Sometime in the distant past, well before humans walked the Earth, the ancestors of modern-day elephants evolved their iconic tusks. Elephants use their bleach-white incisors — they’re technically giant teeth, like ours but longer — to dig, collect food, and protect themselves.
Then Homo sapiens arrived, and elephant tusks became a liability. Poachers kill the massive animals for their tusks, which are worth about $330 a pound wholesale as of 2017. Hunters slaughter roughly 20,000 elephants a year to supply the global ivory trade, according to the World Wildlife Fund.
But just as tusks evolved because they provide a number of benefits, a striking new study shows that some populations of African elephants have rapidly evolved to become tusk–less. Published in the journal Science, the paper’s authors found that many elephants in a park in Mozambique, which were heavily hunted for their ivory during a civil war a few decades ago, have lost their tusks — presumably because tuskless elephants are more likely to survive and pass the trait on to their offspring.
While scientists have known about this trend for a while — it’s not uncommon to see tuskless elephants in places with lots of poaching — the study provides strong evidence that the trait is rooted in genetics, something previous research failed to do, said Andrew Hendry, an evolutionary biologist at McGill University who was not involved in the research. In other words, the study shows evolution in action.
The results also offer a vivid example of how animals can quickly adapt under human pressures such as poaching and climate change. Past research has shown that creatures can evolve new colors, shapes, and even behaviors to better tolerate the increasingly inhospitable world we’ve created for them. The problem is that even rapid evolution has its limits — and many species are already on the brink.
How a civil war caused elephants to lose their tusks
Social conflict and the decline of wildlife are often closely linked, the authors of the Science study write. Few locations reveal a clearer picture of this than Gorongosa National Park, a protected area in central Mozambique where Shane Campbell-Staton, an evolutionary biologist at Princeton University, led the research.
During a 16-year civil war that began in 1977, poachers on both sides of the conflict slaughtered a huge number of elephants in the park for their ivory, which they sold to finance their efforts, according to the study. Over that period, the number of large herbivores (like elephants) at Gorongosa fell by more than 90 percent.
That’s not all that changed in the park. Between 1970 and 2000 — a period that encompassed much of the impact of the long-running war — the portion of female elephants without tusks nearly tripled. The researchers’ best guess was that it had something to do with genetics: A trait visible only in females suggests it might be associated with changes to genes on the X chromosome. (Female elephants have two X chromosomes, whereas males have an X and a Y chromosome.)
This study all but proved it. The first bit of evidence was that female calves born from tuskless mothers were often themselves tuskless, indicating that the trait is passed on from one generation to the next. “A heritable trait is pretty strong evidence of a genetic basis,” said Robert Pringle, a biology professor at Princeton and a co-author of the study.
The authors also identified a couple of regions in the animals’ DNA that appear to be associated with a lack of tusks. Sure enough, “There is strong evidence for mutations on a particular region of the X chromosome,” Pringle said. Mutations, or variations in an organism’s DNA, are an important engine of evolution. If they result in traits that are beneficial — such as tusklessness, for certain populations of female elephants — they’re more likely to get passed to the next generation and drive evolution.
Remarkably, one of the genes associated with tusklessness is also present in humans, where it’s linked to a condition that limits the growth of our lateral incisors. These are essentially the same teeth that, in elephants, evolved into tusks millions of years ago.
What makes this study so fascinating is that it offers evidence of rapid evolution in an animal that has a pretty long lifespan — 50 or 60 years — in the wild, said Hendry and Fred Allendorf, a professor emeritus at the University of Montana who was not involved in the research.
Studies of elephants “rarely can say anything about the genetic basis” of tusklessness, Hendry said. For years, researchers assumed that rapid evolution was common only in small species with short life cycles. Given these results, “Nobody can argue that evolution isn’t occurring, even in the biggest and longest-lived species,” he added.
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