Ivory poaching in Africa drove the rapid evolution of tuskless elephants in some regions, but the good news is that increased protections from poachers are helping the pachyderms get their tusks back.
"In African elephants, tusklessness is very rare," said Brian Arnold, a biomedical data scientist at Princeton University. "But if you look in particular areas, the rate of tusklessness is much higher than average."
To find out why, Arnold and his co-author Shane Campbell-Staton, a biologist at the University of California, Los Angeles, traveled to Gorongosa National Park in Mozambique. In the 1970s, aerial surveys showed about 2,500 elephants lived in the park. Using photos taken during the surveys, Arnold and his colleagues estimated that during that time, roughly 18% of the population lacked both of their tusks, while 9% were missing only one.
In 1977, the aerial surveys stopped due to the outbreak of the Mozambican Civil War, which lasted until 1992. When population surveys resumed in 2000, the conflict's toll on Gorongosa's elephant population was clear: Fewer than 250 elephants remained in the park, and of the survivors, over 50% lacked tusks, a nearly threefold increase in the trait.
Arnold suspects much of the population decline during the war was a direct result of poachers killing elephants, since both sides relied heavily on the ivory trade to finance their war efforts. However, elephants migrating away from the area could have also contributed to the overall decline, he said.
"There was clearly a survival advantage for tuskless elephants," he said. Tusks are typically important for an elephant's survival, as they help them dig for underground water sources and strip the bark from trees, which is an important part of an elephant's diet. But when elephants are hunted for their tusks, this beneficial trait becomes a death sentence.
As they sifted through data, the researchers noticed an interesting pattern: All of the tuskless elephants are female. To understand why, Arnold and colleagues observed the first generation born from the war survivors. For each calf, they recorded whether it had tusks, and then whether its parents had tusks.
On average, they found, 50% of daughters born to a tuskless mother will be tuskless just like her, but all male calves will have tusks. In addition, instead of having male and female calves at a nearly equal frequency, two-thirds of a tuskless mother's calves will be female.
According to Arnold, this pattern suggests that the gene that causes tusklessness is carried on the X chromosome, meaning it is an X-linked dominant trait. Female elephants, like humans, have two X chromosomes. So if one of those X chromosomes carries a mutant tuskless gene while there is a normal gene on the other X chromosome, the female calf will fail to develop tusks. But to survive, the other X chromosome needs to have the normal version of the gene so it can counteract the mutant one to some degree. In this case, one mutant gene is enough to interfere with tusk development, but otherwise, the elephant is reasonably healthy.
Because a mother has a 50-50 chance of passing on either the normal or mutant gene to her offspring, the probability that a daughter of hers will be tuskless essentially comes down to a coin toss. For her sons, things are a bit riskier. Because the male embryos will get only one X chromosome from the mother to go with the Y chromosome they get from their father, inheriting the X chromosome with the mutant gene is a death sentence. The Y chromosome doesn't carry the same genes as X, so it will never have that much needed backup gene to counteract the tuskless gene. And having only the mutant gene is invariably fatal. So half of male embryos — those who inherit the tuskless X — will die before birth, explaining the lopsided sex ratio in the offspring of tuskless moms.
The researchers identified two potential genes that may cause tusklessness, AMELX and MEP1a. One of those genes, AMELX,is involved in mammalian tooth development. In humans, this gene plays a role in the development of lateral incisors, the teeth that elephant tusks evolved from. And intriguingly, in humans this gene is also linked to the development of an X-linked dominant syndrome that is lethal in males. This means much the same thing for human males that it does for male elephants: males that inherit the mutant version on one X chromosome don't have a backup.
"In theory, as tusklessness becomes more common, fertility will decline," as tuskless females pass on a lethal syndrome to half their male offspring, Arnold said.
"There is a bright spot to this story," Arnold said. "Since 1994, elephant populations have been on the rise in Mozambique." At the same time, tusklessness is on the decline, likely due to the fact that tuskless mothers are less fertile. This suggests that the elephants of Gorongosa National Park are on track to return to their former tusked glory.
Originally published on Live Science.
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Cameron Duke is a contributing writer for Live Science who mainly covers life sciences. He also writes for New Scientist as well as MinuteEarth and Discovery's Curiosity Daily Podcast. He holds a master's degree in animal behavior from Western Carolina University and is an adjunct instructor at the University of Northern Colorado, teaching biology.