A deep enough wound will leave a scar, but a traumatic event in the history of an animal population may leave a mark on the genome itself. During the Mozambican Civil War from 1977 to 1992, humans killed so many elephants for their lucrative ivory that the animals seem to have evolved in the space of a generation. The result was that a large number are now naturally tuskless.
A paper published Thursday in Science has revealed the tooth-building genes that are likely involved. One of those same genes is linked to a syndrome in human females that causes abnormal tooth growth. In both humans and elephants, the mutation is lethal to males.
Although evolving to be tuskless might spare some surviving elephants from poachers, there will likely be long-term consequences for the population.
Normally, both male and female African elephants have tusks, which are really a pair of massive teeth. But a few are born without them. Under heavy poaching, those few elephants without ivory are more likely to pass on their genes. Researchers have seen this phenomenon in Mozambique’s Gorongosa National Park, where tuskless elephants are now a common sight.
Female elephants, that is. What no one has seen in the park is a tuskless male.
“We had an inkling,” said Shane Campbell-Staton, an evolutionary biologist at Princeton University, that whatever genetic mutation took away these elephants’ tusks was also killing males.
To learn more, Dr. Campbell-Staton and his co-authors started with long-term data, including prewar video footage of Gorongosa’s elephants.
They calculated that even before the war, nearly one in five females were tuskless. This might reflect earlier conflict and poaching pressure, Dr. Campbell-Staton said. In well-protected elephant populations, tusklessness can be as low as 2 percent.
Today, half of Gorongosa’s females are tuskless. The females who survived the war are passing the trait to their daughters. Mathematical modeling showed this change was almost certainly because of natural selection, and not a random fluke. In the decades spanning the war, tuskless females had more than five times greater odds of survival.
And the pattern of tusklessness in families confirmed the scientists’ hunch: it seems to be a dominant trait, carried by females, that’s lethal to males. That means a female with one copy of the tuskless mutation has no tusks. Half of her daughters will have tusks, and half will be tuskless. Among her sons, though, half will have tusks and the other half will die, perhaps before birth.
The team sequenced the genomes of 11 tuskless females and seven with tusks, looking for differences between the groups. They also searched for places in the genome showing the signature of recent natural selection without the random DNA reshuffling that happens over time. They found two genes that seemed to be at play.
Both genes help to build teeth. The one that best explains the patterns scientists saw in nature is called AMELX, is on the X chromosome, as the team expected. That gene is also involved in a rare human syndrome that can cause tiny or malformed teeth in females — specifically, the top teeth between the front teeth and canines, which are analogous to an elephant’s tusks. The human syndrome also kills males, because it results from a missing chunk of DNA that includes not just the tooth gene but other crucial genes nearby.
In the elephant genome, “We don’t know what the exact changes are causing this loss of tusks, in either one of those genes,” Dr. Campbell-Staton says. That’s one of the things the researchers hope to figure out next.
They also want to learn what life is like for a tuskless elephant. Elephants normally use their tusks to strip tree bark for food, dig holes for water and defend themselves. “If you don’t have this key tool, how do you have to adjust your behavior in order to compensate?” Dr. Campbell-Stanton said.
And the rise of tusklessness may affect not just individual elephants, but the population as a whole, Dr. Campbell-Staton said, since fewer males are being born.
“I think it’s a very elegant study,” said Fanie Pelletier, a population biologist at the Université de Sherbrooke in Quebec who was not involved in the research but wrote an accompanying article in Science. “It’s a very complete story as well. All the pieces are there,” she said.
In her own research, Dr. Pelletier has studied bighorn sheep in Canada. As trophy hunters targeted the males with the biggest horns, the sheep evolved to have smaller horns.
The change in sheep is subtle, she said, unlike the elephants’ total loss of tusks. And the elephants’ genetic change has actually compounded their problems, Dr. Pelletier said. Even if poaching stopped tomorrow, tusklessness would keep indirectly killing males, and it could take a long time for the frequency of this trait to drop to normal levels.
Dr. Campbell-Staton agreed that although the elephants have evolved to be safer from poachers, this isn’t a success story.
“I think it’s easy when you hear stories like this to come away thinking, ‘Oh everything’s fine, they evolved and now they’re better and they can deal with it,’” he said. But the truth is that species pay a price for rapid evolution.
“Selection always comes at a cost,” he said, “and that cost is lives.”