Leaf-Cutter Ants

While there are many things that you might associate with Costa Rica – three-toed sloths, the resplendent quetzal, tamanduas, gallo pinto (that’s rice and beans, not an animal!) – leaf-cutter ants are probably not one of those things. I certainly wouldn’t have put them on the top of my list of ‘things I want to see’. But once you get to Costa Rica, you really can’t miss them. They are everywhere, in very large numbers, and you have to notice them, because stepping on their trails and ending up with them in your socks is not enjoyable. Leaves are not the only thing they can cut with their oversized mandibles.

Once you’ve learnt to step over them, not on them, they are fascinating as a species. They live in colonies of up to 5 million ants, and the nest will take up several square metres underground. The workers (‘media’ workers to be precise) communicate using pheromones, and designate a particular tree or plant for harvesting each day. However, the leaves they cut are not for them to eat. These ants are essentially gardeners, and they only grow one thing – a fungus, on which they are completely dependant. They carry the leaf pieces back to the nest (each ant can carry a leaf pieces up to 3 times its own weight) where the ‘minima’ workers are waiting. The minima act as quality control, rejecting any leaves that are sub-par, and then they chew the leaves up, mix them with saliva and faeces (yummy…) and attach the resulting mixture to the fungus found in the nest. The fungus needs the leaf mixture to grow and reproduce, and the ants then eat the hyphae it produces. It is not just the ants that rely on this relationship – the fungus has lost the ability to reproduce without the ants, and so the two species are completely co-dependant.

Leaf-cutter ants carrying leaves significantly bigger than they are!

Leaf-cutter ants carrying leaves significantly bigger than they are!

This does make things a bit tricky when a queen ant has to start a new colony. Having secreted a little bit of fungus in a pouch in her abdomen, she will mate on the wing with the flying males, and then will find a spot to dig a tunnel and try to keep her fungus alive. If the fungus dies, then so does she, along with all her eggs. These worker eggs will emerge in 4-5 weeks, and then they will take over the feeding of the fungus. The colony will take up to 4 years to reach its full size.

The ants clear the trails they walk on of obstructions they can move, and walk around the ones they can't.

The ants clear the trails they walk on of obstructions they can move, and walk around the ones they can’t.

Given how difficult it is for a new colony to be started, it is surprising and quite impressive to see just how widespread these ants are. We found them everywhere on our trip, from the hotel garden in San Jose to the tropical rainforest on the Caribbean side of the country. And so long as you don’t get bitten, watching them is strangely enjoyable!


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