A Winning Formula

Beneath the rainforests of South America lives a fungi that consumes 50,000 leaves a day without ever coming to the surface. It relies on ants to bring it food in exchange for nutrients. Evolutionary biologist Dr Pepijn Kooij speaks to Amy Barrett about this special relationship...

- Amy Barrett

The leaf-cutter ants rely on their fungus for food. Why don’t they just cut out the middleman and eat the leaves themselves?

Dr Pepijn Kooij Digesting plant material is a difficult process, even for us humans. It’s a hard thing to do. Think about herbivores, carnivores and omnivores, and just look at, for example, the lengths of their intestines. If you take a tiger – a carnivore – the length of its intestines is about six metres. But if you look at a cow – the perfect example of a herbivore – the intestine is about 24m long, and cows also have multiple stomachs. It’s a long process to digest plant material and you need lots of different bacteria to help you do it.

Plants have this defence mechanism: an enormous cell wall around their cells to protect them from being eaten. It takes a lot of energy to get to the parts in the plants that we actually need – the nitrogen and proteins. But there are specialised organisms, such as bacteria [in a cow’s stomach], that are designed to do that kind of work. For the ants, they want those proteins, but it’s very hard for the insects to actually extract them. So they have their fungus help penetrate the cell, then the ants feed on the fungus.

But doesn’t the fungus need the protein? Why would it share?