A new study led by the Dian Fossey Gorilla Fund is shedding light on how mountain gorillas living in different parts of Rwanda’s Volcanoes National Park have considerably different diets despite living only kilometers apart. The study was done in collaboration with the Rwanda Development Board, Smithsonian’s National Zoo and Conservation Biology Institute, The George Washington University, the Max Planck Institute for Evolutionary Anthropology, and the University of Western Australia.
Although the mountain gorilla population living in Rwanda’s Volcanoes National Park (VNP) has been closely monitored since Dian Fossey began her work there in 1967, most scientific knowledge has come primarily from the population living in the park’s west – often referred to as the Karisoke study population, after Fossey’s Karisoke Research Center.
For this new study, researchers tracked the diets of 12 gorilla groups across the majority of the park, from the slopes of Mount Karisimbi in the west to Mount Muhabura in the east. They focused on whether differences in vegetation and elevation influenced what gorillas ate and whether the nutritional quality of their key food might explain regional differences in population growth.
Juvenile mountain gorilla in Volcanoes National Park, Rwanda, feeding on a bamboo shoot. Bamboo is the only key food found across all gorilla groups living in the park yet recent analyses show that it is regenerating less frequently.
Key findings:
The study, published this month in the journal Ecology and Evolution, found:
- Different groups, different diets and it depends on where you live: Gorilla groups in different regions of the park have distinct diets with little overlap, reflecting differences in habitat that are driven primarily by elevation. Of the key foods, which can make up as much as 85% of a gorilla group’s diet, there was only one species of overlap between the western and eastern sub-populations: bamboo. Overall, gorilla groups in the east had significantly more diverse diets than those in the west.
- Same nutrition, different plants: Despite differences in diet diversity, the nutritional value of key foods was found to be similar across regions.
Conservation concerns: Another study has found that bamboo, one of the major key food sources across all gorilla groups, is regenerating less frequently, which could impact long-term food security. Additionally, the eastern groups are incorporating exotic plants outside the park boundaries into their key foods, which may expose them to disease, chemicals and increased human-wildlife conflict.
“Our findings are exciting for many reasons,” says Fossey Fund researcher Honorine Ihimbazwe, lead author on the paper. “First, it’s remarkable to see how much dietary variation there is within a population that lives in such a small area. Second, the VNP mountain gorillas are one of the best-studied animal populations on the planet, and yet we didn’t fully realize until now that these extreme differences in diet existed. For example, wild celery, which is a top food for gorillas in the western part of the park, is almost completely non-existent in the diets of groups in the east.”
In total, 57 new food type-items (includes plant species plus plant part) and at least 19 new plants were added to the dietary list for the Virunga mountain gorillas, representing a 33% and 21%, respectively, increase over what was previously recorded.
“This study really highlights both the value of long-term data and the need for continued research,” says Dr. Tara Stoinski, CEO and chief scientific officer of the Fossey Fund. “Many studies, particularly of primates, are only able to include one or two groups in an area for logistical or expense reasons, and when comparisons are done, they often focus on populations that live in different environments and are studied with different methodologies. This study shows the considerable variability that can exist within a single population, which may have implications for all sorts of aspects of an animal’s life – behavior, ranging patterns, health, reproduction and ultimately conservation,” Stoinski adds.
Female mountain gorilla in Volcanoes National Park, Rwanda, eating the roots of a thistle plant. Thistle is found in diets of groups across the park but makes up a much smaller percentage of the diet in groups that range on the eastern side of the park as compared to the west.
For example, gorillas in the western part of the park have been growing in number faster than those in the east. Since the study found no difference in the nutritional value of the foods they eat, diet alone probably doesn’t explain this. But differences in how gorillas get their food, such as needing to travel farther in the east, might still affect how much energy they spend, which could impact growth over time.
“These results emphasize the need for localized conservation strategies,” says Didier Abavandimwe, a co-author of the study who has led the Fossey Fund’s expansion to include additional gorilla study groups. “As climate change and human pressures reshape their environments, understanding how gorillas use their habitat and can adapt their behavior is more important than ever.”
The researchers hope the findings will help inform conservation strategies for Volcanoes National Park, including the Rwandan government’s plans to restore portions of the park that were converted to agriculture.
