Gorongosa National Park in Mozambique was once home to thousands of elephants. Then, from 1977 to 1992, the country fought a long civil war that lasted 15 years. Fighters killed elephants for their ivory tusks and sold the ivory to pay for the war. By the end of the war, the park's elephant population had fallen by roughly 90 percent.
Decades later, researchers studied the survivors and found something strange. Across Africa, more than 90 percent of female elephants normally have tusks. Before the war, about 18.5 percent of Gorongosa's females were tuskless, already a high number. Among the females that survived the war, about 50 percent had no tusks at all.
Think about a bowl of red and blue candies. Now imagine someone picks out mostly the red ones and eats them. The blue candies did not change at all, but now a bigger share of what is left is blue. Something like this happened in Gorongosa. Hunters wanted ivory, so elephants with tusks were the ones most likely to be killed. Elephants born without tusks had nothing the hunters wanted, so more of them lived. The war did not make elephants lose their tusks. It removed most of the elephants that had them, so tuskless elephants became a bigger share of the survivors.
When one kind of individual is removed from a group, the others become a bigger share of what is left.
The pattern continued in the next generation. Of the females born after the war ended, about 33 percent were tuskless. Tuskless mothers also gave birth to daughters about two thirds of the time, a lopsided ratio. Almost every tuskless elephant in the park was female. Normally, mothers have sons and daughters in roughly equal numbers.
Tusklessness is not something an elephant develops from experience. It is a heritable trait, meaning it is passed from parent to offspring through genes. The scientists linked it to a gene on the X chromosome, called AMELX, and to a second gene on another chromosome. The tuskless version of the X chromosome gene appears to be deadly to male calves before birth.
- Step 1: Every elephant gets chromosomes from each parent. Chromosomes are bundles of genes. Females have two X chromosomes. Males have one X and one Y.
- Step 2: A gene that controls tusk growth sits on the X chromosome. One version makes tusks grow. A changed version stops tusks from growing.
- Step 3: A female with the changed version on one X grows up tuskless. Her other X still has a working copy, so she stays healthy.
- Step 4: A male has only one X. If it carries the changed version, he has no backup copy, and he dies before he is born.
- Step 5: A tuskless mother passes the changed X to about half of her calves. Daughters who get it live and are tuskless. Sons who get it do not survive. That leaves more daughters than sons, and almost no tuskless males.
Why the tuskless trait shows up almost only in female elephants.
Tusks still matter to elephants. They use them to dig for water, strip bark from trees, and defend themselves. Today, poaching in Gorongosa has mostly stopped, and the elephant population is growing again. Researchers continue to track how many calves are born with tusks, and whether tusks will become common again now that the danger has lifted.