Some chimpanzees are smarter than others, and their intelligence differences may largely be explained by the genes they inherit, a U.S. study said Thursday.

For the animals that are highly intelligent and genetically similar to humans, environmental factors might be less important than scientists previously thought, according to the study published in the U.S. journal Current Biology.

"As is the case in humans, genes matter when it comes to cognitive abilities in chimpanzees," lead author William Hopkins, professor in the Center for Behavioral Neuroscience at Georgia State University, said in a statement.

"It doesn't mean that they are the only factor determining cognitive abilities, but they cannot be ignored," said Hopkins, who is also a research scientist in the Yerkes National Primate Research Center at Emory University.

The role of genetics in intelligence has long been debated in scientific circles. It is now clear from previous studies that humans' performances on intelligence tests do depend to a large extent on genetics, but it can be modified by environmental factors, such as formal education and socioeconomic status.

The role that genes play in animal intelligence, however, has received considerably less attention.

The new study involved 99 chimpanzees, ranging in age from 9 to 54, who completed 13 cognitive tasks designed to test a variety of abilities.

The researchers found that some, but not all, cognitive abilities were significantly heritable in chimpanzees.

In addition, they 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 those raised by their chimpanzee mothers.

"The suggestion here is that genes play a really important role in their performance on tasks while non-genetic factors didn't seem to explain a lot. So that's new," said Hopkins.

Since chimpanzee performance on cognitive tests isn't complicated by environmental factors, studies of chimpanzees could add significantly to scientists' understanding of intelligence, the researchers said.

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 added.

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