Violence Between Gorillas Slows Population Growth, Scientists Say

Gorilla population growth over the past 50 years in the Virunga Mountains of East Africa may have a negative side effect — an uptick in gorilla infanticide. Scientists say more gorillas in the same small territory means more violent encounters between groups.

Scientists at the Dian Fossey Gorilla Fund analyzed decades of data on gorilla populations and found that violent encounters increased threefold and infanticide increased fivefold after gorilla density increased in the 2000s because of better protections. According to a new paper published in Science Advances, that violence contributed to the population’s annual growth rate declining by half between 2000 and 2017.

Tara Stoinski, chief scientific officer for the Dian Fossey Gorilla Fund and an author on the paper, was quick to clarify that gorillas are generally peaceful creatures. 

Chimpanzees won’t share territory between groups and will actively patrol their areas, said Stoinski. Gorillas often have a partial overlap in territories. But as a population expands within a fixed geographic area, groups are forced to share their entire territories with each other. The increased density means more encounters between groups, some of which turn violent. Similar issues might occur with other wildlife in the future as species are confined to smaller areas, Stoinski said.

Male gorillas will often target infants because female gorillas won’t reproduce while they are nursing, said Stoinski. This male reproductive strategy is shared by other animals, such as lions.

As the number of gorillas increases, the animals are confined on the same “island” of protected land amid human populations, said Stoinski. The Rwandan government is interested in expanding protected habitat for the gorillas, which could help prevent gorilla violence. Making sure gorillas can expand into new regions that are already protected could also help, she said. Gorillas only shift their range about 5-10% each year. It is important that they feel safe exploring new areas and do not run into things like snares or human encroachment that cause them to retreat.

The long-term data used in the study allowed scientists to get a more complete picture of gorilla behavior than shorter studies would allow, said Stoinski. She said long-term studies are especially important for long-lived animals like gorillas. Infanticide rates were high in the 70s and 80s, because males were being targeted by poachers, leaving their babies unprotected. In the 90s and early 2000s, there was almost no infanticide, because the gorilla population stabilized at a relatively low density. The infanticide rate increased again after 2007 due to increased population density. 

“If you had come at any point in time you could have made very different conclusions (about infanticide rates),” she said.

Jessica Mayhew, director of the Primate Behavior and Ecology Program at Central Washington University, said the sort of long-term data used in the study is critical for testing larger ecological and evolutionary hypotheses, some of which remain fairly theoretical. She said the study is also a reminder of how challenging conservation work really is. 

“The Dian Fossey Gorilla Fund has championed the conservation of mountain gorillas by working incredibly hard to change the conversation around this species over the past half century,” she said. “If you only look at one metric, population growth, the program is undeniably a success story. But this study reminds us of the sheer complexity of biological systems. When you pull on different levers of the natural world, there are cascading effects, some of which are predictable and manageable, and others unintended or unforeseen.”

All animal species are being forced to contend with changing ecosystems and “unrelenting human neighbors,” said Mayhew. In order to design effective conservation strategies, it is important to understand how different social, political, ecological, and behavioral variables impact one another.

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