If you've ever felt jealous watching a monkey climb effortlessly up a tree, don't be. You're better at moving on the ground than he is. Researchers have found that one reason larger primates climb less than smaller animals do is not because they're particularly inefficient in the trees; it's because they're more efficient on land.
As a graduate student, Jandy Hanna was surprised to find that no studies had been done in nonhuman primates to measure the amount of energy needed to climb up a tree or wall. So she built vertical treadmills with ropes that move down on a pulley: Primates essentially climb in place, like a human running up a down escalator. Hanna tested five species of primates of various sizes and shapes, ranging from slender lorises no bigger than rats to mongoose lemurs the size of small Chihuahuas at the Duke University Lemur Center in Durham, North Carolina, and at the Center for Neotropical Primate Research and Resources in Mobile, Alabama. By measuring the amount of oxygen the primates consumed while climbing at maximum speed for 15 to 30 minutes or walking in a straight line (they were enclosed in Plexiglas metabolic chambers in which the amount of oxygen consumed from room air was monitored during climbing), Hanna and her colleagues calculated how much energy it took for each animal to climb or walk.
Size mattered. The team found that both small and large primates consumed the same amount of oxygen to lift 1 kilogram of body mass 1 meter while climbing; but the larger the animal, the less oxygen it consumed while walking in a straight line. The lemur used 60 units of energy to walk 1 meter, for example, and the loris used 140 units. (Bipedal humans use just 5 units of energy to cover the same ground.) "Larger animals can save energy while traveling horizontally--on the ground, for example--by taking long strides," says Hanna, now a biological anthropologist at the West Virginia School of Osteopathic Medicine in Lewisburg. Hence, it makes sense for them to stay out of the trees.
The findings, reported tomorrow in Science, refute a long-standing view that it was energetically costly for the first shrewlike primates to head to the trees and go vertical around 65 million years go. Instead, the study suggests there was no added cost to moving out on a limb, so to speak, if the first primates were smaller than 450 grams. As a result, the first primates were likely small and flexible, able to invade a new arboreal habitat without giving up energy. But as primates came down from the trees, they, like nearly all terrestrial animals, benefited from energy-saving tricks and longer stride lengths that reduce the cost of walking in larger animals. This made it costly for large primates to return to life in the trees, because they had become more efficient when grounded--not because they were using too much energy to climb.
"[Hanna] gets credit for being the first person to demonstrate this," says biological anthropologist Herman Pontzer of Washington University in St. Louis, Missouri. He and Hanna both say the next step is to run larger primates, such as macaques and chimpanzees, on the vertical treadmill to see if the trend holds for bigger body sizes. That might offer clues as to why human ancestors stayed relatively small until after they had adopted a fully modern style of bipedalism 1.8 million years ago.