Acacia Trees Prevent Elephant Attacks With Armies of Ants
Ants in your pants? That's nothing compared with ants up your snout. And that's what elephants in the African savanna must contend with when trying to snag a meal from a certain type of acacia tree.
Ants protect the tree in exchange for both housing that the tree produces in the form of bulbous hollow thorns, as well as food in the form of a sugary nectar that is secreted at the base of the leaves
It’s a classic David and Goliath story, except there are 90,000 Davids and they all have stings. On the African plains, the whistling-thorn acacia tree protects itself against the mightiest of savannah animals – elephants – by recruiting some of the tiniest – ants.
Elephants are strong enough to bulldoze entire trees and you might think that there can be no defence against such brute strength. But an elephant’s large size and tough hide afford little protection from a mass attack by tiny ants. These defenders can bite and sting the thinnest layers of skin, the eyes, and even the inside of the sensitive trunk Researchers found that ants are such a potent deterrent that their presence on a tree is enough to put off an elephant.
African plants have many defences against elephants. Some try to match them for strength by developing thick buttresses. Others rely on physical defences like thorns and poisons. Yet others abandon the fight and concentrate on being able to re-grow quickly. None of these measures is foolproof; despite these efforts, elephants frequently damage trees, severely and sometimes fatally.
The whistling thorn acacia (Acacia drepanolobium) is a striking exception. Even in the heart of elephant country, it’s rarely touched and can grow in dense forests. It’s no coincidence that the whistling thorn has a partnership with ants. It provides them with homes in the form of swollen thorns, and food in the form of nectar. In return, the ants attack any invaders, regardless of their size.
In a recent study, researchers found that six young elephants were happy to feed on branches from either the whistling-thorn acacia or a related species – the blackthorn – if they didn’t have any ants. If ants were about, the elephants avoided both types of branches. These results showed that there’s nothing about the whistling-thorn that’s inherently off-putting to elephants; it’s the ants that make the tree a no-go zone.
Whistling-thorn cover stayed the same both inside and outside of the “exclosures”. In contrast, related acacias without the protection of ants became far less common outside the protected areas.
Over a long period of time, these small differences can lead to substantial changes in the African landscape.
It is extraordinary to think that these tiny insects can decide the fates of entire forests, at the hands of the savannah’s largest animal.
"The elephants wouldn't even touch the branches with ants on - they could smell the ants and knew it would be painful to eat them" Professor Jake Goheen, University of Wyoming
