Some Elephants Are Evolving to Lose Their Tusks

 by Estella Sum-Campbell

Tusks are continuously growing elongated teeth that are utilised for a variety of tasks, including digging for food and nutrients, clearing paths through vegetation, debarking trees to obtain fibrous food, and helping males in the fight for females. In addition, other creatures rely on the work elephants do with their tusks too. For many other lesser species that depend on them, elephants play a crucial role as a keystone species, toppling trees and digging holes to access water. Habitats are created by the tusk action from the elephants as shown by certain lizards who favour living in trees that elephants have knocked over or damaged whilst browsing. The characteristics of the tusk’s main material, ivory, gives it its strength and stiffness to allow it to fulfil such a wide range of functions. 

The remarkable qualities of ivory are attractive to humans. Traditionally used to create ornaments and artwork with cultural significance, it has now gained importance as a status symbol. However, research indicates that the demand for ivory has contributed to the multibillion-dollar wildlife trade, which in turn promotes illegal hunting. Furthermore, recent studies have suggested that elephant evolution may have been impacted by this as well, putting those large incisors at a liability during periods of intense ivory poaching. 

In the Gorongosa National Park of Mozambique, Africa savannah elephants were the subject of a recent study conducted by scientists at Princeton University. During a civil war that raged from 1977 to 1992, more than 90% of large herbivores, including elephants, were slaughtered. As a result, the number of elephants fell from more than 2,500 individuals fifty years ago to less than 250 in 2000. The survivors were likely to share one important trait: about a third of younger females simply never acquired tusks - while, prior to the war, less than a fifth of younger females had no tusks. Elephants in Gorongosa that did not develop tusks gave them a biological advantage from hunting. The tuskless female survivors of the war passed on their genes, with predictable and yet unexpected outcomes. About half their daughters had lost their tusks. Even more confusing, out of their children, two-thirds were female. 

Mammals have two chromosomes that determine sex: males have XY and females have XX. A group of scientists in Gorongosa collected blood samples from seven female elephants with tusks and eleven without, and compared the DNA samples. The data from the elephant survey gave them an idea of where to look: they surmised that the X chromosome was responsible for the tuskless characteristic because the tuskless elephants were female. Additionally, they also suspected that the relevant gene was dominant, which means that a female only needs to change one gene to become tuskless and that this gene might stunt the development of male embryos if it is passed on to them. The hypothesis was supported by the pattern of genetic inheritance for the tuskless trait shown. 

Upon examining the genomes of the eleven tuskless elephants for indications of recent evolution, the scientists discovered a relevant DNA sequence on the X chromosome: AMELA, which is linked to the production of cementum and enamel, the minerals that cover teeth and tusks. Mutations that eliminate the corresponding gene in humans can result in a hereditary condition where females are born without 'maxillary lateral incisors', which are the teeth that become tusks in elephants. Given that one in ten female elephants have either on or no tusk, another genetic element must also play a role in determining this feature. The researchers then discovered MEP1a, a gene involved in the production of dentin, the main mineral in ivory, by comparing the DNA of tusked and tuskless elephants. So, in reaction to poaching, at least two tusk genes have rapidly developed. Given that tusklessness predates the Mozambican war, the characteristic probably isn't caused by new mutations, but by rare genetic variants that have become more prevalent in the gene pool. Due to the ivory trade, the tuskless characteristic has increased in frequency in the population due to the ivory trade, as females born without tusks have a higher chance of surviving and reproducing. 

This tuskless trait is not exclusive to Mozambique, either. Similar changes are also observed among female survivors and their daughters in other nations where there has been significant ivory poaching in the past. The impact has apparently been particularly severe in South Africa, when in the early 2000s, 98% of the 174 females in Addo Elephant National Park were said to be tuskless. 

Despite being practical as multipurpose tools, the fact that females have the ability to survive without tusks suggests that they are not essential for survival.