An elephants' set of tusks is usually an advantage since they grant them the ability to dig for water, strip bark for food, and joust with other elephants. During periods of intensive ivory poaching, however, those large incisors become a burden.

Researchers have now identified how years of civil conflict and poaching in Mozambique have resulted in a higher proportion of elephants that will never develop tusks.

During the battle, which lasted from 1977 to 1992, militants on both sides massacred elephants for ivory to fund military activities. Around 90% of the elephants in the area that is now Gorongosa National Park were killed.

The survivors were likely to have one thing in common: half of the females were naturally tuskless          - they never acquired tusks - whereas, before the battle, less than a fifth lacked tusks.

Genes, like eye color in humans, determine whether elephants acquire tusks from their parents. Although tusklessness was historically uncommon in African savannah elephants, it is becoming increasingly frequent, much like a unique eye color becoming more common.

Following the conflict, the surviving tuskless females passed on their genes, with both expected and unexpected effects. Half of their daughters lacked tusks. What's more confusing is that two-thirds of their offspring were female.

The years of unrest “changed the trajectory of evolution in that population,” said evolutionary biologist Shane Campbell-Staton, based at Princeton University.

He and his colleagues set out to figure out how the ivory trade's pressure had skewed the scales of natural selection. 

Over several years, researchers in Mozambique, including biologists Dominique Goncalves and Joyce Poole, studied the national park's around 800 elephants to compile a registry of mothers and children.

“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.

Poole had previously seen elephant populations with a disproportionately large number of tuskless females following intensive poaching in Uganda, Tanzania, and Kenya. “I’ve been puzzling over why it’s the females who are tuskless for a very long time,” said Poole, who is a co-author of the study.

The scientists obtained blood samples from 7 tusked and 11 tuskless female elephants in Gorongosa, then studied their DNA in an attempt to spot any differences.

The elephant survey results pointed them in the right direction: Since the tuskless elephants were female, they concentrated their efforts on the X chromosome. (Females have two X chromosomes, whereas men have one X and one Y.)

They also hypothesized that the relevant gene was dominant, implying that only a female requires one changed gene to become tuskless and that when passed on to male embryos, it might stunt their development.

“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.

Their genetic investigation showed two crucial regions of the elephants' DNA that scientists believe play a role in the characteristic of tusklessness transmission. In other mammals, the same genes are linked to tooth development.

"They've established the smoking gun proof for genetic changes," said Chris Darimont, a conservation scientist from Canada's University of Victoria, who was not involved in the study. The effort "assists scientists and the general public in understanding how our culture can have a significant impact on the evolution of other living forms."

Most people conceive evolution as a slow process, yet the matter of fact remains that humans have the ability to 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 what the presence of more tuskless elephants means for the species and its savannah environment. Their preliminary investigation of fecal samples indicates that the Gorongosa elephants are changing their diet, as they no longer have long incisors to remove bark from trees.