The civil war that ravaged Mozambique between 1977 and 1992 left marks on the evolutionary trajectory of the elephants that inhabit one of the country’s national parks. During the years of conflict, the indiscriminate hunting of the species, in search of its coveted ivory, meant that females without tusks became the majority in the area, something rare in natural populations of elephants.
Everything indicates that the process is linked to a set of mutations in the DNA of pachyderms, reveals research published in a recent issue of the specialized journal Science. These are genetic alterations that appear to be lethal for the development of male offspring, but would not have such severe effects on females, which explains why they have spread so much through the female population of the species.
This is one of the most intense and rapid effects of human action on the evolution of a wild species, say the study’s authors, led by Robert Pringle, from Princeton University (USA). The team combined long-term observations in Gorongosa National Park in central Mozambique with DNA analysis of elephants and computer simulations to understand how the mammal population had transformed over the past few decades.
During the civil war, elephant ivory became an important source of income for both sides of the conflict — as a result, the elephants lost 90% of their population in the park.
At the same time, another change occurred: whereas before the war only 18.5% of females in the reserve had no prey, this proportion jumped to 50.9% of them in the first decade of the 21st century (there are also females with only one prey, characteristic still poorly understood).
The fact that there were no males without ivory led scientists to suspect that the trait was linked to the X chromosome, one of the components of the genome responsible for determining sex. As with humans, male elephants carry an X and a Y chromosome, while female elephants have two X chromosomes.
This means that a potentially harmful DNA mutation on this chromosome tends to have milder effects on females: if one of their X chromosomes is affected, the other can carry a “healthy” version of the same gene, which avoids more serious problems for the female. body. Males, on the other hand, who only have one X chromosome, would be unprotected in the face of a mutation with deleterious effects.
An important piece of data that matched exactly this hypothesis had to do with the offspring of females without fangs. Two-thirds of their babies were female – while the expected proportion of sexes generated by the species’ mothers is roughly fifty-fifty, as in the case of humans. And two-thirds was exactly the expected number if the mutation in a single mother’s chromosome were lethal to developing male embryos (if the other maternal chromosome is normal, she can still produce male babies, but to a lesser extent).
The final piece of the puzzle was fitted with the analysis of the animals’ genomes. The researchers used well-established techniques to identify areas favored by natural selection — that is, stretches of DNA that are very similar in different individuals, indicating that they carry versions of genes that helped their carriers reproduce more efficiently than other members of the species.
With that, they arrived at a gene on the X chromosome that is precisely associated with the formation of teeth – which, of course, is something to be expected in the case of animals with abnormalities in the formation of prey. In humans, alterations in this same region of DNA are associated with malformations in the skull and face of women, and are lethal in the case of male embryos. A version of another gene, on a different chromosome, also appears to be linked to the lack of tusks in female elephants.
The good news is that, with the end of the war, the situation appears to be slowly returning to normal. “We found that the frequency of preyless females in the generation born after the war was lower than in the generation that went through the conflict,” Pringle told sheet. To be exact, females without ivory are now 33% of the total.
“Fangless females tend to have fewer offspring than fanged ones because some of their pregnancies [as de embriões machos] they end up being unfeasible,” he explains. “In the absence of a strong selection in favor of the absence of ivory, that is, when hunting is restricted, this characteristic should decrease in frequency over time.”
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