Gorongosa National Park in Mozambique was once home to thousands of African elephants. But during the Mozambican Civil War, which lasted from 1977 to 1992, soldiers and poachers killed elephants to sell their ivory tusks and fund the fighting. By the war's end, the elephant population had dropped by roughly 90 percent. The survivors were not a random sample of the original group.
Across most African elephant populations, more than 90 percent of females have tusks. Tusklessness is naturally uncommon. Before the war, about 18.5 percent of Gorongosa's females were tuskless — already higher than average. But among the females that survived the war, about 50 percent had no tusks at all. Elephants with tusks had been targeted and killed at much higher rates.
Natural selection is like sorting a jar of mixed marbles. Imagine you have a jar with mostly red marbles and a few blue ones. Now imagine someone reaches in and removes almost all the red marbles, leaving the blue ones behind. The jar didn't make new blue marbles — the blue ones were always there. But now, blue marbles make up a much bigger share of what's left. That's what happened with Gorongosa's elephants. Poachers removed tusked elephants, so the tuskless ones made up a much larger share of survivors. When those survivors had babies, they passed on their traits — including tusklessness.
Tusklessness is a heritable trait, meaning it is passed from parent to offspring through genes rather than learned or developed during an individual's life. Researchers from a 2021 study published in Science found that tusklessness is linked to a dominant allele — a version of a gene — on the X chromosome, near a gene called AMELX, along with a second gene called MEP1a on a different chromosome. The tuskless allele appears to be lethal to males, which explains why tusklessness shows up almost entirely in females.
A diagram showing two versions of the X chromosome side by side. Left side labeled 'Typical allele' shows an X chromosome with the AMELX gene region highlighted, with a label reading 'Tusk development proceeds normally.' Right side labeled 'Tuskless allele' shows an X chromosome with the same AMELX region highlighted in a different color, with a label reading 'Tusk growth is blocked.' Below, two boxes: one shows a female elephant symbol (XX) with the note 'Females can survive with one tuskless allele and one typical allele, or two tuskless alleles — they are tuskless but alive.' The other shows a male elephant symbol (XY) with the note 'Males who inherit the tuskless allele on their single X chromosome do not survive — this is why tusklessness appears almost only in females.'
The tuskless allele sits on the X chromosome; females have two X chromosomes, but males have only one, which is why the allele is lethal to males.
The effects of this selective pressure carried into the next generation. Of the female elephants born after the war ended, about 33 percent are tuskless — lower than the survivors' rate of 50 percent, but still far above the pre-war rate of 18.5 percent. Tuskless mothers also give birth to daughters about two-thirds of the time, a skewed sex ratio likely connected to the male-lethal effect of the tuskless allele.
Tusks are still useful tools for elephants. They use them for digging water holes, stripping bark from trees, and defending themselves. The rapid rise in tusklessness shows how intense selective pressure during the war reshaped the traits of Gorongosa's elephant population within a single generation.