As with people, some chimpanzees are smarter than others, and as with people, a lot of that variation in intelligence depends on the genes that individuals carry and pass on from one generation to the next.

A new study found no effect of either sex or rearing history on the cognitive skills of chimpanzees. That is, chimpanzees raised by human caretakers performed no better on cognitive tests delivered to them by humans than did individuals raised by their chimpanzee mothers.

The role of genetics in intelligence has long been debated in scientific circles, the researchers say. It is now clear from previous studies that humans' performances on IQ tests do depend to a large extent on genetics, even if it can be modified by environmental factors. But the role that genes play in animal intelligence had received considerably less attention.

The new study included data on the cognitive abilities of 99 chimpanzees in all, from age 9 to 54. The researchers' analysis found that about 50% of the variation in the chimps' performance on a series of standardized cognitive tests could be attributed to genetic factors.

"As is the case in humans, genes matter when it comes to cognitive abilities in chimpanzees," says William Hopkins of the Yerkes National Primate Research Center. "It doesn't mean that they are the only factor determining cognitive abilities, but they cannot be ignored."

Studies of chimpanzees could add significantly to scientists' understanding of intelligence, the researchers say. That's in part because, unlike humans, chimpanzee performance on cognitive tests isn't complicated by factors related to school systems or other sociocultural complexities.

The findings suggest that differences in cognition may have arisen in the common ancestor of humans and chimpanzees about 5 million years ago. The findings may also lead to the discovery of particular intelligence-related genes.

"What specific genes underlie the observed individual differences in cognition is not clear, but pursuing this question may lead to candidate genes that changed in human evolution and allowed for the emergence of some human-specific specializations in cognition," Hopkins says. "It is also intriguing to consider what changes in cortical organization might be associated with individual differences in cognition and whether common genes might explain their common variance."