In Mozambique there are more and more elephants without tusks. Cause is poaching, which makes an actually disadvantageous mutation to the survival advantage.
Mutation and selection are the drivers of evolution. At the – sad – example of African elephants in Mozambique, this phenomenon, once recognized by Charles Darwin, can currently be observed again. There have always been some in the population of pachyderms living there that did not have tusks from birth due to a genetic mutation.
In a world without poaching, this is more of a survival disadvantage. The teeth serve the animals for defense as well as a tool. And males, which inherit the mutation, die already as embryos, so that only toothless females are born.
But the selection factor humans changed the disadvantage into a survival advantage: The bloody civil war in Mozambique in the years 1977 to 1992 was financed by the opposing camps with poaching and ivory sales.
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Selection pressure on elephants with tusks was so great that the number of pachyderms in Gorongosa National Park declined massively. Only the toothless females were spared, so that after the war half of the female animals no longer had tusks. That reports a research team around Shane Campbell-Staton of the University of Princeton in New Jersey in the technical periodical "Science".
Not having tusks increases the chances of survival
"We have also been observing in the Serengeti for some time that elephants are walking through the savannah with smaller tusks or no tusks at all more frequently than in the past," says Dennis Rentsch, who worked for the Frankfurt Zoological Society (ZGF) in Tanzania for 15 years and has been deputy head of Africa at the ZGF in Frankfurt since the beginning of 2021. "Elephants with large tusks nowadays live mainly in the best protected areas".
But until now, there has been a lack of comprehensive hard data and background for a connection between poaching and elephants without tusks, says ZGF managing director Christof Schenck. This has now provided the US team. While there were 2542 elephants living in Gorongosa National Park before the civil war in 1972, after the waning of war-related poaching, not even a tenth of that population remained at 242 animals. At the same time, the proportion of elephant cows without tusks tripled from 18.5 percent to 50.9 percent.
Because elephants without ivory were not of interest to poachers, they were about five times more likely to survive the civil war than their tusked counterparts, Campbell-Staton and his team determined. "This meant that an important basis for the evolution of elephants had changed," says Schenck.
The mutation-related survival disadvantage of tusklessness was outweighed by the massive survival advantage in an environment dominated by poaching – although the tusks are certainly an important tool for the animals: For example, they use them to dig holes to access groundwater, Rentsch says, and peel the bark off baobab trees in times of shortage or take down nutritious leaves and twigs to get food. Bull elephants also need tusks for fights against competing conspecifics.
But poaching made all these features that animals had developed over millions of years meaningless against the background of pure survival. Suddenly, mutants, the minority of elephants without tusks, had an advantage and their share of the total population increased.
A piece of the X chromosome is missing
The AMELX and MEP1a genes apparently play a central role in this process, Campbell-Staton’s team has found through elaborate genetic analyses. Both genes control the development of teeth in mammals. While MEP1a is located on elephant chromosome 1, AMELX is located on the X chromosome, of which female mammals normally have two, while male mammals have only one X chromosome, which is complemented by a Y chromosome – much like in humans.
In which there is a clinical picture of teeth that are reduced in size and decayed because of poorly developed enamel, which is caused by the fact that the affected women lack the very section of the X chromosome in which the AMELX gene is located. Since the missing section on the X chromosome also contains other vital genes, male embryos that, unlike females, lack a second intact X chromosome with these vital genes, die during embryonic development. You can not compensate the gene loss. This is probably also the case in elephants and could explain why only elephant cows but no bulls without tusks are observed.
Fish also shrink in size and bighorn sheep shrink in horns
The study confirms what conservationists are observing: Poaching and other human impacts not only decimate species, but also alter their evolution – sometimes at breathtaking rates.
And not only in elephants. For example, fish species that are heavily fished have become smaller in recent decades, biologist Robert Arlinghaus of the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin has observed: Small fish are more likely to escape through fishing net meshes than larger ones. Researchers at the University of Victoria have also observed a gradual shrinking of horns in heavily hunted bighorn sheep in Canada, they write in Science.
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"Reversing such evolutionary steps triggered by us humans in just a few years can take many decades or centuries," says Christof Schenck, "or the initial state can never be reached again."To hope that endangered species could save themselves from extinction by "saving" mutations would be mistaken. In rhinos, for example, which are also highly threatened by poaching, there are no known hornless mutants that deter poachers.