Science

Civil war drove these elephants to lose their tusks—through evolution

Mozambique’s civil war from 1977 to 1992 had a grim outcome for elephants: During that time, some 90% were killed for the ivory in their tusks, which were sold to finance the war. Now, researchers report this intense hunting dramatically altered a major elephant population there, favoring female elephants born without tusks. Although the adaptation comes with a price—an associated genetic mutation kills male elephants before they’re born—the emerging trait may have helped save the population.

“It’s another example of the imprint of [human] effect on nature,” says wildlife ecologist George Wittemyer of Colorado State University, who was not involved in the research. Albert Roca, a geneticist at the University of Illinois, Urbana-Champaign, calls the shift “striking” in its speed—and because it is driven by people. “This paper is going to lead to a lot of speculation and modeling.”

When humans hunt, they can cause their quarry to evolve by targeting individuals with particular traits, like big fish or sheep with hefty horns. But it’s rare to figure out the genetics behind this human-caused evolution, experts say. Shane Campbell-Staton, an evolutionary biologist now at Princeton University, was curious about the elephants of Gorongosa National Park in Mozambique, where tuskless elephants—which are all female—are unusually common.

First, Campbell-Staton wanted to make sure the proportion of tuskless elephants in Mozambique had indeed changed. He and his colleagues analyzed videos, taken before the civil war, of elephants in the park. At that time, about 18% of females there were born without tusks. But in the generation born after the war, the rate was 33%, according to decades of observations by the nonprofit group ElephantVoices.

Researchers took blood samples from elephants in order to sequence their genomes.Robert Pringle

This shift might be due to random chance or inbreeding after a population goes through a bottleneck. But a computer simulation, which examined the probability of tusked and tuskless females surviving the war, suggested the increase in tuskless females was far more likely to have resulted from selection, the team reports online today in Science. “It’s rare that you get such a dramatic example,” says co-author Robert Pringle, an ecologist at Princeton.

Next, researchers wanted to pin down the mechanism of inheritance for tusklessness. Because there were no tuskless males, the researchers suspected the trait might be caused by genes on the x chromosome, including a dominant one for tusklessness and a recessive one lethal only to males. Tusk inheritance patterns in the ElephantVoices database backed up that assumption. For example, tuskless mothers—who would have had one copy of the dominant tuskless gene, from their own mothers—had the same number of daughters with and without tusks. They also had twice as many daughters as sons. Such a biased sex ratio would be expected with a recessive gene that kills males when they inherit it.

To look for genes that might be involved, the team took blood from 18 female elephants in the park and sequenced their genomes. Two genes stood out: MEP1a and AMELX, which are active in tooth development in other mammals, were present in seven elephants with tusks, but had unique mutations in 11 tuskless elephants. In humans, a mutated version of AMELX is linked to male death before birth; in females, that version stunts the growth of the upper incisors, the same teeth that become tusks in elephants. It’s not clear why a mutated version of AMELX—which is located on the x chromosome—would be fatal to males, but researchers suspect one or more nearby genes come along for the ride. The results suggest that by killing elephants for their tusks, poachers selected for mutated versions of AMELX and MEP1a, which spread in the population and made tuskless elephants more common.

The study is “extremely thorough” and “ticks off all the boxes,” Roca says. It also raises many questions. The biggest is why a dominant gene associated with deadly effects for males would persist in the population during periods without poaching. Such genes ought to disappear, Roca says, because females that lack them would have more offspring. One possibility is that surges of intense hunting have occurred on and off in Gorongosa over millennia, letting the genes occasionally provide a benefit. Wittemyer wonders whether a similar phenomenon happened long ago in Asia, because both male and female fossil elephants there have tusks, but among living Asian elephants, only females have tusks.

Today, poaching has stopped in Gorongosa, and the elephant population is recovering. But it could take a long time for tusked females to become as common as they once were. “This is the long shadow of that human imprint that is going to take generations to erase,” Pringle says.

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