The increasing occurrence of tuskless female elephants in certain regions of Africa has long been a mystery.

But now a team led by Princeton University researchers has shed light on the phenomenon by implicating a gene pair that is responsible for the mutation. These genes, one of which is connected to the X chromosome and is lethal to males, are associated with tooth development in mammals.

The increased incidence of tuskless elephants is a response to a population bottleneck resulting from the decades-long Mozambican Civil War. During this conflict, elephants were poached for their ivory and meat. Using statistical analyses, the researchers found that tuskless female elephants were five times more likely to survive during the war than females with tusks.

“Tusks suddenly became a liability, even though in natural circumstances, tusks are very useful organs for elephants,” said Brian Arnold, an author of the study and a biomedical data scientist at Princeton. “There was intense hunting pressure on tusked females. Specifically targeting tusked females gave tuskless females a huge competitive advantage.”

“Elephants are such an iconic species that is so important for the savanna ecosystem and now we have a better understanding of how human activity is impacting them,” added Shane Campbell-Staton, assistant professor of ecology and evolutionary biology at Princeton.