Researcher observes the unexpected: nut-eating gorillas
Despite their large size, gorillas are known to have a vegetarian diet composed almost exclusively of leafy vegetation and fruit. Their teeth are large and high crested compared to those of other great apes, which is generally considered an adaptation to spend a lot of time chewing resistant fibrous plant material. However, their teeth are not well adapted to the consumption of hard objects, such as nuts embedded in a woody shell, because the high peaks of their molars may be damaged.
Imagine the surprise of the researchers when they found gorillas bombarding African nuts.
Louis and the Max Planck Institute for Evolutionary Anthropology observed a population of western lowland gorillas in Loango National Park, Gabon, using their teeth to open the woody shells of Coula edulis nuts.
Adam van Casteren, professor of biological anthropology in arts and sciences, is the first author of a new study published in the American Journal of Physical Anthropology that combines direct feeding observations with mechanical tests of seed shells to show that gorillas, unlike previous science, can put their teeth at risk for years to access this high energy food source.
"I've never tasted the grain myself. It looks and feels like a wet coconut, but the grain is a small package of nutrients," van Casteren said. "Compared to leaves and fruits, it is rich in protein and fat, which means that when available, these nuts are probably a good stimulant for gorilla feeding."
"Other animals that are known to crack nuts are elephants and bush pigs, both of which have very powerful bites," he says. "It is also known that rodents use their sharp incisors to gnaw at the hard outer shell. Chimpanzees open the nut with tools. We also know that humans use nuts in the kitchen. Now we know that gorillas use their powerful chewing muscles to pierce the woody shell."
Researchers were surprised to learn that Loango gorillas regularly play with their teeth and tax them near their expected mechanical limits. While some primates, such as chimpanzees, protect their teeth by using tools to crack open nuts, it seems that gorillas in Loango National Park rely on brute force to pierce the woody shells of Coula edulis nuts. The fact that they do so year after year indicates that gorilla teeth can be stronger than previously thought.
Finding that some gorillas regularly participate in nut cracking with their teeth could also influence how researchers interpret the fossil remains of human ancestors, the researchers said.
Although the teeth appear to have the shape of a leaf diet, the study shows that western lowland gorillas are able to regularly crack nuts, which has important implications for how researchers predict the diet of human ancestors based on the shape of their teeth.
"I was amazed when we first saw gorillas eating nuts," said Martha Robbins, a research scientist at the Max Planck Institute for Evolutionary Anthropology and lead author of this article. "We can not only see it, but also hear it, because the shell gives way to the incredible strength of their bite. Gorillas obviously have large and powerful jaws, but we did not expect to see this because their teeth are not well adapted to such behaviour."
Coula edulis nuts, also known as Gabon nuts or African nuts, are wrapped in a hard, woody shell that takes about 271 kg of strength to split. Yet, during the three months of the year when nuts are available, gorillas in Loango National Park concentrate their food on energy-rich almonds, spending up to three hours a day chewing nuts.
This is surprising, because animals that eat very hard foods tend to have strong, rounded molars that act like a pestle and mortar and are very effective at cracking fragile foods. Like other foliage eaters, gorilla teeth have higher ridges that provide additional cutting edges for cutting resistant materials. Under the monumental occlusion force required to crack nuts, teeth with sharp edges are prone to break, which means they can wear out quickly.
Research also suggests that Western Lowland Gorillas have a much wider diet than previously thought. The absence of nutcracker behaviour in other western gorilla populations where nuts are also present suggests that the behaviour may be cultural, if the gorillas are to observe and learn the behaviour of other members of the group.
