Written by: R. Reeder
Woolly monkeys don’t simply walk along branches to get from tree to tree. These arboreal monkeys also climb, leap, and even suspend (hang from the arms, legs and/or tail). Woollies, as well as spiders, howlers, and muriquis, are the four monkey species in the entire world that have prehensile tails, which means that their tails act like a fifth limb, making them extremely mobile in the canopy. Although they share this anatomical trait, all four species otherwise differ slightly in their anatomy and dominant modes of locomotion, giving each other a little more space as they move about in the forest.
Spider monkeys, a close relative of the woolly monkey, predominantly use suspensory locomotion to travel between substrates, as opposed to the climbing and quadrupedal walking patterns used by woollies . This is due to the fact that a spider’s anatomy is highly specialized for extensive arm and trunk rotation between handholds, whereas a woolly’s anatomy, and locomotion, is more generalized . One of these anatomical differences is visible in their hands: woollies have thumbs, but spiders do not. A thumb serves a woolly well as it moves more slowly through the canopy, using it to grip larger substrates, but a thumb would be a greater risk than benefit to a spider monkey that is brachiating very quickly through the canopy. Woollies and spiders use different modes of locomotion as a means of niche separation . “The more generalized body plan, locomotion, and postural behavior of Lagothrix argues for a broader niche, with more generalized foraging abilities”, whereas “The relatively more exaggerated suspensory locomotion of Ateles…suggests that an advantage may exist in harvesting more distal fruits than Lagothrix is able to harvest economically” . The differences in their anatomies and modes of locomotion allow them to exploit different resources, and therefore reduce competition between them.
As researchers here at TBS, we are fortunate enough to get to observe these modes of locomotion and resource partitioning strategies in action. I vividly remember one of the first times I saw a woolly do a leap. “Leaping or jumping and dropping are locomotive patterns used to move between discontinuous supports” . This woolly was a mom, she had an infant riding on her back, and she was at the edge of a branch, preparing to leap into a huge palm frond ten meters away. She held onto the edge of the branch tightly, rocked back and forth three times, and then let go. The momentum she had built up soared her over the empty space, and safely onto the spine of the palm leaf. I can only imagine how the infant must have felt. I have also observed in action the more general foraging habits of woollies in comparison to spiders, which as Defler (1999) suggests, may be a result of their differences in body plan and locomotion. Both forage mainly of fruits, but woollies will also eat insects, leaves, and stems. However, it is not uncommon to see woolly and spider monkeys foraging together in the same tree. Their extreme tolerance of one another is in part due to niche separation, but is also aided by the abundance of food at TBS, which allows them to live sympatrically and happily here in the Amazon.
 Defler TR. 1999. Locomotion and posture in Lagothrix lagotricha. Folia Primatol 70:313-327.