Elephants with large tusks have an advantage in digging for water, stripping bark for food, and jousting with other elephants. Those large incisors, on the other hand, become a liability during periods of severe ivory poaching.
Researchers have now identified how years of civil war and poaching in Mozambique have resulted in a higher proportion of elephants never developing tusks.
Fighters on both sides murdered elephants for ivory to fund war operations throughout the conflict, which lasted from 1977 to 1992. Around 90% of the elephants in the area that is now Gorongosa National Park were slaughtered.
Half of the females were naturally tuskless – they just never acquired tusks — whereas less than a fifth lacked tusks before the battle.
Genes determine whether elephants receive tusks from their parents, just as they do in humans. Tusklessness was originally uncommon in African savannah elephants, but it's now more prevalent, much as a unique eye color becoming more common.
Those tuskless surviving females passed on their genes after the battle, with both expected and unexpected effects. Approximately half of their daughters lacked tusks. What's more confusing is that two-thirds of their children were female.
The years of turbulence "altered the track of development in that group," according to Princeton University evolutionary researcher Shane Campbell-Staton.
He and his colleagues set out to figure out how the ivory trade's pressure had skewed the scales of natural selection. Their findings were published in the journal Science on Thursday.
Over several years, researchers in Mozambique, including biologists Dominique Goncalves and Joyce Poole, studied the national park's around 800 elephants to compile a list of moms and offspring.
“Female calves stay by their mothers, and so do males up to a certain age,” said Poole, the scientific director and co-founder of the nonprofit ElephantVoices, according to the Associated Press.
Poole had previously seen elephant populations in Uganda, Tanzania, and Kenya with a disproportionately large number of tuskless females following intensive poaching.
“I’ve been puzzling over why it’s the females who are tuskless for a very long time,” said Poole, who is also a co-author of the study.
The scientists obtained blood samples from seven tusked and 11 tuskless female elephants in Gorongosa, then studied their DNA to see whether there were any changes.
The results of the elephant survey pointed them in the right direction: The tuskless elephants focused on the X chromosome since they were female. Males have one X and one Y chromosome, whereas females have two X chromosomes.)
They also hypothesized that the relevant gene was dominant, meaning that a female requires only one changed gene to lose her tusks and that passing it on to male embryos may cause them to develop too quickly.
“When mothers pass it on, we think the sons likely die early in development, a miscarriage,” said Brian Arnold, a co-author and evolutionary biologist at Princeton.
Scientists discovered two important parts of the elephants' DNA that they believe play a role in the characteristic of tusklessness being passed along. In other mammals, the same genes are linked to the production of teeth.
Most people conceive of evolution as a gradual process, yet humans can accelerate it.
“When we think about natural selection, we think about it happening over 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 selection for tusklessness happened over 15 years is one of the most astonishing findings.”
Scientists are now investigating the implications of more tuskless elephants for the species and its savannah habitat. Without lengthy incisors to peel bark from trees, their early examination of feces samples reveals the Gorongosa elephants are altering their diet.