ETHE VOLUTION ENSURES that the animals are well adapted to their situation. Sometimes, as with predators and prey, these circumstances include the behavior of other creatures. And, as an article has just appeared in Science described, which includes the behavior of human beings, which can force drastic changes on a species in the blink of an evolutionary eye.
Shane Campbell-Staton, a biologist at Princeton University, studies how animals adapt to human creations like cities and pollution. His interest was piqued by a film about the helpless female elephants of Gorongosa National Park, Mozambique. Their lack of tusks was believed to be the consequence of another human creation: the Mozambican civil war, which lasted from 1977 to 1992 and was partly paid for by the slaughter of elephants for their ivory. About 90% of the pachyderms living in Gorongosa were reportedly killed. Biologists have therefore questioned whether increasing the absence of tusks could be an adaptation to make elephants less attractive to human hunters.
It was a plausible theory, says Dr. Campbell-Staton, but no one had actually tested it. Through a mix of old video footage and surveys, he and his colleagues concluded that around 18% of female elephants in Gorongosa had no tusks before the war. Three decades later, after its end, that number had risen to 50%. Computer simulations suggested that the likelihood of such a rapid change to happen by chance, even in a small population, was minimal.
In addition to confirming the change, the researchers were able to unravel its genetic roots. The absence of defenses is caused by a mutation of a gene on the elephantine X chromosome. (As with humans, two X chromosomes make a female, while a X and one Yes make a male.) Unfortunately for males, the mutation is a bundle, causing changes in neighboring genes that interfere with embryonic development. Males who inherit the mutant gene die before birth. Women can avoid life-threatening side effects if either of their two X the chromosomes contain an unmutated gene, but they will continue to grow defenseless.
Fortunately for females, the specifics of how the mutant gene is inherited prevent them from inheriting two copies. Since mutant males die before being born, those who survive to childbearing age carry only unmutated versions of the X chromosome, ensuring that their daughters also have at least one copy.
At present, the continual reintroduction of non-mutants X male chromosomes limit the spread of lack of defenses in the female population. But, given the time and genetic recombination, explains Dr. Campbell-Staton, evolution could disentangle the mutation for the lack of defenses from the maladaptive mutations in its neighboring genes, opening the door for males to come together. also get rid of their tusks. There are occasional rumors, he says, of defenseless male elephants in the wild, but – so far at least – no solid evidence.
Finding one now seems unlikely. With the end of the war, the evolving pressure of poaching eased. Tusks became useful tools again, helping their owners remove bark from trees and dig for water. In recent years, the prevalence of helpless females has dropped to around 33%. But the speed of change is a reminder that wars can change the history of evolution as well as mankind. â
This article appeared in the Science & Technology section of the print edition under the title “Spoils of war”