AUDIE CORNISH, HOST:
Our next story begins in a place many of us are familiar with - up awake, watching a YouTube video at 3 in the morning. That's where Princeton evolutionary biologist Shane Campbell-Staton found himself a few years ago.
SHANE CAMPBELL-STATON: The Howard Hughes Medical Institute - they had this video that was called "The Tuskless Elephants Of Gorongosa." And I was like, ooh, what's this?
SARAH MCCAMMON, HOST:
Gorongosa - that's a national park in Mozambique.
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UNIDENTIFIED PERSON: Some elephant populations seem to be missing their tusks.
CORNISH: You see; while most African elephants have tusks, some female African elephants are born without them and never grow them.
MCCAMMON: But the number of tuskless elephants was multiplying in Mozambique during and after the country's decades-long civil war, which ended in 1992.
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UNIDENTIFIED PERSON: Large-tusked elephants in Gorongosa were killed for their ivory, which was sold to buy arms and ammunition.
MCCAMMON: Around 90% of the elephants there were killed, but many female elephants without tusks survived and thrived.
CORNISH: Campbell-Staton had heard all of this before.
CAMPBELL-STATON: But then I realized that there wasn't actually a lot of empirical data about what the response was from, you know, what the genetic basis of the trait was. Was it genetically inherited at all?
CORNISH: So he made some calls and assembled a team.
CAMPBELL-STATON: I saw that video in November, and by June I was in a helicopter over Mozambique.
MCCAMMON: Campbell-Staton and his team worked with local researchers who had tracked elephants for decades. They found that the tuskless trait was genetic, found in the X chromosome, and it was deadly in males. So if a tuskless mom had babies...
CAMPBELL-STATON: She has a 50% chance of passing the trait on to her offspring. So 50% of her daughters will be tusked. Fifty percent will be tuskless. Fifty percent of her sons will be tusked, but that other 50% would die.
CORNISH: The upshot, which they published this week in the journal Science, is that females without tusks are pretty likely to have tuskless babies, which is why the trait was becoming so prevalent.
FANIE PELLETIER: It's one of the most detailed example of how human activity can influence the genes of a population.
CORNISH: That's Fanie Pelletier, a wildlife biologist in Canada who wasn't involved in the work.
PELLETIER: The reason why that's important is because if you stop the killing right now, you know, the time it would take for the population to, you know, restore that traits would be much longer now that there's been a change in the genes than if it wasn't.
MCCAMMON: Those changes can ripple through ecosystems. Campbell-Staton points out that other species in Gorongosa rely on elephants having tusks to dig up holes for water and strip bark from trees.
CAMPBELL-STATON: You know, people during the Mozambican Civil War were not thinking about the evolution of elephants, I imagine, in the slightest. You know, yet those actions - right? - during that 15-year period had all these cascading consequences, all the way up from, you know, a single gene to an entire ecosystem health.
CORNISH: Consequences that are still continuing nearly 30 years after the war.
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