Elephant hunting alters the gene pool

Ivory poaching in Africa has led to elephants in some regions evolving without tusks.

The good news, however, is that with increased protection from hunters, these large mammals are regaining their tusks.

Tusklessness as a survival advantage

«In African elephants, the absence of tusks is very rare", says Brian Arnold, a biomedical data scientist at Princeton University. "But when you look at certain areas, the rate of tusklessness is much higher than average.»

To find out why, Arnold and his co-author Shane Campbell-Staton, a biologist at the University of California, Los Angeles, travelled to Gorongosa National Park in Mozambique.

From 1977 to 1992, a civil war raged in Mozambique. Both sides financed their weapons and other equipment through illegal ivory trading and thus the illegal hunting of elephants. As a result, the number of elephants in Gorongosa National Park dropped from an estimated more than 2,500 animals in 1972 to fewer than 250 by the year 2000. Using photographs, Arnold and his team estimated that at that time approximately 18% of the population was missing both tusks, while 9% had only one tusk.

Before the war, four out of five female elephants had tusks. Afterwards, fewer than half did. Males without tusks, on the other hand, were virtually non-existent both before and after the war. In other regions of Africa as well, the number of female elephants without tusks has increased significantly in recent decades wherever poachers illegally harvest ivory by slaughtering elephants.

Only females affected

«There was a clear survival advantage for elephants without tusks», said Arnold. Tusks are generally important for the survival of elephants, as they help them dig for underground water sources and strip bark from trees. However, when elephants are hunted for their tusks, this useful trait becomes a death sentence.

When reviewing the data, the researchers noticed an interesting pattern: all tuskless elephants are female. They found that, on average, 50% of the daughters of a tuskless mother are born tuskless just like her, but all male calves have tusks.

According to Arnold, this pattern suggests that the gene causing tusklessness is located on the X chromosome, meaning it is an X-linked dominant trait. Since male embryos receive only one X chromosome from their mother alongside the Y chromosome, inheriting the X chromosome carrying the mutated gene is a death sentence. Half of all male embryos therefore die before birth.

A silver lining: tusks are returning

«There is a silver lining to this story», said Arnold. «Since 1994, elephant populations in Mozambique have been on the rise. At the same time, tusklessness is declining, likely because tuskless mothers are less fertile. This suggests that the elephants in Gorongosa National Park are well on their way to recovering their former tusk splendour.»

What is clear is that even a period as brief as 15 years of ivory hunting — short by evolutionary standards — represents a significant and immediately visible selective pressure. Moreover, the changes to the gene pool have long-lasting consequences for the elephant population. To this day, more than a third of females roam without the characteristic tusks so coveted by poachers .