Invincible Gibbons?

By contributing writer Sarah Buckleitner


Scientists have long wondered how gibbons--a good-sized meal for the big cats, eagles, and snakes that share their habitat--have largely been able to avoid filling the bellies of those above them on the food chain. Have they evolved specific behaviors that allow them to escape unscathed from encounters with their natural predators?

This question is of particular interest to researchers because it suggests that predation may be a driving factor in the evolution of tropical primates.

In order to answer this question, researchers Clarke, Reichard and Zuberbuhler put together an experiment that would allow them to examine wild white-handed gibbons (Hylobates lar) that live in the canopies of Khao Yai National Park, in Thailand (Figure 1). In order to mimic the interactions that the gibbon families were likely to have with predators, they created models of an Asiatic tiger, clouded leopard, crested serpent eagle, and reticulated python with the help of patterned fabrics, their creativity, and an unlucky field assistant (the tiger model was essentially a field assistant hunched over and draped with a striped cloth). The models were presented (along with a brightly colored control) to various gibbon groups throughout a one-year period.

Thomas Tolkien
Figure 1. A white-handed gibbon (Hylobates lar). (From Thomas Tolkien/Flickr)

After the gibbons had spotted the "predator" researchers recorded their responses based on various predicted short term reactions: defecations, distance moved, predator inspection, branch dropping, vocal behavior, drop in canopy height and vigilance. They also continued to observe the group's long-term behavior based on distance between mated pairs, post-predator vocalizations and daily activity and strata use.

The researchers found that the gibbons' responses varied from predator to predator, and that the gibbons responded more readily to the perceived predators than the control, which suggests that not only did they believe the researchers' models were a threat, but that they have also adapted specific responses to the different predators--which may explain the low rates of predation on gibbons in the wild (Figure 2).

gibbon
Figure 2. Graphs of (a) the median distance groups moved away from predator presentations and (b) the closest a group member moved to a predator model during presentation. (From Clarke et al., 2012)

Researchers believe that by dropping sticks, defecating, singing, and acting vigilante--or keeping a keen eye out--the gibbons intend to show predators that they are aware of their presence, which may dissuade them from continuing their hunt. Other tactics, such as climbing lower in the canopy and ushering juveniles toward the trunks of trees, have also been observed.

So it seems that gibbons are not immune to predation, but have become experts at avoiding it. Their behavioral responses to predators may provide framework for future studies on the connection between predation and primate behavior.

References

Clarke, E., Reichard, U., & Zuberb├╝hler, K. (2011). The anti-predator behaviour of wild white-handed gibbons (Hylobates lar) Behavioral Ecology and Sociobiology, 66 (1), 85-96 DOI: 10.1007/s00265-011-1256-5