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A deep enough wound will leave a scar, but a traumatic event in the history of an animal population can 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 within a generation. The result was that many people were now naturally toothless.
A Article published Thursday in the journal Science revealed the dentition genes likely involved. One of the same genes is linked to a syndrome that causes abnormal tooth growth in human females. In both humans and elephants, the mutation is lethal to males.
Despite Evolving to be toothless may save some surviving elephants from poachers, there will likely be long-term consequences for the population.
Normally, both male and female African elephants have tusks that are really a pair of large tusks. But few are born without them. Under heavy hunting, a few elephants without ivory are more likely to pass on their genes. Researchers have discovered this phenomenon Mozambique’s Gorongosa National Park, where toothless elephants are now a common sight.
Female elephants, that is. A toothless male that no one has ever seen in the park.
“We had an intuition that whatever genetic mutation these elephants had that destroyed their tusks, it was also killing the males,” said Shane Campbell-Staton, an evolutionary biologist at Princeton University.
To learn more, Dr. Campbell-Staton and his co-authors started with long-term data, including pre-war video footage of Gorongosa elephants.
They calculated that even before the war, about one in five women was toothless. Dr. Campbell-Staton said this may reflect previous conflicts and poaching pressure. Edentosis can be as low as 2 percent in well-protected elephant populations.
Half of Gorongosa females today are toothless. Females who survived the war pass this trait on to their daughters. Mathematical modeling showed that this change was almost certainly due to natural selection and not a random coincidence. In the decades that covered the war, edentulous females had more than five times the chance of survival.
And the familial pattern of edentulism confirmed the scientists’ hunch: It appears to be a dominant trait carried by women, deadly for men. This means that a female with one copy of the toothless mutation does not have teeth. Half of her daughters will have teeth and half will be toothless. But half of their sons will have teeth, and the other half will die, perhaps before they are even born.
The team sequenced the genomes of 11 edentulous females and seven females with teeth, looking for differences between the groups. They also looked for locations in the genome that showed the signature of recent natural selection, without the random DNA reshuffling that occurs over time. They found two genes that seem to be at play.
Both genes help form teeth. The one that best explains the patterns scientists see in nature is called AMELX, and it’s on the X chromosome, as the team expected. This gene also plays a role in a rare human syndrome that can cause small or damaged teeth in females, particularly the upper teeth between the front teeth and canines, which resemble an elephant’s tusks. The human syndrome kills men as well, because it is caused by a missing chunk of DNA that contains not only the tooth gene but other important genes nearby.
Dr. “We don’t know what the exact changes are in any of these genes that cause this tooth loss,” Campbell-Staton says. That’s one of the things the researchers hope to figure out next.
They also want to know what life is like for a toothless elephant. Elephants normally use their tusks to peel bark 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 for it?” said. Dr. Campbell-Staton.
Dr. Campbell-Staton said the rise of edentulism could affect the population as a whole, not just individual elephants, because 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 Québec, who was not involved in the study but wrote: An accompanying article in Science. “It’s also a very complete story. All the pieces are there,” he said.
Dr. Pelletier in his research big horn sheep in Canada. While bounty hunters target males with the biggest horns, sheep evolved to have smaller horns.
The change in sheep is subtle, he said, as opposed to elephants losing their tusks entirely. Dr. Pelletier said that the genetic change of elephants actually exacerbates their problems. Even if poaching stopped tomorrow, toothlessness would continue to indirectly kill males, and it could take a long time for the frequency of this trait to return to normal levels.
Dr. Campbell-Staton agrees that while elephants have evolved to be safer from poachers, this is not a success story.
“I think when you hear stories like this, it’s easy to walk away saying, ‘Everything’s fine, they’ve 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.
“Choice always has a price,” he said, “and that price is life.”
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