From 1977 to 1992, the southeastern African country of Mozambique was gripped by a civil war during which both sides financed their efforts by hunting African savanna elephants for their ivory. This intense poaching slashed the elephant population of Gorongosa National Park by 90 percent.
It also had another unintended impact on the animals, scientists reported this week. Female elephants without any tusks became much more common after the conflict. When the researchers examined population records and genetic material from park-dwelling elephants, they found that this trait rose swiftly as a response to poaching, and they pinned down several genes that may cause it.
“This study highlights the ubiquity of human influence across the tree of life,” says Shane Campbell-Staton, an evolutionary biologist at Princeton University, who published the findings on October 21 in Science. “Even the largest organisms on the planet are not just being affected by population decline…we’re literally changing the trajectory of their evolution moving forward.”
There are numerous cases of animals evolving in response to human activities, from moths darkening amidst the smog of the Industrial Revolution to lizards upping their heat tolerance to survive in sweltering cities and insects becoming resistant to pesticides. “Most of what we know comes from relatively small animals that are very abundant and have very quick generation times,” Campbell-Staton says. These changes are harder to track in large, long-lived animals that reproduce slowly, such as elephants.
After the Mozambican Civil War ended, people began to notice that tuskless elephants had become more common in Gorongosa National Park. However, Campbell-Staton realized, little was known about what genetic changes might have actually caused the phenomenon.
To get to the bottom of this mystery, he and his team pored over photos and videos of elephants in the park from before the conflict, as well as more recent observations. Generally, it’s rare for elephants to be born without the ability to develop tusks, and the trait is only seen in females. The park’s elephant population already had an unusually high number of tuskless females before the war, perhaps as a consequence of past hunting practices. Afterwards, though, the proportion of females without tusks had nearly tripled to 50.9 percent.
“The question is whether or not this is actually natural selection favoring tuskless females, or whether or not it’s just because of the entire population decline that you get this rise simply by chance,” Campbell-Staton says.
He and his colleagues ran computer simulations of the waning population from 1972 to 2000. They concluded that the odds of winding up with a population in which half the females were missing tusks were very slim, unless this characteristic was giving the elephants a survival advantage.
The researchers suspected that the mutations responsible for tusklessness would be dominant—meaning that only one copy of the abnormal genes would be needed to result in a lack of tusks—in female elephants, and lethal for males. Supporting this idea, they observed that tusked mother elephants didn’t have tuskless daughters.
On the other hand, tuskless elephants should have one abnormal copy of the gene inherited from their own mothers, and one normal copy inherited from their fathers. This means that a tuskless mother has a 50 percent chance of passing the trait to her offspring. Sure enough, the researchers found that elephants born to tuskless mothers were more likely to be female (the gene is deadly in males), and about half of these daughters were tuskless.
Campbell-Staton and his team next searched for differences in the genomes of tusked versus tuskless elephants. They identified two genes related to tooth development that might have contributed to the rise of tuskless females. One, called MEP1a, plays a role in forming dentin, the layer of tissue surrounding the tooth’s pulp.
The other is known as AMELX and is involved in enamel production. In humans, abnormalities in this gene and several of its close neighbors collectively cause a syndrome that’s lethal to males and can prevent the maxillary lateral incisors, which correspond roughly to elephant tusks, from growing in females.
The precise nature of the genetic underpinnings of tusklessness in the park’s female elephants have yet to be determined, Campbell-Staton says. Another open question is whether the same thing could happen in other elephant species that are also threatened by poaching, adds John Poulsen, an ecologist at Duke University who studies how African forest elephants are affected by human disturbances and was not involved with the new research. Still, he says, finding such quick evolution in the elephants of Mozambique “is remarkable.”
“When I first read the paper, my first thought was, ‘Wow this inspires hope for biodiversity conservation; maybe species like the elephant that are really slow-reproducing can actually adapt quickly to human pressures,” Poulsen says. However, the evidence that males cannot develop this trait is troubling, particularly since male elephants are already targeted more by poachers because of their larger tusks. It’s possible that the abundance of tuskless females could focus more poaching pressure on male elephants.
“That puts us in a real pickle,” Poulsen says. “It also means that we can’t sort of depend on evolution to get us out of this problem of unsustainable killing of elephants…we still need management and we still need policy efforts to try to conserve elephants.”
The evolution of tusklessness among elephants may also have far-reaching consequences for other species. Tusks are a bit like a Swiss army knife for elephants, Campbell-Staton says. Elephants use their tusks for a variety of tasks, including stripping bark off of trees and digging for subterranean minerals and water sources. These activities can open up habitat for other animals, destroy trees, and allow other plant species to grow—and have a myriad of other complex ecological effects.
“If elephants don’t have this key tool to provide these important services, then what happens to the rest of the ecosystem?” Campbell-Staton says.
In recent years, the elephant population of Gorongosa National Park has grown and the prevalence of tusklessness has begun to drop. Still, the animals grow slowly and take nearly two years to gestate their young. The elephants may have evolved quickly in response to poaching, Campbell-Staton says, but it will take them much longer to rebuild their numbers and play the same role in the ecosystem as they did before the war.