Extreme ivory poaching led to tuskless elephants in Mozambique
As the country’s civil war decimated elephant populations, the proportion of tuskless females rose dramatically. A new study explains why the tuskless trend continued in peacetime.
African elephant infant and adult. Image Credit: Michelle Gadd/USFWS, Flickr
Mozambique’s devastating civil war, fought between 1977 and 1992, seems to have had unexpected consequences: the rapid evolution of tuskless elephants.
Both sides of that war financed themselves largely through ivory trade, fueled by the rapid slaughter of Mozambican elephants. In just 15 years, elephant populations in Gorongosa National Park declined by 90%. By the early 2000s, there were only 200 elephants in the whole country, Nature reports. Among them were some individuals that, thanks to a rare genetic mutation, lacked tusks. With no ivory to offer, they were more likely to be spared and survive to pass on their tusklessness to their offspring.
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Since the end of the war, observers at the park have noted increased numbers of elephants with no tusks. A study published today in the journal Science dives deep into the elephant genome to show yet another unexpected way human affairs can sculpt our biological world. “It’s more than just numbers,” Rob Pringle, an ecologist at Princeton University and coauthor of the study, told The Guardian. “The impacts that people have, we’re literally changing the anatomy of animals.”
The study authors started by analyzing historical video footage from prior to the civil war and contemporary elephant sighting data kept by local NGOs. That data showed that the dramatic decline in elephant populations in Gorongosa meant an equally dramatic increase in the proportion of surviving tuskless female elephants. Whereas at the start of the war, tuskless females made up 18% of the female population in the park, they now represent more than half, and about a third of female elephants born after the war were tuskless. In total, the authors estimate that across the 28-year period the study analyzed, tuskless females were some five times likelier to survive than tusked individuals.
The researchers then drilled down into the selection mechanism by observing that there was no record of tuskless male elephants in the park. They hypothesized that any evolutionary mechanism acting on Gorongosa tusks would likely be an “X chromosome–linked dominant, male-lethal trait.” That means the mutation would be passed exclusively through female elephants, with just one copy needed to cause tusklessness in females and with male tuskless elephants dying in utero. If that were to be the case, tuskless mothers in the park would be much more likely to give birth to daughters. The data bore out that hypothesis. In the first decade after the war, they found that rates of tusklessness among female offspring of elephant war survivors continued at a rate almost twice that of pre-war populations. And not only that, but they found that tuskless mothers were giving birth to almost 66% daughters.
Finally, the authors moved to more nitty-gritty genomics to try to pinpoint the exact genes responsible for this wave of tusklessness. Comparing whole-genome scans for 18 Gorongosa elephants with and without tusks, they zeroed in on mutations on two possible genes: AMELX and MEP1a, both of which play an important role in tooth development in many mammals. AMELX is even associated with a similar “X-linked dominant, male-lethal” syndrome in humans—one that limits growth of our lateral incisors, our equivalent of tusks.
There’s plenty of precedent for the kind of rapid evolution the study authors are arguing for at Gorongosa. Bighorn sheep in Alberta, Canada; crickets in Hawaii; and lizards in the Caribbean have all been shown to exhibit remarkably fast transformation in response to evolutionary pressures. Still, to see a trait like tusklessness evolving within less than two decades, in a “long-lived, slow-reproducing species like the elephant, is incredible,” John Poulsen, a tropical ecologist at Duke University who was not involved in the study, told The Atlantic.
And Gorongosa is not the first place where elephants at high risk of poaching have evolved away from tusks. New Scientist reports that fewer than 5% of male Asian elephants in Sri Lanka still have tusks. The Atlantic notes that Zambia’s South Luangwa National Park and South Africa’s Addo National Park have both seen a dramatic rise in the proportion of tuskless female elephants.
But pinning down the exact genetic mechanisms at play and differentiating them from other pressures like climate change is always difficult in studies like this one, Chris Darimont, a conservation scientist at the University of Victoria, Canada, who was not involved in the study, told Nature. “It’s hard to prospect for these genes.” Plus, he added, there’s longstanding controversy about whether harvest pressure like hunting matters in the first place. Still, he called the genomic data offered in the new study “compelling,” saying the conclusions should serve as “a wake-up call in terms of coming to grips with humans as a dominant evolutionary force on the planet.”
A loss of tusks is not just a loss for elephants. Elephant tusks are “basically a Swiss Army knife for African elephants,” Pringle told New Scientist, helping them strip trees of bark, dig holes, find water. And many other animals indirectly depend on those tusks, snacking on bugs from barkless trees or getting water from those holes. “This is what maintains biodiversity,” co-author Shane Campbell-Staton, an evolutionary biologist at Princeton University, told New Scientist. “There are all these cascading consequences that can result from our actions that are quite surprising.”
With proper ecological protections in place, tusklessness will gradually disappear in Gorongosa, Pringle told The Guardian. “We actually expect that this syndrome will decrease in frequency in our study population, provided that the conservation picture continues to stay as positive as it has been recently,” he said. “There’s such a blizzard of depressing news about biodiversity and humans in the environment and I think it’s important to emphasise that there are some bright spots in that picture.”
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