Poaching ivory fuels the evolution of toothless elephants

Toothless elephants have an advantage in areas where ivory poaching is prevalent. Image credit: Peter Chadwick/Getty

During the 1977-1992 civil war in Mozambique, Africa, people sold ivory to supplement military expenses, during which about 90% of elephants were hunted. A human war has had a serious impact on elephants. Since then, ivory poaching has also been common.

On October 22, a study published online in Science found that this extreme hunting practice had dramatically changed the main elephant population there, which was more inclined to give birth to female elephants without ivory.

However, this adaptation comes at a cost, with associated genetic mutations that can cause male elephants to die before birth, but this new trait may help save the population.

When humans hunt, they usually target individuals with specific traits, such as large fish or sheep with long horns, which prompts prey to evolve. But few have figured out the genetic manifestations behind this man-induced evolution.

Shane Campbell-Staton, an evolutionary biologist at Princeton University in the United States, is curious about elephants in Gorongosa National Park in Mozambique, where female toothless elephants (larger African elephants and both male and female elephants have tusks) are very common.

First, Campbell-Staton wants to determine whether the proportion of toothless elephants in Mozambique has indeed changed. He and his colleagues analyzed videos of elephants in the national park taken before the civil war. At the time, about 18 percent of female elephants were born without ivory. But according to decades of observations by the nonprofit conservation group ElphantVoices, the percentage of female elephants born after the Civil War rose to 33 percent of female elephants born toothless.

This shift may be due to random or inbreeding during population bottlenecks, but the team's study of the likelihood of toothed and toothless females surviving the war using computer simulations suggests that the increase in the proportion of toothless females is more likely to be caused by selection.

Next, the researchers wanted to figure out the genetic mechanisms behind the birth of toothless female elephants. Because there are no toothless males in the elephant population, the researchers suspect that this trait may be caused by genetic mutations on the X chromosome, including a dominant toothless gene and a recessive gene that is only fatal to males.

The ivory inheritance pattern in the ElephantVoices database supports this hypothesis. For example, if a toothless female elephant carries a dominant toothless gene from the mother, the number of toothless and toothless female baby elephants in their offspring is the same. In addition, the number of female elephants in their offspring is twice that of males. This sex ratio may be due to a recessive gene that dies when the offspring male inherits it.

To find these genes, the team collected blood from 18 female elephants in Gorongosa National Park and sequenced them, finding two genes that stand out — MEP1a and AMELX (which are active in tooth development in other mammals). Both were expressed in 7 toothed elephants, but mutations occurred in 11 toothless elephants.

In humans, mutations in AMELX are associated with male prenatal death; in females, this type of mutation hinders the growth of upper incisors, which are the tusks of elephants.

For now, researchers don't know why mutant AMELX, located on the X chromosome, is fatal to males.

Taken together, the findings suggest that poachers' behavior of acquiring ivory by hunting elephants creates a selection of mutated AMELX and MEP1a, allowing these two genes to spread through the herd, eventually leading to toothless elephants becoming more common.

Today, poaching in Gorongosa National Park is no longer practised and elephant populations are recovering. But it may take a long time for the tusked female to return to the same number as before.

"This is the shadow of the human imprint that will take generations to erase." Robert Pringle, an ecologist at Princeton University and one of the authors of the study, concludes. (Xu Rui)

Source: China Science Daily