A strong set of tusks is often an advantage for elephants, allowing them to dig for water, remove bark for food, and compete with other elephants. But during episodes of intense ivory poaching, those large incisors become a liability.
Now, researchers have pointed to how years of civil war and poaching in Mozambique have led to a higher proportion of elephants never growing tusks.
During the conflict from 1977 to 1992, fighters on both sides slaughtered elephants for ivory to fund the war effort. In the region that is now Gorongosa National Park, around 90% of the elephants were killed.
The survivors likely share a key characteristic: Half of the females were naturally fanless, simply never developing fangs, whereas before the war less than a fifth were fangless.
Like eye color in humans, genes are responsible for elephants inheriting tusks from their parents. Although tusklessness was once rare in African savanna elephants, it has become more common, as rare eye color is becoming more widespread.
After the war, those surviving tuskless females passed on their genes with expected as well as surprising results. About helped her daughters to be fangless. More puzzling, two-thirds of his offspring were female.
The years of unrest “changed the trajectory of evolution in that population,” said evolutionary biologist Shane Campbell-Staton of Princeton University.
With his colleagues, he set out to understand how the pressure of the ivory trade had tipped the balance of natural selection. His findings were published Thursday in the journal Science.
Researchers in Mozambique, including biologists Dominique Goncalves and Joyce Poole, observed the approximately 800 elephants in the national park over several years to create a catalog of mothers and calves.
“The female calves stay with their mothers, just like the males, up to a certain age,” said Ms Poole, chief science officer and co-founder of the non-profit organization ElephantVoices.
Ms Poole had previously seen other cases of elephant populations with disproportionately large numbers of tuskless females following heavy poaching, including in Uganda, Tanzania and Kenya. “I’ve been wondering for a long time why it’s the females that don’t have tusks,” said Ms Poole, co-author of the study.
In Gorongosa, the team collected blood samples from seven female elephants with tusks and 11 without tusks, then analyzed their DNA for differences.
The elephant survey data gave them an idea of where to look: Because the tuskless elephants were female, they focused on the X chromosome. (Females have two X chromosomes; males have one X and one Y.) .
They also suspected that the relevant gene was dominant, meaning that a female needs only one altered gene to become fangless, and that when it is passed on to male embryos, it can short-circuit their development.
“When mothers pass it on, we think the offspring probably die early in development, a miscarriage,” said co-author Brian Arnold, an evolutionary biologist at Princeton.
Their genetic analysis revealed two key parts of the elephants’ DNA that they believe play a role in the transmission of the tuskless trait. The same genes are associated with tooth development in other mammals.
“They have produced irrefutable proof of genetic changes,” said Chris Darimont, a conservation scientist at the University of Victoria in Canada, who was not involved in the research. The work “helps scientists and the public understand how our society can greatly influence the evolution of other forms of life.”
Most people think of evolution as something that moves slowly, but humans can step on the gas.
“When we think of natural selection, we think of it going on for hundreds or thousands of years,” said Samuel Wasser, a conservation biologist at the University of Washington, who was not involved in the research. “The fact that this dramatic fangless selection has occurred for 15 years is one of the most surprising findings.”
Now scientists are studying what more tuskless elephants mean for the species and its savannah environment. Their preliminary analysis of fecal samples suggests that the Gorongosa elephants are changing their diet, without long incisors to peel the bark off the trees.
“The tusked females ate mostly grass, while the tusked animals ate more legumes and hardy woody plants,” said co-author Robert Pringle, a biologist at Princeton University. “These changes will last for at least several generations of elephants.”
This story was reported by The Associated Press.