Tracking down leafcutter ants: Mission impossible?

You’re in the rainforest. Its 30 degrees plus hot and humid, you’re sweating from every pore, and you have a very specific goal: to find leafcutter ants. On first appearances this seems a lot like finding a needle in a haystack…

Before you head out you need to grab a selection of digging tools (unexpected use for a dessert spoon), something to cut with (large branches to thread-like roots), containers of different shapes and sizes, and a medley of problem-solving tools (forceps, tape, bin bags…). Plus a good supply of water, long sleeves, wellies, and don’t forget the bug spray – everything that can bite you, will.

And then you’re off.

The basic strategy is ‘search for small moving things, carrying pieces of leaf-litter’: yep, that’s right, you’re looking for leaves among leaf litter on the forest floor.
I imagine it’s a bit like tracking a bear (I’ve never done it) except much, much smaller. Or the fieldwork equivalent of those police stakeouts in the movies… but with fewer rounds of coffee and doughnuts: you prop yourself up, you wait, and you watch.

Very carefully.

When you spot something promising the fun really starts, trying to follow the twists and turns of the ant’s foraging path all the way back to its colony entrance. It’s not uncommon that you’ll step on the ant’s odour trail before you spot the ant, leaving the ant running in circles and you back at square one. If you’re lucky the ant you spot doesn’t drop so deep into the leaf litter that you lose it, the colony is close, and the entrance is obvious – in reality, one out of these three is good going.

A few cardinal rules – e.g. don’t dig in the rain – and a lot of patience later, you hit upon an ant colony.

The attine ants (leafcutters and their relatives) share a novel way of getting food: they are all fungus-farmers. They vary in size from species of a few millimetres, to others where the large soldiers can be an inch or three (not to mention the monster Queens in the big leafcutter Atta species).

They vary in the places they build their nests and how big a colony is. There are the ones you can’t miss: the large leafcutter species, possibly familiar from natural history documentaries. You don’t have to hunt so hard for these but they can take hours of digging to collect, while defending yourself from the bites of angry soldier ants. But many other species you have to hunt down – just finding a colony can take hours. Some you find in cavities in the banks along rainforest creeks. Some have 10-plus separate chambers, up to a metre underground. Others hang their colonies in a kind of net, under fallen trees or rocks. And make no mistake, a longer hunt or a smaller ant species make little difference to how long you have to dig or how dirty you get in the process.

So why are we interested?

No doubt about it, fungus-farming ants are cool (they were developing agriculture before the great apes evolved). But these ants are also one of the most useful systems we have to try and better understand how mutualism – the name for cooperation that happens between two or more different species – works in nature.

Fungus farming ants, as with any successful farmers, need to both tend their crop and protect it from natural enemies. This led to three principle players as this mutualism developed: the ants, the fungus they cultivate as a crop, and unique bacteria that grow on the ant cuticles while producing antibiotics that the ants use as a pesticide on their fungus garden.
Mutualism is important in unexpected places: crop plants that fix nitrogen from the soil in their roots, coral bleaching or the conservation implications of this phenomenon, and human diseases affected by gut bacteria all involve mutualisms (or mutualism breaking down).

We’re interested in mutualism, how it evolves, whether and how these different species have, over evolutionary time, developed adaptations that increase the stability of this mutualism. We’re interested in the bacterial community that forms on the ants, and the antibiotics they produce. And much more besides.
Plus I defy anyone to hunt down a colony and not be taken aback by the complexity, curiosity, and unbelievable degree of organisational efficiency.
So, why are we interested in leaf cutter ants? My best answer is why on earth wouldn’t we be.

Tabitha Innocent is a Marie Curie postdoctoral research fellow at the Centre for Social Evolution, University of Copenhagen; CSE researchers are collaborating with the Hutchings group on this work.

http://socialevolution.ku.dk
http://www.uea.ac.uk/leafcutter-ants
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