Climbing: A Cake Walk for Some Primates

May 16, 2008 -- Scientists have long thought that walking is easier than climbing for primates -- explaining why we humans ended up on our feet all the time.

But researchers at two U.S. universities have found that, at least for smaller primates like squirrel monkeys and lemurs, climbing is no more difficult and energy-consuming than walking.

And that could explain the evolutionary conundrum of why some 65 million years ago the tiniest ancestors of humans, apes and monkeys climbed into the trees and never came down.

In research published in the May 16 issue of the journal Science, researchers at Duke University in North Carolina and the University of South Alabama in Mobile said they sought to find out if climbing with hands and arms was more energy-intensive than walking for several small primate species.

They tested five species: the slender loris (Loris tardigradus) found in Sri Lanka; the pygmy slow loris (Nycticebus pygmaeus) of Indochina; the fat-tailed dwarf lemur (Cheirogaleus medius) and the mongoose lemur (Eulemur mongoz), both of Madagascar; and the Bolivian squirrel monkey (Saimiri boliviensus).

Duke University's Daniel Schmitt, one of the leaders of the study, said scientists had assumed that gravity always made walking less energy-intensive than climbing.

"We thought climbing would always be more expensive" for the animals, Schmitt said.

The researchers had to design a "climbing treadmill" -- essentially a loop of rope around two pulleys -- to measure the animals' exertion while climbing.