Elephants: Females have evolved to lose tusks due to ivory poaching

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A family of tuskless elephants

Russell Millner/Alamy

Female elephants in Mozambique rapidly evolved to become tuskless as a result of intense ivory poaching during the country’s civil war, even though one of the mutations involved kills male offspring.

During the war, from 1977 to 1992, both sides hunted elephants for ivory, and the elephant population of Gorongosa National Park plummeted. Now an analysis of historical video footage and contemporary sightings by Shane Campbell-Staton at Princeton University and his colleagues has shown that the proportion of tuskless females rose from 19 to 51 per cent during the conflict, and a statistical analysis indicated this was extremely unlikely to have occurred in the absence of a selective pressure. The proportion of tuskless elephants has been declining since the war ended.

This loss of tusks due to ivory hunting or poaching has happened in many other places too. For instance, in Sri Lanka less than 5 per cent of male Asian elephants still have tusks.

Oddly, though, all male African elephants have retained their tusks despite the pressure of hunting. This appears to be the result of a genetic quirk.

The team hasn’t yet found the precise genetic changes that cause tusklessness in females, but it appears two mutations are involved. One is probably in a gene on the X chromosome called AMELX, which plays a part in tooth formation.

It appears this mutation also affects other, crucial genes nearby. Females have two copies of the X chromosome, so if one copy isn’t mutated, the genes it carries will still function normally and the elephant will still be healthy. But males have only one X chromosome, so this mutation is lethal to any males that inherit it.

Much the same genetic condition can occur in people, says Campbell-Staton. Women with it lack upper lateral incisors – the equivalent of tusks – and male fetuses that inherit the mutation are usually lost in the third trimester.

It is possible that further genetic changes may compensate for the lethality and result in males losing tusks, too. For now, there is no sign of that happening. But even the loss of tusks in females can have all sorts of knock-on effects, says Campbell-Staton.

“Tusks are basically a Swiss army knife for African elephants,” he says. They use them to strip bark off trees, to dig holes for underground water or minerals, and so on, says Campbell-Staton, so the loss of tusks may spare females from poachers, but make it harder for them to survive in other ways.

What’s more, many other animals indirectly depend on tusked elephants – for instance, to get water from the holes dug with tusks. “This is what maintains biodiversity,” Campbell-Staton says. “There are all these cascading consequences that can result from our actions that are quite surprising.”

Journal reference: Science, DOI: 10.1126/science.abe7389

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