ELEPHANTS: WHY ARE THESE ICONIC AFRICAN ANIMALS LOSING THEIR TUSKS?
A genetic analysis suggests that African elephants are evolving to be tuskless, and it seems that poaching could be to blame
List an elephant's most iconic characteristics and tusks should be right behind the long trunk, and arguably ahead of big ears and thick skin. Tusks are elongated teeth that grow continuously and are used to dig forfood and nutrients, clear paths through vegetation, mark or remove tree bark, and for fighting between males. The tusk can serve those diverse purposes thanks to the properties of its main material, ivory, which makes it strong and tough.
Ivory's impressive properties make it attractive to humans. Traditionally used to make art and ornaments of cultural value, ivory has become a valuable status symbol. But as studies have shown, demand for ivory has helped to fuel a multibillion-dollar wildlife trade that encourages illegal hunting. And now new research has found that this may also have left an evolutionary mark on elephants.
A study by biologists at Princeton University looked at African savannah elephants in Mozambique's Gorongosa National Park. During a civil war that lasted from 1977 to 1992, more than 90 per cent of large herbivores were slaughtered, including elephants. The elephant population dropped from more than 2,500 individuals 50 years ago, to less than 250in 2000.
And while the population decreased, the proportion of female elephants without tusks increased: comparing historical videos to modern footage showed a rise from 19 per cent to 51 per cent. Among females born after the war, one-third are tuskless.
In mammals, sex is dictated by a pair of chromosomes: females are XX, males XY. Because nearly all male elephants have tusks, the Princeton biologists suspected that the tuskless mutation was linked to the X chromosome. After searching the genomes of 11 tuskless elephants for signatures of recent evolution, the biologists found one relevant DNA sequence on the X chromosome: AMELX, a gene that helps produce enamel and cementum, two minerals that coat tusks and teeth.
A pattern of inheritance predicts that all daughters of two-tusked mothers should be born with two tusksor none at all, but 1 in10 female elephants have one tusk or zero, so the trait must also be influenced by a second genetic factor. By comparing DNA from tusked and tuskless elephants, the researchers identified another gene, MEP1a, which is involved in producing dentin, the core mineral in ivory.
As tusklessness existed before the war, the trait probably isn't caused by new mutations, but by rare variants that are now more common in the genepool. Driven by the harvest of elephants for ivory, the tuskless trait has become more common as females born without tusks are likelier to survive and reproduce.
Though humans are technically part of nature, calling that process ‘natural selection' is vague. The evolution of tuskless elephants is an example of ‘harvesting selection' or ‘human-driven selection'.
Despite tusks being useful, the fact that females can cope without them suggests they aren't essential for survival. Indeed, of the three living species of elephants, females of the forestand savannah elephants tendto be tusked, but female Asian elephants might only have short protrusions called ‘tushes'. Tusks don't seem to be vital to Asian males either: in Sri Lanka just 10 per cent are tusked.
One explanation is that 3,000 years of huntingand domestication favoured the loss of tusks in Asia, whereas African elephants have only recently been exposed to human-driven selection.
So yes, elephants are losing their tusks. But while it's tempting to focus on how we are altering another animal's anatomy, it's difficult to untangle the feature from the dramatic dropin population size: according to figures from the World Wide Fund For Nature, African elephants numbered three to five million in the early 20th Century; today there are just415,000.
The issue extends beyond elephants. The world's largest living land animal will bulldoze its way through habitats, which can trigger a transition from forest to grassland and change the local composition of species. Elephants are ‘ecosystem engineers' whose behaviour has knock-on effects. Compared to the human-driven selection that's driving the loss of tusks, which has taken mere decades, the ecological functions played by elephants can't be restored quickly. As the Princeton biologists conclude: “Restoration of these functions may require disproportionately longer time scales than the initial selection event.”
“As tusklessness existed before the war, the trait probably isn’t caused by new mutations, but by rare genetic variants that are now more common in the gene pool”