests that intelligence evolved to enable animals to anticipate, respond to and perhaps, even manipulate the actions of others in their social groups. If the social brain hypothesis can predict success at solving nonsocial problems, then we would expect that species that live in larger social groups should be more intelligent. However, we did not find any support for that prediction in this study.”
Gregory Stricker, MSU integrative biology graduate student, also contributed to this study. Ben Dantzer, University of Michigan, and Eli Swanson, University of Minnesota, were part of this research team as well.
Holekamp, one of the world’s leading behavioral ecologists, has accumulated more than 25 years of data with her long-running hyena study, covering nearly 10 generations, of spotted hyenas. She and her students have published more than 150 scientific papers.