Washington, May 16 : Duke University researchers have found in a study that smaller primates do not have to invest more energy in climbing than in walking.
Jandy Hanna, a faculty member at the West Virginia School of Osteopathic Medicine in Lewisburg who was a Duke graduate student at the time of the study, has revealed that the new finding is based on a study of energy consumption by five different primate species while negotiating vertical and horizontal treadmills.
She said that the surprising finding may help gain a better understanding as to what encouraged the tiny ancestors of modern humans, apes and monkeys to climb into the trees about 65 million years ago, and stay there.
"We assumed it would be more energetically expensive for all of them to climb than to walk, so this finding was unexpected. There's this longstanding assumption that it should cost more to go up," said Hanna.
She built a novel climbing treadmill for her study, essentially a pool of rope around two pulleys, to measure the animals' efforts.
She used sensors to measure oxygen level changes amongst the animals as they moved at their highest sustainable speed, and thereby derived the primates' energy consumption.
While climbing was not significantly more demanding for heftier primates than lighter ones, "the energetic cost of walking decreased with size," said Timothy Griffin, a medical instructor at the Duke Medical Center's Orthopaedic Bioengineeing Laboratory.
As a result, species weighing more than half a kilogram might have more incentive to walk than to climb.
For the animals weighing less, however, "there was no difference," Griffin added.
Daniel Schmitt, a Duke associate professor of biological anthropology and anatomy who was Hanna's doctoral dissertation advisor, said that scientists thought that the earliest primate ancestors, who were only the size of large rats, underwent a number of fundamental evolutionary changes as they adapted to moving and feeding on thin branches of trees 65 million years.
"Those changes included developing grasping hands with nails instead of claws. They were climbing up into the canopy and staying there. What we have shown is that they could have made this shift into a rich environment with insects and fruits without increased energetic cost," Schmitt said.
An article describing the study has been published in the journal Science.