Small primates like squirrel monkeys and lemurs expend no more energy climbing than they do walking, according to a new study.

On the other hand, the energy required for a primate to walk in a straight line actually decreases as the primate's size increases, which implies that it makes more sense for larger primates to stay on the ground while, metabolically, it does not matter if smaller primates walk or climb.

The findings may 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.

For their experiment, researchers designed and built a novel climbing treadmill -- essentially a loop of rope around two pulleys -- to measure the animals' efforts. As the animals moved at their highest sustainable speed, sensors measured oxygen level changes within a chamber to derive the primates' energy consumption.