A study of lemurs has provided important clues about how primates adapted to life in the trees. When hanging beneath branches, lemurs rely on their front limbs to do most of the work, which helps explain how some monkeys took to swinging Tarzan-like through the forest.
There are a lot of advantages to an arboreal lifestyle. Living in trees offers access to fruits as well as protection against many predators. We spent a lot our evolution up there. However, it is a complex environment to navigate, and no one method of transport serves all purposes. Species have found different ways to get around, from the hand-over-hand swinging of orangutans to running over the tops of branches much like a creature on the ground.
A paper in the Journal of Experimental Biology notes that “the capacity of animals to change… altering gait, limb posture or body position fluidly and efficiently” is valuable in many contexts, but most strongly in complex three-dimensional environments like trees, raising questions about how flexible species do it.
Lemurs often get around like sloths, hanging beneath branches with all four limbs. However, they also walk above the branches.
The paper, by graduate student Michael Granatosky and two Duke University colleagues, argues: “The ability to effectively adopt both above- and below-branch locomotion is seen as critical for successful arboreal locomotion.” However, the authors add: “Little information exists on the ways in which limb mechanics change when animals shift from above- to below-branch quadrupedal locomotion.”
To fill this gap, Granatosky put various observing instruments on tree branches and watched what happened when two lemur species, Varecia variegata and Lemur catta, made their way around. The two species of lemur were chosen because V. variegata uses the hanging position as a matter of course, while L. catta spends a lot of time on the ground, and even when in the trees is more likely to leap than crawl below branches. "At first we tried covering the runway with fruits, but most of the time the lemurs just knocked the food off and ate their meal on the ground," Granatosky said in a statement. Eventually, fruits were hung below branches to get the lemurs to do their thing.
Nothing if not thorough, the team measured “values for peak vertical, breaking and propulsive forces, as well as horizontal impulses for each limb” for each species when they did travel both above and below the branch.
Too much under-branch hanging with four limbs can lead to this. Curioso/Shutterstock
Despite describing below-branch movement using all four limbs as “superficially resembl[ing]... above-branch walking," the team found the two to be very different. With below-branch movement, most of the weight-bearing and propulsive force was done by the forelimbs, while the hindlimbs were largely used to stop. Moreover, the forelimbs also bore most of the weight. This, the authors argue, “represents a shift toward mechanics associated with bimanual suspensory locomotion.”
The advantages of being able to swing from tree to tree are obvious, but why hang under a branch if it takes all four limbs to do so, particularly when you need to learn different ways of moving to do it? It has been proposed that hanging beneath branches provides access to additional fruit, and may also make balancing easier.