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Inbreeding helps mountain gorillas adapt to small populations

Mountain gorillas are a critically endangered species. For those living in the Virunga volcanic mountain range on the borders of Rwanda, their numbers dropped below 300 in 1981 due to habitat loss. Even with conservation efforts in place, scientists worried that such a small population would spell doom. Now, over 30 years later and with their numbers rising, the first genomic study of mountain gorillas reveals the curious way they’ve avoided the common pitfalls of small populations: Inbreeding.

“We worried that the dramatic decline in the 1980s would be catastrophic for mountain gorillas in the long term, but our genetic analyses suggest that gorillas have been coping with small population sizes for thousands of years,” says Dr Yali Xue, first author from the Wellcome Trust Sanger Institute. “While comparable levels of inbreeding contributed to the extinction of our relatives the Neanderthals, mountain gorillas may be more resilient. There is no reason why they should not flourish for thousands of years to come.”

The researchers were able to gather blood samples from the Rwanda Development Board, The Institut Congolese pour la Conservation du Nature and by Gorilla Doctors, which treats wild gorillas injured by snares. In doing so, they managed to sequence the genome of seven gorillas, which had never been done before. The blood samples were uniquely robust, as previous DNA from hair and fecal samples was of poor quality.

Their analysis found that compared to larger groups from other parts of western Africa, mountain gorillas from the Virunga region were as much as three times less genetically diverse. Typically, this is a bad thing – low genetic variation is associated with susceptibility to disease and environmental changes. However, for whatever reason, that’s not the case with these small, inbred populations, and in fact they have fewer of the harmful loss of function gene variants found in larger populations.

The findings demonstrate that these populations of mountain gorillas were never a thriving species obliterated by human or environmental causes. In fact, it’s just the opposite: The research indicates that gorillas in this region have likely numbered in the hundreds for thousands of years. While most species evolve to embrace genetic diversity to grow their strength in numbers, these gorillas opted to build a small, yet hardy population.

“Our dedicated programme of clinical monitoring and intervention in cooperation with the Rwanda Development Board, the Institut Congolese pour la Conservation du Nature and local communities is helping to ensure the health and sustainability of endangered mountain gorillas,” says Dr Mike Cranfield, Co-Director of Gorilla Doctors.