a A deep wound will leave a mark, but a traumatic event in the history of an animal population can leave a mark on the genome itself. During the Mozambican Civil War (1977–92), humans killed so many elephants for their attractive ivory that the animals seem to have evolved in the space of one generation. Result: A large number are now naturally toothless.
A paper published last week Science have revealed tooth-forming genes that are potentially involved, and for male elephants, the mutation is lethal.
Although being toothless may have saved some surviving elephants from predators, there would probably be long-term consequences for the population.
Normally, both male and female African elephants have teeth, which are actually a pair of large teeth. But some are born without them. Under heavy poaching, some elephants without ivory are more likely to pass on their genes. Researchers have observed this phenomenon in Mozambique’s Gorongosa National Park, where toothless elephants are now a common sight.
The female elephant, ie. What no one saw in the park is a toothless male.
“We had the impression that whatever genetic mutation took away these elephants’ teeth, it was also killing the males,” says Shane Campbell-Staten, an evolutionary biologist at Princeton University.
To learn more, Campbell-Staton and her co-authors started with long-term data, including video footage from Gorongosa’s elephants from the Civil War, before the Civil War.
He calculated that even before the war, almost one in five women were toothless. Campbell-Staten says this may reflect earlier conflict and the pressure of poaching. In well-preserved elephant populations, toothlessness can be as low as 2 percent.
Today, half of Gorongosa females are toothless. Women who survived the war are passing this trait on to their daughters. Mathematical modeling suggests that this change is almost certainly due to natural selection and not a random fluke. In the decades after the war, women without teeth were more than five times as likely to survive.
The pattern of tusklessness in families confirms the scientists’ hunch: it appears to be a dominant trait carried by females, fatal to males. This means that a woman with one copy of the mutation without teeth has no teeth. Half of his daughters will have teeth, and half will be without teeth. Among her sons, however, half will have teeth, and the other half will die, probably before birth.
The team sequenced the genomes of 11 tuskless females and seven tusks, looking for differences between the groups. They also discovered locations in the genome showing signatures of recent natural selection without the random DNA shuffling that occurs over time. He found two genes that seemed to be at play.
Both genes help in making teeth. The one that best explains the pattern observed by scientists in nature is called AMELX and occurs on the X chromosome, as the team expected. That gene is also involved in a rare human syndrome that can cause small or distorted teeth. AMELX is adjacent to other important genes whose absence from the X chromosome can kill males.
“We don’t know what exact changes are causing this loss of teeth in either of those genes,” says Campbell-Staten. This is one of the things the researchers hope to explore further.
They also want to know what the life of a toothless elephant is like. Elephants usually use their teeth to remove tree bark for food, to dig holes for water, and to defend themselves. “If you don’t have this important tool, how are you going to have to adjust your behavior to compensate?” Campbell-Staton asks.
And the rise of toothlessness could affect not only individual elephants but also the population as a whole, as fewer males are being produced.
“I think it’s a very beautiful study,” says Fanny Pelletier, a population biologist at the Université de Sherbrooke in Quebec, who was not involved in the research but wrote an accompanying article. Science, “It’s also a very complete story. All the pieces are there.”
In his own research, Pelletier has studied wild sheep in Canada. As trophy hunters targeted males with the largest horns, the sheep had smaller horns.
The change in sheep is subtle, she says, unlike the total loss of teeth in elephants. And the elephants’ genetic variation has actually exacerbated their problems, Pelletier says. Even if poaching stops tomorrow, tusklessness will continue to kill males indirectly, and it may take a long time for the frequency of this trait to drop to normal levels.
Campbell-Staton agrees that, although elephants have evolved to be safe from predators, it is not a success story.
“I think when you hear stories like this it’s easy to think, ‘Oh, everything’s fine. They evolved, and now they’re better, and they can deal with it,'” he says. But the truth is that species pay a price for rapid evolution.
“Selection always comes at a cost,” he says, “and that cost is life.”
This article originally appeared in the new York Times,
Credit: www.independent.co.uk /