A sodium craving may be the reason endangered mountain gorillas (Gorilla beringei beringei) from Volcanoes National Park in Rwanda leave the park to raid nearby eucalyptus plantations, according to a recent paper published in Biotropica.
Tourists from around the world pay $1,500 for the once-in-a-lifetime opportunity to spend just an hour with these majestic great apes. With the protection afforded by their high-earning potential, the gorillas of Volcanoes National Park are thriving and the park is often heralded as a model for conservation.
However, for the communities adjacent to the park, living alongside their famous neighbors can often feel more of a trial than a triumph. Gorillas make regular forays into nearby farms to feed, damaging vital crops. “Crop raiding is the main source of conflict between local dwellers and wildlife,” says Cyril Grueter, a biological anthropologist from the University of Western Australia and lead author of the study.
To minimize conflict, the study team made up of researchers from the Karisoke Research Center, Volcanoes National Park and the Max Planck Institute for Evolutionary Anthropology looked into what drives the gorillas to leave the safety of the park.
“We found that when the gorillas exit the national park, they often head straight to eucalyptus trees that have been planted outside the park,” Grueter says.
Previous studies have found that the park’s gorillas select food based on protein content and digestibility. Compared to food sources readily available in the park, eucalyptus bark scores poorly on both counts. This led Grueter and his team to hypothesize that some other nutritional quality was drawing the gorillas.
One possibility was sodium, a vital micronutrient all animals need to maintain crucial physiological processes. Humans aren’t alone in our salt cravings. “Sodium deficit can cause a specific hunger for it, which causes animals to go out of their way to obtain it,” Grueter says. Between 2009 and 2010, the team monitored the diets of 22 gorillas in three different family groups, and then analyzed the sodium content of the foods they ate.
“Our nutritional analyses showed that eucalyptus are more than a hundred times richer in sodium than gorillas’ staple foods inside the park,” Grueter says. The team found that up to two-thirds of the sodium in the gorillas’ diets came from consuming eucalyptus bark from outside the park. As well as raiding eucalyptus plantations, some gorillas also braved the cold conditions of the higher-altitude sub-alpine zone to feed on sodium-rich plants like giant groundsels and lobelias.
All animals must maintain a certain level of sodium in their diet, and when their normal food sources fail to provide enough, many find novel ways to supplement their diet. A similar study found that the mountain gorillas of Bwindi National Park in Uganda obtain 95 percent of the sodium in their diet from dead wood, a food source with little other nutritional value. In Mount Elgon National Park, which straddles the Kenya-Uganda border, elephants are known to use their tusks to “quarry” sodium-rich rock deposits found in underground caves.
With such a high proportion of their sodium coming from the relatively small portion of time the gorillas spend feeding on eucalyptus, Grueter says he’s convinced that sodium is the driving force behind the apes’ crop raiding. He says he hopes this new information can be used to help minimize human-gorilla conflict.
Shane McGuinness, a conservation researcher from the National University of Ireland, who was not involved with the study, witnessed human-wildlife conflict firsthand around the borders of Volcanoes National Park. Although gorillas were far from the most prevalent of the park’s crop-raiding inhabitants, they were still regular culprits.
McGuinness also noted the gorillas’ preference for eucalyptus when outside of the park. Mature eucalyptus trees are relatively resilient to de-barking by gorillas, and their timber can still be used after gorillas feed on them. However, gorillas favor young trees, which are much easier to peel, McGuinness says. When young eucalyptus trees lose their bark, it limits their growth and chances of survival.
Because of the risk of disease transmission between humans and gorillas, local people are not allowed to chase the gorillas away from their crops. Park authorities have introduced measures to deter crop raiding, such as walls and ditches, that have been effective against buffalo but have proved no match for nimble primates.
The area surrounding the park has exceptionally high human population densities for a rural area, with more than 700 people per square kilometer in some places. Local people survive through subsistence farming on small plots of land, and even a small amount of crop damage can be devastating.
McGuinness says the situation is exacerbated by the high-profile nature of gorilla conservation. Mountain gorilla conservation is a rare success story, with the continued growth in the species’ population recently prompting the IUCN to reclassify mountain gorillas from critically endangered to endangered.
Because of the high rates tourists are willing to pay to trek with gorillas, the park is able to generate a huge amount of revenue: $17.1 million in 2017. The gorillas bring in enough money to fully fund Volcanoes National Park and to subsidize the running of Rwanda’s other national parks.
To bolster support for conservation, 5 percent of the income from gorilla permits is also granted to community projects in the areas surrounding the park. However, McGuinness says the locals closest to the park, whose crops are raided, don’t see the benefits yet still bear the cost — a perspective echoed by other studies that have evaluated the impact of community conservation around Volcanoes National Park. “Unfortunately, many farmers seemed to think that the government cares more for conservation than development,” he says.
With such a high density of humans surrounding the park and an increasing density of gorillas within the park, tensions between the two are likely to continue. Grueter says he’s concerned that the impact of climate change and increased gorilla numbers could deplete food reserves within the park, forcing gorillas to leave the area in search of food, thereby further increasing problems. One solution he proposes based on his research is a buffer zone of nutritionally unattractive plants surrounding the park to deter the gorillas from raiding crops.
No matter the difficulties, Grueter says he believes Rwanda’s mountain gorillas are worth fighting for: “They are simply stunning creatures without which this world would be a poorer place.”
Cyril C. Grueter et al, “Going to extremes for sodium acquisition: use of community land and high-altitude areas by mountain gorillas Gorilla beringei in Rwanda,” Biotropica (2018).