Unexpected nut eating by gorillas

Despite their large body size, gorillas are known to have a vegetarian diet consisting almost exclusively of leafy vegetation and fruit. Their teeth are large and high crested when compared to other great apes which is traditionally seen as an adaptation to them spending a large amount of time chewing tough fibrous plant material. In contrast, their teeth are not well adapted to eating hard objects, such as nuts encased in a woody shell, because the high crests on their molar teeth would be at risk of damage. "I was amazed when we first observed the nut eating by the gorillas," states Martha Robbins, senior author on the paper. "We can not only see it, but also hear it, as the shell gives way to the incredible strength of their bite. Gorillas obviously have large, powerful jaws, but we did not expect to see this because their teeth are not well-suited to such behavior."

The nuts of Coula edulis are encased in a hard, woody shell that takes around 271 kg of force to crack. Yet for the three months the nuts are available, the gorillas of Loango National Park concentrate their feeding on the energy rich kernels, spending up to three hours a day chomping through nuts. This is surprising as animals that eat very hard food items tend to have strong, rounded molars that act like a pestle and mortar and are very efficient at cracking brittle foods. Like other foliage eaters, gorilla teeth have higher crests providing extra cutting edges for slicing tough material. Under the monumental bite force required to crack nuts, teeth with sharp edges are prone to break meaning they may be worn away quickly. The researchers were surprised to learn that the gorillas at Loango are regularly gambling with their teeth and taxing them close to their predicted mechanical limits. While some primates, like chimps, protect their teeth by using tools to crack open nuts, it appears that the gorillas at Loango National Park reply on brute strength to break through the woody shells of Coula edulis nuts. The fact they do this year after year indicates that gorilla teeth may be stronger than previously thought.

The research also implies that western lowland gorillas have much greater dietary breadth than previously believed. The absence of nut cracking behavior in other populations of western gorillas where the nuts are also present suggests the behavior may be cultural, if gorillas need to observe and learn the behavior from other group members. "The fact that this nut eating is observed in Loango but not in other forests in central Africa where the nut occurs stresses the importance of studying and conserving gorillas throughout the habitat where they are found," says Robbins.

Discovering that some gorillas routinely partake in nut cracking with their teeth could also influence the way researchers interpret the fossil remains of human ancestors. Despite having teeth seemingly shaped for a leafy diet the study shows that western lowland gorillas are capable of routinely cracking nuts, which has important implications for the ways researchers predict the diet of human ancestors based on the shape of their teeth.