With their long trunks, fanlike ears and prominent ivory tusks, elephants are one of the most iconic animals on the planet. But for certain elephant populations, those signature tusks have become increasingly scarce.
In a new study published Oct. 21, 2021, in the journal Science, researchers were able to analyze an unprecedented example of elephant evolution in action. In the span of just 15 years, the number of elephants born without tusks in Gorongosa National Park in Mozambique more than doubled. This was unusual, since tusks are typically advantageous for elephants: The massive animals use the tusks for everything from defense to stripping bark from trees. Stranger still, nearly all the tuskless elephants born were female.
"That's one of the mysteries of tusklessness," says Brian Arnold, a biomedical data scientist at Princeton University and co-lead author of the study, "Why is it largely present in females?"
Selective Pressures and Civil War
When English biologist Herbert Spencer coined the term "survival of the fittest," he wasn't talking about which animals would win in a pushup contest (though clearly, it's the western fence lizard). In Darwinian evolution, "fitness" refers to how well-suited an organism's particular set of traits are for surviving and thriving in a given environment. Which means that fitness is subject to change, depending on a huge range of environmental factors.
Everything from drought to air quality to foliage color can influence an organism's fitness — this is called selective pressure, and it's one of the main drivers of evolution. In the case of the Gorongosa elephants, selective pressure came in the form of war.
Between 1977 and 1992, Mozambique was locked in a brutal civil war. The conflict claimed the lives of between 600,000 and 1 million civilians in just 15 years. And humans weren't the only casualties; the environmental toll was also devastatingly high.
A 2018 study published in Nature determined that up to 90 percent of the large mammals in Gorongosa National Park — a 1,500 square mile (3,885 square kilometer) preserve in Mozambique's portion of the Great African Rift Valley — were killed during this period, too. Elephants in particular became a favorite target of poachers, who could sell the ivory tusks for a hefty sum and use the profits to buy more ammunition.
During this time, elephants with tusks were nearly five times more likely to be killed than elephants without tusks, according to Arnold. While having tusks is a huge advantage during normal circumstances, "under certain conditions, tusks are a liability," Arnold says. "During the Mozambique Civil War, they were especially a liability."
Fortunately, since 1994, Gorongosa's elephant population has been making a strong recovery, and instances of tusklessness are on the decline.
Intense poaching explains why tusklessness suddenly became an advantage. But that still left another mystery: Where were all the tuskless male elephants?
Killer X-linked Genes
Think back to freshman biology. You might remember making Punnett squares to solve practical everyday problems, like the odds of a pea plant having green or yellow pods. It's a classic example of dominant versus recessive inheritance, also known as Mendelian inheritance. If present, the "dominant" version of the trait is always expressed over the "recessive" version — this is why green peas outnumber yellow peas three to one.
You may also recall from freshman biology that, at least in mammals, two chromosomes (often) determine sex: the X and the Y chromosomes. A pair of Xs means female, while an X and a Y equals male. When a trait is located on an X chromosome, it is said to be an X-linked gene.
In the case of Gorongosa's elephants, the researchers noticed a peculiar pattern of inheritance unfolding. The tuskless trait was showing up in female elephants in what looked like a simple X-linked dominant pattern. But tuskless males were basically nonexistent.
"It suggests that this trait is largely controlled by a simple inheritance pattern involving an X-linked dominant, male lethal mutation," Arnold says. In other words, inheriting the tuskless trait kills off male elephants in the womb. X-linked dominant male lethal mutations are rare in nature, mostly because they can lead to population declines if they become widespread. However, some examples include a short-faced mutation in mice and Rett syndrome in humans.
Arnold and his co-researchers had answered the how and the why of tusklessness, but they weren't quite finished. To answer the question of where, they needed to dive deeper into the elephants' genetic code.
Locating the Tuskless Gene
It's hard to get a blood sample from a wild elephant. Luckily for the researchers, they weren't the only scientists studying Gorongosa's elephant population. Another team of ecologists from Princeton were conducting tracking studies, fitting elephants with GPS collars after tranquilizing them from a helicopter. "We kindly asked them: 'When you tranquilize elephants, could you target females?'" Arnold says, "And they said, 'sure.'"
Equipped with blood samples from a number of both tusked and tuskless female elephants, the team started sequencing genomes. They found several genes that indicated strong selective pressure in the population. But when they homed in on the X chromosome, they only found two. "And one of those is a gene that is known to be involved in tooth development in mammals," says Arnold.
While that in and of itself isn't definitive proof that it is responsible for tusklessness, it is a "tantalizing" candidate, Arnold says. Ultimately, further investigation is needed to fully unravel this pachyderm puzzle. And the researchers think that such work may be on the horizon.
"We hope that this study kind of starts a conversation about this mystery of tusklessness," Arnold says.