"We already know that the bigger you get, the more efficient you are at walking," said Brian Richmond, a biological anthropologist at George Washington University in Washington, D.C., who was not involved in the study.
"You can see that with little kids taking a lot of little steps and struggling to keep up with their parents."
(Related: "Humans Beat Chimps at Walking Efficiently" [July 16, 2007].)
"It turns out that efficiency gain is not realized when you are climbing. No matter what size you are, it's going to cost the same relative to your body size."
The metabolic scorecard may explain why larger species tend to be more terrestrial, although some, such as gorillas, can climb trees.
"The cost to them is not necessarily a barrier, but there may be a cost-benefit ratio in terms of how much climbing they can do," study co-author Hanna explained.
Small primates, on the other hand, rarely spend much time walking, Richmond said.
"There is no hurdle or barrier against climbing up and down in trees [where it's] easier to avoid predators and there is a lot of food to be found."
The new study, recently published in the journal Science, may also be important for understanding transitions in the primate fossil record.
Many experts theorize that evolutionary adaptations for climbing were associated with the origin of primates and the beginnings of bipedalism.
The study authors believe that their findings bolster theories that the earliest primates were small animals—perhaps weighing less than about a pound (half a kilogram)—that lived in trees.
"This suggests that early primates, when they were invading this new arboreal niche, were moving around on branches, and doing so involved more climbing," Hanna said.
"If the earliest primates were small, then they weren't paying any extra energetic costs to move into that environment."
SOURCES AND RELATED WEB SITES