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.
Should all elephants ditch their tusks?
In theory, it’s advantageous to be born without tusks in areas where poachers are active, Hendry said. But tusklessness also has its downsides. Elephants need their tusks to dig, lift objects, and defend themselves. The hulking incisors are not useless appendages.
The genes that seem to make female elephants tuskless also appear to prevent mothers from giving birth to male calves — that’s why all the tuskless elephants in the park are female, Pringle said. (Some mothers did give birth to males with tusks, who likely didn’t inherit the gene.) Over time, a shift in the sex of elephants could have consequences for population growth.
There are also potential costs to African grasslands, which are among the rarest and most biodiverse ecosystems on Earth, the study authors write. By turning over soil in search of food and minerals and gouging trees with their tusks, African savanna elephants prevent forests from growing too dense and help maintain grasslands. That’s why they’re considered “engineers” of the ecosystem. If they lose their tusks, a whole web of plants and animals may feel the impact.
“This evolutionary change could have massive cascading ecological influences,” Hendry said.
How humans are changing animals
Humans have shaped the environment around them for centuries, down to the very genetics of wild plants and animals. The tuskless elephants in this study are just one example in a long list of species that have adapted in response to the pressures we’ve placed on them.
“Human-induced changes are creating conditions for fast biological evolution — so rapid that its effects can be seen in only a few years or even more quickly,” a team of scientists wrote in a landmark intergovernmental biodiversity report in 2019.
One of the earliest and most famous examples is the peppered moth in the UK. Before the Industrial Revolution, most of the moths flitting about England were white with speckles of black, which helped them blend in with lichen and tree bark. Then, in the mid- to late-1800s, coal-fired power plants and mills started belching dark soot that blackened trees in parts of the country. White moths stood out against the newly dark background and were more likely to be eaten by birds, whereas the once-rare black ones were camouflaged and survived. In a matter of years, some populations of peppered moths inverted from white to mostly black. The phenomenon was deemed “industrial melanism.”
Scientists have measured similar changes in recent decades. One study from 2003, for example, found that bighorn sheep in Alberta, Canada, evolved smaller horns in roughly 30 years. The reason? Trophy hunters tend to target rams with larger horns. Another study, published in November 2020, suggests that a type of lily found in the mountains of China is evolving less-colorful leaves so it doesn't stand out in regions where it’s harvested as a traditional herb.
Rising temperatures from climate change also appear to be making some animals, including birds and mammals, smaller, as I previously reported. Smaller bodies cool off more easily than larger ones, so shrinking could be an adaptive response in warming environments (though it’s not yet clear whether these particular changes are genetic).
Then there are species changing in less conspicuous ways. In Japan, populations of mamushi snakes that are heavily hunted for their perceived medicinal and nutritional benefits seem to be better at evading predators, compared to snake populations that hunters have ignored. Many species, including plants and insects, have evolved resistance to pesticides, which is why farmers often use several at once and chemical companies must constantly develop new solutions.
There’s something like hope behind the idea of rapid evolution. Humans are deforesting, polluting, and exploiting the Earth at an alarming pace, yet in some cases, animals are adapting to live another day. There’s even a term for this resilience, Hendry said: “evolutionary rescue.”
Still, this evolution, as fast-tracked as it may be, still often isn’t quick enough to overcome the many threats species face. And because adaptations can also come with drawbacks, there are untold and unpredictable consequences for the ecosystem at large.
Plus, not all species can adapt their way out of crisis. Consider rhinoceroses, which poachers kill for their horns. Three of the five rhino species have been hunted almost to extinction, yet none appear to have evolved hornlessness.
In Gorongosa National Park, the ecosystem has largely recovered from the war, Pringle said. Poaching has subsided, but tusks haven’t bounced back. After the war, the park successfully rebuilt its infrastructure, ramped up law enforcement, and put social development programs into place. The presence of tuskless elephants is now akin to a scar from an injury that’s healed, Pringle said. So while evolution may have helped these creatures survive, the real remedy is putting an end to the underlying forces that triggered it in the first place.