Leafcutter ant colonies operate like super-organisms | ECOVIEWS

Whit Gibbons
·3 min read

Leafcutter ants of the American tropics exemplify animals that exhibit super-organism status, advanced social organization and symbiosis.

As you walk through the jungle you become aware of these insects when you notice a parade of green leaf parts marching across the trail. Closer examination reveals that each leaf fragment is held aloft by a large reddish ant, a tiny Hercules. You follow the trail of moving leaves to a mound of soil populated by a colony of ants, as many as a million or more.

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The almost 50 known species of leafcutter ants are confined to the Western Hemisphere but range from Argentina through South and Central America into Mexico. At least two species can be found in the U.S., from Louisiana to Arizona.

The broad definition of symbiosis includes interactions of unrelated species in which one or both benefit from the association. Leafcutter ants are the paragon of a form of symbiosis known as mutualism in which each species enhances the welfare of the other. With leafcutters ants, neither the ants nor a particular fungus could survive without the other.

Whit Gibbons is professor of zoology and senior biologist at the University of Georgia’s Savannah River Ecology Laboratory.
Whit Gibbons is professor of zoology and senior biologist at the University of Georgia’s Savannah River Ecology Laboratory.

The ants cut living tree leaves into pieces small enough to carry. Designated workers take the fresh leaf parts to the mound where they provide nourishment for a type of fungus that serves as a food source for the ants.

According to T.R. Schultz and S.G. Brady (Smithsonian Institution) in Proceedings of the National Academy of Sciences, “Agriculture is a specialized form of symbiosis” known in “only four animal groups: humans, bark beetles, termites, and ants.”

To appreciate the overall impact of leafcutter ants, the authors call them “the dominant herbivores of the New World tropics.” Some reliable estimates indicate that leafcutter ants consume more than 20% of the leafy vegetation in the American tropics.

You do not become top-notch agriculturists without an organized production program. Although high species diversity among leafcutter ants has created variability in the details of farming techniques across geographic regions, a common denominator is an extraordinary division of labor within a colony.

Foraging, one of the specialized tasks for colony members, is carried out by ants that find a source of leaves, cut the leaves and carry the cuttings to the mound. Some ants are soldiers that challenge potential predators, whereas others have the equally critical job of tending the fungus gardens.

Parker Gibbons stands atop a leafcutter ant mound in Belize. The 15-by-20-foot mound houses an ant colony with a queen that could be 20 years old and have laid 200 million eggs.
Parker Gibbons stands atop a leafcutter ant mound in Belize. The 15-by-20-foot mound houses an ant colony with a queen that could be 20 years old and have laid 200 million eggs.

A vital part of taking care of the gardens is to remove decaying leaves as well as a pathogenic fungus that is harmful to the kind of fungus the ants are harvesting. The little sanitation engineers also remove dead ants from the colony. Waste management involves an exclusive part of the workforce that does not switch jobs with other workers, presumably because they become tainted for the rest of their lives.

Most leafcutter ants have underground dumps to which they transport their waste. In the journal Behavioral Ecology, A.G. Hart and F.L.W. Ratnieks (University of Sheffield, UK) described a species of leafcutter ants in Panama that has waste dumps outside the colony.

Further division of labor occurs among the waste managers themselves. Some are full-time transporters and others mind the waste heap.

The researchers noted that all outdoor waste dumps are downhill from the mound where the colony lives. Clever ants. Furthermore, to avoid contamination, the paths that workers use to bring leaves to the fungus gardens never cross the paths that waste managers use to go from the gardens to the dump. Very clever ants.

Leafcutter ants indisputably qualify for super-organism status: different individuals perform different tasks to the benefit of the entire population, and a lone individual cannot survive.

Much of the success of super-organisms, which include most of the more than 10,000 kinds of ants in the world as well as honeybees, is that individuals function selflessly, always placing the welfare of the colony ahead of their own.

Whit Gibbons is professor of zoology and senior biologist at the University of Georgia’s Savannah River Ecology Laboratory. If you have an environmental question or comment, email ecoviews@gmail.com.

This article originally appeared on The Tuscaloosa News: Leafcutter ant colonies operate like super-organisms | ECOVIEWS