Apes Rule the Planet

Smart ape: chimpanzee. David Trood

The great apes are the smartest of all nonhuman primates, with orangutans and chimpanzees consistently besting monkeys and lemurs on a variety of intelligence tests, Duke Medical Center researchers have found.

"It's clear that some species can and do develop enhanced abilities for solving particular problems," says Robert O. Deaner Ph.D. '01, who led the study as part of his dissertation. "But our results imply that natural selection may favor a general type of intelligence in some circumstances. We suspect that this was crucial in human evolution."

Experts in psychology broadly define intelligence as general problem-solving abilities—"domain-general cognition," in the parlance of the field. This intelligence allows an animal to tackle new and unpredictable situations.

Intelligence testing of animals has repeatedly revealed that some species perform better than others, suggesting that some animals have better domain-general skills, says Deaner, now an assistant professor of psychology at Grand Valley State University. However, scientists have been hard-pressed to prove that these differences could be attributed to intelligence.

"The trouble is that one species may outperform another in a problem-solving test not because it's smarter, but because one species is poorly suited to that particular testing situation," he says. For example, one species may be more comfortable grabbing a joystick.

Deaner and his colleagues reasoned that they could refute this premise by demonstrating that some primate species surpassed others across a range of problem-solving tests. Primates are an excellent comparison group because their similar perceptual and motor skills mean that the same tests are generally appropriate for all of them, Deaner says.

The team examined hundreds of published studies by comparative psychologists and assigned each testing situation or experiment to one of nine overall paradigms. For example, one paradigm was patterned strings. During the test, a primate is shown an array of crossing strings, only one of which is tethered to a treat. The subject is allowed to pull only one of the strings and must decide before pulling which string is actually attached to the food. The paradigm tests the ability to form spatial representations.

The results were clear: The smartest species were the great apes—orangutans, chimpanzees, and gorillas—which, over all, outperformed monkeys and prosimians.

"The fact that great apes performed better than other primates in these laboratory tasks is reassuring," says Carel van Schaik, a study co-author and director of the Anthropological Institute and Museum at the University of Zurich. "After all, in absolute terms, their brains are the largest, and they show the most sophisticated behavior." The study was published online in the journal Evolutionary Psychology.