On the trail of a sad selection effect: Researchers shed light on the genetic cause behind the spread of tusklessness in female elephants in the course of ivory poaching. According to this, a genetic peculiarity that is mediated by the X chromosome and specifically prevents the formation of tusks in females was able to gain acceptance. It is a striking example of the strong evolutionary selection effects that humans can exert on living beings, say the researchers.
“Survival of the fittest” – the individuals who are best adapted to the challenges of their way of life survive and pass on their characteristics to subsequent generations. According to Darwin’s theory of evolution, this basic principle leads to the development of species and the development of their typical characteristics. In the case of elephants, this is how the tusks developed in the course of their development history: They are useful for the animals in obtaining food and are therefore subject to positive selection. But in the age of man these practical tools became known to be a curse for the pachyderms: Thousands of them still fall victim to the greed for ivory.
Protective loss
A particularly bad episode of the slaughter of elephants took place during the civil war in Mozambique from 1977 to 1992. During this conflict, the armed forces on both sides relied heavily on the ivory trade for funding. In order to “harvest” tusks, elephants were therefore hunted on a large scale in the Gorongosa National Park. The population decreased by more than 90 percent. As the researchers working with Shane Campbell-Staton report, this was linked to a development that is already known from other heavily hunted elephant populations: By comparing it with recordings before the caesura, the team was able to document that intensive poaching in Gorongosa has increased sharply which led to tusklessness in female elephants. About half are now affected. It is noteworthy that there is no lack of tusks in male animals.
Using population models, the researchers were initially able to substantiate the assumption that the phenomenon was an evolutionary development due to the selection pressure caused by poaching: Since elephants without tusks were not the target of the shotguns, the feature was apparently able to establish itself surprisingly quickly in the population . But what is typical female tusklessness based on? The researchers then pursued this question through a genetic test. They sequenced the genomes of seven female elephants with tusks and eleven individuals who are missing them. Through genetic comparisons, they were able to look for differences in the genetic material that could be the basis of the trait.
Tracking down the genetic basis
As the scientists explain, the lack of tusklessness in male elephants showed that the trait is based on a genetic characteristic that has a gender-specific effect. Thus, the female X chromosomes as the carriers of the responsible genetic make-up came into focus. The investigations made it clear that these genetic material carriers in female elephants without tusks show a strikingly high genetic variability in an area that includes a relevant known gene: AMELX is known to have a function in the formation of teeth in mammals.
It is particularly interesting: In humans, mutations that affect AMELX and neighboring genes lead to a tooth anomaly in women: The development of the upper lateral incisors is disturbed. As the researchers explain, these are the human counterparts to the tusks in elephants. It is therefore obvious that mutations affecting the AMELX system underlie the toothlessness of female elephants. The scientists emphasize that more precise mechanisms and the spread of certain mutations with an effect on tusk development must now be clarified in further studies.
According to them, however, the study makes it fundamentally clear that humans have become a significant evolutionary factor in nature: By selecting certain characteristics in the context of exploitation, they can shape the development of many species. As the scientists finally emphasize, this can also have consequences that go beyond the species directly affected. In the case of elephants, for example, widespread tusklessness can have consequences for the development of the ecosystem. Because without these tools, the animals may be less able to keep the development of woody plants in their habitat in check, which can change the vegetation and thus many other features of the ecosystem. In conclusion, Campbell-Staton and his colleagues write: “Understanding the dynamics of rapid evolution in the Anthropocene is important in order to show the biological effects of human activities and to develop strategies to mitigate the consequences”.