Creature Feature: Leafcutter Ants

The first Homo sapiens were expert foragers, keen at finding edible plants and chasing after prey animals. While many people today still live as hunter-gatherers, most people acquire their food through agriculture. We have been farming for over ten thousand years, time enough to become so good at it that we’ve reshaped the surface of the Earth. However, we’re not the only animals who farm, not by a long shot. As it turns out, the ants got there long before us.

Across the Americas, the foragers go about their work, carrying bits of snipped leaves and grass across the forest floor and down into their colony chambers. There, down in the musky dark, a towering, asteroid-shaped spire of whitish fungus stretches to the chamber wall, and its loyal ant caretakers tend to their crop, cultivating like a group of careful gardeners. The ants check the fungus for disease, coat it with antibacterial goo, then feed it the chewed-up remnants of leaves collected aboveground. For these leafcutter ants, the fungus– a still-unidentified species in the Agaricaceae family– is the lifeblood of the colony, and their larvae’s only food source. Fungi and ants have lived together like this for over 50 million years [1].

Leafcutter ants harvesting their fungus. [Source]

The relationship between these ants and the fungus is called a “mutualism,” where two species benefit from working together. After millions of years, the mutualism that ant and fungus share runs deep. Leafcutter ant larvae don’t eat anything other than the fungus they help to grow, and the fungus can’t survive without the ants to help care for it. These two partners are in it for the long haul.

There are dozens of species of fungus-growing ants, each with different divisions of labor in their ranks. Even though every worker in the colony shares the same mother (the queen), they can differ greatly in behavior and appearance. For example, in the leafcutter ant species Atta sexdens, the smallest workers play the role of gardeners; their job is to make sure the fungus is well cared-for and free from diseases [2]. Medium-sized worker ants diversify: some are foragers, responsible for exploring outside the colony and finding leaves. Other midsize workers are engineers and construction workers, tasked with carving out new chambers and passageways. Slightly smaller midsize workers become trash-handlers, removing harmful waste from the colony. The largest of the workers become soldiers, massive-jawed defenders of the colony, who guard the nest from potential threats. All of leafcutter ant society is built around their relationship with the white fungus; by having a dependent food source, these leafcutter ants have been free to specialize into a broad array of forms. As such, they have the most complex division of labor of any species other than humans.

Sitting atop the white fungus her workers cultivate, the queen ant dwarfs her ant children. [Source]

Long ago, a group of agricultural humans heading off to settle a new place would bring a bag of seeds with them, carrying the crops that would one day provide them with sustenance in their new home. Fungus-growing ants are much the same. When a young queen is ready to leave and start a new colony, she takes a piece of the family fungus with her. One day, if all goes well, that piece will grow as tall as the fungus it was taken from. 

It wasn’t always such an inseparable partnership. The ants’ ancestors once lived without farming. The fungus’ ancestors once could grow on their own. As their relationship developed, evolution caused the two organisms to change one another [3]. The ants acquired mutualistic bacteria that helped them adapt to eating the fungus. The fungus developed specialized nutrient-rich structures called gongylidia for ants to harvest and eat [4]. Over millions of years of coevolution, the ants and fungus that worked together best were rewarded: the queens that cultivated more fungus laid the most eggs. The fungi that fed more ants created the most new colonies. Farmer ants spread across the continents.

In many ways, humans are much the same. Since the invention of agriculture, we have forged many unique relationships with plants, animals, fungi, and bacteria. For example, our relationship with cows has caused some of us to develop lactase persistence, a genetic change that allows us to continue drinking milk long after early childhood. Around the world, we have formed special relationships with symbiotic bacteria that help us digest foods; we have become so dependent on these bacteria that without them, we can miss out on vital nutrients.

We may think we have changed our foods, but just like the leafcutter ants, our foods have also changed us over the course of just ten thousand years. Who knows what our relationships with domesticated crop species will look like after 50 million years? Some mutualisms run so long, it may be easy to forget how we ever survived without them. Maybe one day, evolution will tie us as closely to our crops as leafcutters to their fungus. For now, despite how far we’ve come, Homo sapiens still gardens in the shadow of an ant.


Jacob Johnson is a second-year animal behavior PhD student at UC Davis studying how animals respond to rapid environmental changes. When he’s not chasing birds or ants, he enjoys playing jazz saxophone, running board game nights, and writing about nature. His first book, Are You There God? It’s Me, Darwin will be available late next year.


References:

[1] Holbrook, T., R. Clark & B. Haney. (2014). A Closer Look at Castes. ASU – Ask A Biologist. Retrieved November 4, 2021.

[2] Wilson, E. O. (1980). Caste and Division of Labor in Leaf-Cutter Ants (Hymenoptera: Formicidae: Atta): I. The Overall Pattern in A. sexdens. Behavioral Ecology and Sociobiology, 7(2), 143–156.

[3] Schultz, T. R., & S. G. Brady. (2008). Major evolutionary transitions in ant agriculture. Proceedings of the National Academy of Sciences, 105(14).

[4] Aylward, F. O., C. R. Currie & G. Suen. (2012). The Evolutionary Innovation of Nutritional Symbioses in Leaf-Cutter Ants. Insects, 3(1), 41–61.

Main image source

[Edited by Alexandra Dwulit]

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