Gorillas are highly sociable animals — at least until their mountain gets crowded, new research finds.
Mountain gorillas spend most of their time sleeping, chomping leaves and wild celery stalks and grooming each other’s fur with their long, dexterous fingers.
These big vegetarian apes generally are peaceful — unless you’re a rival gorilla.
Researchers who analyzed 50 years of demographic and behavioral data from Rwanda found that, as the number of gorilla family groups living in a habitat increased, so did the number of violent clashes among them.
Most often, dominant males — silverbacks — led the fights, which sometimes ended in death, especially for infant gorillas. And that slowed population growth.
“Encounters between groups can be violent,” said Damien Caillaud, a behavioral ecologist at the University of California, Davis, and co-author of the new study published in the journal Science Advances. “Males will fight to protect the females and infants in their group and to acquire new females.”
The frequency of gorilla family feuds was determined not by the total number of individuals but by the number of family groups in a region, the study found.
Mountain gorillas have been a focus of intense research and conservation efforts in central Africa since the late 1960s. After teetering near the edge of extinction in the 1970s, the population has rebounded to just over 1,000 animals — considered to be endangered by scientists.
“Normally when thinking about conservation, scientists focus on tangible, ecological issues like food availability, degradation of habitat, hunting by humans — rarely do we think about how an animal’s behavior and social structure can influence population size,” said Rich Bergl, a primatologist at the North Carolina Zoo, who also works on great ape conservation in Africa and wasn’t involved in the research. “But it turns out we should, especially for social animals like gorillas.”
Mountain gorillas live in forested parks on mist-covered volcanoes, spanning parts of Rwanda, Uganda and the Democratic Republic of Congo, where industries such as logging and mining are banned.
“Everyone wants to know how many gorillas can live inside the protected habitat area,” said Tara Stoinski, a primatologist and co-author of the new paper who is also president of the Dian Fossey Gorilla Fund, a nonprofit research and conservation group. “It turns out the answer depends partly on how they organize themselves socially.”
During fieldwork in Rwanda, Stoinski said she witnessed changes in gorilla behavior beginning around 2007. Around that time, three large family groups splintered into multiple smaller units, and the overall population also grew.
As a result, there were then about 10 family groups in the study area. The groups spread out to occupy more territory within Volcanoes National Park, but the number of violent clashes among them still increased threefold, the number of infant deaths increased fivefold, and the population growth rate was halved.
The dissolution of the larger family groups — one had included 65 individuals — was probably related to the deaths of charismatic silverback leaders, Stoinski said.
“When these ‘elder statesmen’ gorillas got older and died, the younger males weren’t able to keep the groups together,” she said. “We still don’t understand all the factors that go into determining leadership qualities in gorillas.”
The study was based on demographic and behavioral data for about 400 gorillas in Rwanda between 1968 and 2017.
“This work draws on such a beautiful, well-curated data set — it’s a very impressive effort that really shows the importance of long-term research,” said Susan Alberts, a behavioral ecologist at Duke University who wasn’t involved in the paper.