The source of intelligence in chimpanzees is in genes rather than how they're raised, firmly putting nature over nurture as the basis of smarts in our closest primate cousins, a study indicates.

Since genes influencing chimp behavior probably have links to similar genes tied to human intellect, the study is considered important for an understanding of human cognitive abilities and intelligence, the researchers said.

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

For their study, the researchers administered standardized primate cognitive tests to 99 chimpanzees of ages between 9 and 59 years.

At least half the variations in performance on the tests were down to genetic factors, they found.

"We found that some but not all cognitive traits were significantly heritable in chimpanzees," the researchers said in their paper published in Current Biology.

The gender of the chimps made no difference in their performance in the tests, the researchers reported, nor was there any difference between chimps raised by their own mothers and those raised by human caretakers.

"What specific genes underlie the observed individual differences in cognition is not clear," Hopkins says, but he added he believed further research could lead to the identification of genes that evolved in humans and resulted in some human-specific advances in cognition.

Previous studies have shown genetics has a large impact on humans' performances in IQ tests, even though that performance can be enhanced by environmental factors.

However, until now recent studies of the role genes might play in determining animal intelligence had received less attention.

The study findings suggest differences in intelligence may have first appeared in a common linking ancestor of people and chimps around 5 million years ago.

Cognitive skills in the chimps, which can be involved in problem solving and foraging, may have become inheritable since they have an impact on mating chances and survival, Hopkins says.

"Smarter chimps might gain access to more food resources and mates," he says.

Researching intelligence in chimps is easier than in humans because the chimps' behavior is not modified by the socio-economic factors that can have an effect on human performance in IQ tests, the researchers said.