By Fiona Hodge

Mercenary ants tip the balance in the plant war against elephants.

Plants are under constant attack by hungry insects, birds, and mammals. These animals can damage or destroy plants, depending on their size and feeding style. Elephants, the largest land mammal, are particularly brutal feeders. They swarm the trees in the savannas of Africa despite their foul tasting compounds, defensive spines, and buttress roots.

However, not all trees are equally attacked. Scientists Goheen and Palmer noticed that one species of tree, the Whistling Thorn Tree, always survived intact when elephants went through the neighbourhood. Closer inspection of the tree revealed an army of ants, housed in swellings like barracks at the base of the leaves. Each ‘barrack’ was catered with nectar, produced by the tree.

Were the ants really acting as an army to protect the Whistling Thorn Trees from the rogue elephants? Or did the ants simply live on the foulest tasting tree to avoid the elephants? Goheen and Palmer decided to do a range of experiments to find out.

They visited a Kenyan elephant orphanage, offering a range of tasty branches to elephants and recorded what was eaten and in what order. The branches on offer included Whistling Thorn Trees with and without ants, and a closely related tree Blackthorn with and without ants. The elephants didn’t distinguish between the different tree species, but definitely noticed and avoided ant-covered branches “like a kid avoids broccoli" according to Palmer.

Goheen and Palmer then went on to do field experiments, looking at how wild Whistling Thorn Trees with ants removed fared against elephants. They didn’t do well. Next they looked at vegetation trends in areas with elephant exclosure fences. Unlike other tree species, the Whistling Thorn Trees were not affected by protection from elephant damage.

So it is the tiny ants that, in return for catered barracks, protect the Whistling Thorn Trees from elephants over a billion times their size. Palmer now has a few ideas about how the ants might deter elephants… He says the inside of an elephant’s trunk is lined with nerve endings, making them very sensitive to ants, "It seems that elephants simply do not like ants swarming up the insides of their trunks, and I can't say I blame them".