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(CNN) -
An elephant's tusks are among its distinguishing features: They help the animal lift heavy limbs, knock down trees, pluck bark, fight and dig holes for water and minerals.
But a growing proportion of female elephants in Mozambique's Gorongosa National Park were born without these crucial tools, and scientists say it's an evolutionary response to the brutal slaughter of elephants for their ivory tusks during the 15-year war. civil of the country.
Elephant experts working in the park began to notice the phenomenon after the end of the war in 1992. Field data and analysis of old video recordings of the park found that the proportion of female elephants without tusks increased by more than from three between 1972 and 2000. It was a period when the elephant population declined dramatically, from about 2,000 to about 250, said Ryan Long, associate professor of wildlife sciences at the University of Idaho.
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"During the war, Gorongosa was essentially the geographic center of the conflict," Long said by email.
"As a result, there were a large number of soldiers in the area and a lot of associated motivation ... to kill elephants and sell the ivory to buy weapons and ammunition. The level of poaching was very intense."
Genetic signature
Scientists now have a better understanding of the genetic basis for this lack of fangs and why it only appears to affect females, according to a study published Thursday in the journal Science.
The analysis showed that the fanged-less females were more than five times more likely to survive over the 28-year period than their fanged counterparts, so adaptation is highly unlikely to be a fluke.
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Tusklessness occurs naturally - and only in females - even when there is no incidence of poaching, but typically only occurs in a small minority of elephants.
In Gorongosa in the 1970s, 18.5% of female elephants did not have tusks, while three decades later 51% did.
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"Evolution is simply a change in heritable characteristics within a population over successive generations, and based on the results of our study, the shift toward the absence of tusks among female elephants in Gorongosa fits this definition. perfectly, "said Long, an author on the study.
"The fact that it happened so quickly is really rare, and is a direct function of the strength of the selection," he said by email.
"In other words, it happened so quickly because the fangedless females had a MUCH greater chance of surviving the war and therefore a MUCH greater potential to pass their genes on to the next generation."
Researchers take genetic samples from a female elephant with tusks in 2018.
But what about male elephants?
After taking blood samples from 18 female elephants, with and without tusks, the researchers sequenced their genomes.
They found that the fangless females had a genetic variation in a very specific region of the X chromosome, which plays a role in the development of fangs.
"Females have 2 X chromosomes. In fangless females, one of those chromosomes is 'normal' and the other contains the deleted information," Long explained.
"When a female without fangs conceives a male, that male has a 50% chance of receiving the affected X chromosome from her mother. If she receives the 'normal' chromosome, she will survive and be born with the genetic information necessary to produce fangs."
However, if the male elephant fetus receives the chromosome with the genetic variant, it dies in utero because the variant that produces tuskless females is lethal to males, Long said.
According to the study, the exact genetic and developmental mechanism that causes the lack of tusks in females and the loss of male elephants during an elephant's 22-month gestation period is not yet known.
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Elephant numbers have rebounded in Gorongosa to about 800, Long said.
Not having fangs does not appear to be a major obstacle for females, but it is something the researchers want to study further.
Long notes that diet analysis suggests that fanged-less females eat a higher proportion of herbs.
"The population is doing well and there are many elephants without tusks. It is clear that they have adapted to life without them, but there are many things that we do not know."
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