Chimpanzees and other great apes are known for their intelligence: They can learn words, play with objects, and even seem to mourn the deaths of their friends. But just as for humans, cognitive abilities vary from one animal to the next.

Now, in one of the largest studies ever conducted on chimp cognition, researchers report that those individual differences are due in no small part to genetic makeup. The study appears Thursday in Current Biology.

Genes determine about half of the variability in chimp intelligence and environmental factors the other half, according to primatologist William Hopkins, of the Yerkes National Primate Research Center in Atlanta, Georgia, and colleagues. (Pictures: "How Smart Are Planet's Apes? 7 Intelligence Milestones.")

Research on animal learning has been focused almost entirely on the contribution from the environment. For most of the 20th century, scientists held that animals were like robots, behaving in predictable ways based only on environmental cues, such as reward and punishment. This new study adds to growing evidence that animals are not passive machines but rather are sharp, active thinkers.

Studies of humans have produced similar estimates to the primate study, suggesting that intelligence is approximately 50 percent heritable. But human development is heavily influenced by cultural factors, such as formal education systems, and so nature and nurture are difficult to tease apart, Hopkins said.

Being one of the closest relatives of humans, he said, "chimps offer a simpler way to think about that question."

Social Survival

Chimps can be surprising in their cognitive abilities, Hopkins noted. Several decades ago, a chimpanzee Hopkins was studying figured out that he could watch real-time video of himself on a nearby television monitor. A video camera was recording the chimp's actions.

The chimp, named Austin, opened his mouth wide to look at his teeth, but couldn't see them well.

"So he went and got a flashlight and shined it into his mouth to see farther down his throat," said Hopkins.

Austin was the smartest chimp Hopkins ever encountered, he said. But he also saw a lot of variability among the animals. "When you're out there working with them all the time, you definitely form some opinions about whether you think they're smart or not so smart," he said.

To find out how much of that variability is due to genetics, Hopkins and his team assessed the cognitive abilities of 99 captive chimpanzees. They used a battery of 13 tests measuring various manifestations of intelligence, such as how the animals dealt with the physical world, reacted to sound, and used tools.

The group of chimps tested had an expansive family tree, ranging from full siblings to fourth and fifth cousins. This allowed the researchers to calculate how well scores on cognitive traits aligned with genetic relatedness.

Two categories of tasks were significantly heritable: those related to spatial cognition, such as learning physical locations, and those that required social cognition, such as grabbing a person's attention. Some chimps are quite clever, making kissing sounds or clapping their hands to draw an experimenter's attention, Hopkins said. "This one is a real measure of intelligence and innovative behavior."

Performance on the two types of tasks did not correlate with sex or with whether the chimps were reared by their mothers or human caretakers, the researchers found.

The findings lend support to the so-called social brain hypothesis, which postulates that human intelligence evolved because it helped our ancestors to manage relationships in large and complex groups, Hopkins said.

That the study found spatial cognition to be heritable also makes sense, noted Josep Call, a comparative psychologist at the University of St. Andrews in Scotland, who was not involved in the study. "Thinking about space is extremely important for a number of animals. It's evolutionarily ancient," he said.

Puzzlingly, though, the study found that other cognitive skills, such as understanding causality and using tools, are not particularly heritable. "Why do they find it for space, why do they find it for social cognition, but they don't find it for tool use?" Call wondered. The ability to use tools, after all, would have also been an important skill for survival.

Environment Matters

Although Hopkins and colleagues found a strong genetic component to chimp intelligence, there were equally strong effects from environmental influences, which are malleable over time.

These results are similar to those in human studies, noted Ajit Varki, distinguished professor of medicine at the University of California, San Diego, who was not part of the new study.

What's more, the power of the environment may be underestimated by studies like these, Varki said. "In the impoverished and stereotyped setting of long-term captivity," he said, "the critical influence of environmental variability could be markedly blunted."