Do Animals, Like People, Favor One Side?

Most people are right-handed, but what about other animals?

A growing body of evidence suggests that chimps, rhesus monkeys, cockatoos, humpback whales and even toads favor one hand, paw, claw or fin over the other. And now it appears another animal, the crow, or at least one species of crow, may be predominantly oriented to the right like people.

Right-Eyed Tool-Makers

The clues are in the elaborate hooking tools the famously clever New Caledonian crow crafts for poking into trees' crevices and yanking out spiders, millipedes, larvae and cockroaches.

This species of crow, which populates the island of Grand Terre, New Caledonia (a Pacific island east of Australia) manufactures the tools from leaf twigs and from the long, stiff narrow leaf of the pandanus tree.

When animal psychologist Gavin Hunt analyzed the tools and markings on leaf remnants left behind after the crows had cut away their instruments, he was able to determine if the birds had used their bills to cut the tool from left to right or from right to left. As he reports in this week's journal of Nature, the vast majority of the nearly 4,000 leaves sampled indicated the birds cut much more often from left to right.

To Hunt that suggests the birds mostly use their right eyes to guide their work as they carved out their instruments. And he points out activities carried out on the right are usually directed by the left hemisphere of the brain.

That scenario rings familiar with people who are predominantly right-handed (and right-eyed and right-legged) and whose language speaking hardware is located in the left side of the brain.

Since energy is a premium in nature, scientists have suggested that people evolved to host their motor and speaking tools on one side the brain to streamline development. The same could be true for the crows.

"Just as people appear to have developed a specialization in the left side of the brain for language, these crows seem to have a specialization on their left side for tool making," said Hunt, who is based at the University of Auckland in New Zealand.

Chimps, Handedness and Peanut Butter

Most individual creatures from dogs to cats to horses and toads show a tendency to favor one side over the other. But scientists are still divided over whether entire species of animals favor one side over the other like the human right-handed majority.

For example, William McGrew, a professor of anthropology and zoology at Miami University in Oxford, Ohio, has studied handedness among chimps in the wild and determined that about half the animals are left-handed and half are right-handed.

"Each individual chimp seems to commit itself to one side or another," he said. "But chimps don't seem to show any overall leanings."

William Hopkins, a psychologist at Berry College in Mount Berry, Georgia, disagrees. His studies with chimps in captivity show the animals overwhelmingly favor using their right hands.

A common experiment he uses to test for handedness is giving a chimp a long tube with peanut butter lodged inside. If the chimp holds the tube with its left hand and probes for the peanut butter with its right hand, the animal is likely right-handed. Hopkins has found that the chimps almost always attack the peanut butter this way.

M. K. Holder, a biological anthropologist at Indiana University in Bloomington, has found no right-handed majority among primates in the wild and argues the artificial environment of captivity and the presence of human influences likely change behavior.

"When animals are housed in artificial social and environmental conditions, eat atypical foods and perform artificial tasks, this can affect behavior," she said.

Humans Not Unique?

But recently new evidence has been mounting that people aren't alone in favoring one side.

A study on humpback whales shows these advanced mammals favor their right sides when snatching prey and when slapping the sea surface. A study on toads found the creatures mostly used their right legs when removing a plastic balloon that researchers had wrapped around their heads. And scientists have found Australian cockatoos appear overwhelmingly left-footed.

The most recent addition of apparently right-oriented New Caledonian has led some to wonder if, when it comes to sharing a preference for one side, most animals are much more like people than previously thought.

"The old idea that this is only in humans and not in other animals is clearly not true," said Hopkins. "I think you could argue a handedness is present in many many animals and it's just a matter of finding them."