Deadly poop

Leaf-cutting ants, as their name implies, cut leaves. Instead of hunting or scavenging for prey, they actively cut leaves and bring them back to their nest. These leaves are processed by smaller workers, and incorporated in to the fungus they cultivate. Furthermore, they manure the fungus by defecating on it. In turn this fungus happily produces little edible structures called gongylidia, which the ants eat.

Leaf-cutting ant cutting a leaf. Picture by Alex Wild.

However, now a conflict of interests arises. To illustrate, we humans grow many different crops for two reasons:
1) We like variety.
2) If we grow only one crop, and a disease breaks out, we would be left without food.

Number two is also very important for the ants. For them it would be great to grow different strains of fungus to prevent diseases from ruining their food-supply. However, the fungus doesn’t agree with this, as it also has its wants and needs, and it mostly wants to have as many offspring as possible. Therefore, it tries to secure its position and be the only fungus that the ants can cultivate!

Leaf-cutting ants defending their fungus garden. Picture by Alex Wild.

To do this, the fungus exploits the ants’ way of manuring. It adds compounds to its gongylidia, which do not get broken down in the gut of the ants when they eat them. These compounds remain present in the manure, and will attack any fungus except for the one that made them. Therefore, even if the ants would try to grow a new fungus by manuring it, they would actually kill it!

This leaves the ants with only one fungus to cultivate. How do they prevent it from getting sick all the time? That will be the topic for next time!

Source:
Poulsen & Boomsma (2005). Mutualistic fungi control crop diversity in fungus-growing ants. Science 307: 741-744