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 US 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 robe around two pulleys -- to measure the animals' exertion while climbing.
Sensors on the treadmill measured the oxygen levels in the test chamber to determine how much energy was expended by the primate.
Timothy Griffin, a medical instructor at the Duke Medical Center's Orthopaedic Bioengineeing Laboratory and another leader of the study, said they found that for an animal weighing less than 500 grams (1.1 pound), "there was no difference" between walking and climbing, energy-use-wise.
Moreover, for the small primates weighing more than 500 grams, climbing was not significantly more demanding.
Humans' earliest primate ancestors, barely the size of large rats, evolved in special ways for life in the treetops, including developing grasping hands with nails instead of claws, Schmitt notes.
But, he said, they did not have to expend any more energy for the constant climbing, compared to walking on the ground.
"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.
