The team sequenced the genomes of 11 females without tusks and seven females with tusks, looking for differences between the groups. They also looked for places in the genome showing the signature of recent natural selection without the random DNA shuffling that occurs over time. They found two genes that appeared to be at play.
Both genes help build teeth. The one that best explains the patterns scientists have seen in nature is called AMELX and is found on the X chromosome, as the team expected. This gene is also involved in a rare human syndrome that can cause tiny or badly formed teeth. AMELX is adjacent to other crucial genes whose absence of the X chromosome can kill males. In the elephant genome, “we don’t know what exact changes are causing this loss of defenses, in either of these genes,” says Dr. Campbell-Staton. This is one of the things the researchers hope to find out next.
They also want to learn what the life of a helpless elephant is like. Elephants normally use their tusks to tear bark from trees for food, dig holes for water, and defend themselves. “If you don’t have this key tool, how should you adjust your behavior to compensate? said Dr Campbell-Staton.
And the increased lack of tusks can affect not only individual elephants, but the population as a whole, Dr Campbell-Staton said, as fewer males are being born.
“I think it’s a very elegant study,” said Fanie Pelletier, a population biologist at the University of Sherbrooke in Quebec who was not involved in the research but wrote a companion article in Science. “It’s also a very complete story. All the pieces are there, ”she said.
In her own research, Dr. Pelletier studied the bighorn sheep in Canada. While trophy hunters targeted males with larger horns, sheep evolved to have smaller horns.
The change in sheep is subtle, she said, unlike the elephants’ total loss of tusks. And the genetic change in the elephants actually made their problems worse, Dr Pelletier said. Even if poaching stopped tomorrow, the lack of defense would continue to indirectly kill males, and it could take a long time for the frequency of this trait to drop back to normal levels.