Duke University researchers said their finding might explain the evolutionary edge that encouraged the tiny ancestors of modern humans, apes and monkeys to climb into the trees about 65 million years ago and stay there.

The researchers compared the energy consumed by five primate species while negotiating vertical and horizontal treadmills.

"We assumed it would be more energetically expensive for all of them to climb than to walk, so this finding was unexpected," said Jandy Hanna, a Duke graduate student at the time of the study. "There's this longstanding assumption that it should cost more to go up," she added.

While climbing is not significantly more demanding for heftier primates than lighter ones, "The energetic cost of walking decreased with size," said Timothy Griffin of the Duke Medical Center's Orthopaedic Bioengineeing Laboratory. Consequently, species weighing more than about 1 pound might have more incentive to walk than to climb. But for those weighing less, there was no difference, Griffin said.

The study appears in the journal Science.