Credit: Jarrod J. Scott
One of the most amazing spectacles in all of nature is the community formed by leaf-cutter ants. Most of us have seen nature videos of these ants trooping down trees with their recently-harvested bits of leaves. Many probably realize that the ants don’t actually eat the leaves themselves. Instead, they use the leaves as food for underground fungus farms. But few are aware of the complex system the ants have evolved to care for their fungi.
Fungus-farming ants are, next to humans, perhaps the most elaborate examples of an organism that actively cultivates another organism for food. Microbiologist Moselio Schaechter wrote last week about a pair of studies examining the role of anti-fungal bacteria the ants rely on to help cultivate their fungus.
Since the fungus and its leaf-food is a rich source of nutrition, it’s only natural that other, undesirable fungi might grow in the ants’ underground farms, either as “weeds” consuming the plant matter, or parasites on the fungus itself. In their vast colonies, with populations of up to 8 million ants occupying underground spaces larger than many New York apartments, pest management is essential. When a queen migrates to form a new colony, she carries with her not only a small sample of the fungus her offspring will soon cultivate, but also at least one type of bacteria, Pseudonocardia, which produces an antifungal chemical that helps keep the weeds at bay. Other bacteria also play a role in keeping unwanted fungi out of the ants’ garden, and the ants may even adjust the balance of fungicides they use depending on the particular leaves they’re providing as food for their fungus.
But while the leaf-cutter ant’s system of farming is remarkable, it’s by no means the only organism that raises its own food. The “aspiring protistologist” who blogs as Psi Wavefunction wrote last July of a protist that also seems to create its own food. In this case, a single-celled foraminiferan known as T. blocki manages to promote the growth of bacteria for later consumption. T. blocki lives on seagrass, crawling slowly along the seaweed’s surface and ingesting bacteria as it moves along. Behind it is a trail of “waste,” which bacteria see as a nutritious mix of mucus and sugar. A 1993 study by Martin Langer and Chris Gehring found that within 48 hours after the T. blocki laid its trail, the tracks were populated by a dense mix of bacteria. The foraminiferans then turned around and harvested their “crops,” once again laying a trail for the next batch of bacteria. The study was published in the Journal of Foraminiferal Research.
“Farming” like this isn’t limited to animals and protists: Even plants sometimes get into the act. Last summer, the microbiologist who blogs as “Lab Rat” wrote about a 2010 study showing how trees can influence the populations of microbes living in the soil around their roots. The trees encourage the growth of microbes that deposit useful minerals, although the precise way they do this isn’t yet known. Lab Rat suspects that small-molecule signals, similar to those used by bacteria to communicate with each other, may be at work. Essentially, trees may hijack the bacterial communication system for their own ends, thus getting the nutrients they need for survival.
While many species from throughout the living world engage in farming, there is little doubt that leaf-cutter ants are the most visible wild creatures that use farming to survive. It has been estimated that they consume up to 20 percent of the greenery in the Amazon rain forest. Even if you don’t get a chance to visit the rain forest in person, see if you can find a nearby leaf-cutter ant exhibit. I visited the leaf-cutter exhibit in the at the Montshire Museum of Science in New Hampshire with my daughter last year. The plexiglass-encased exhibit, like many others in museums and zoos around the world, showed all aspects of colony life, from harvesting to fungus farming to waste management, and we both spent the better part of an hour watching the remarkable division of labor in this amazing insect’s community.
Dave Munger is editor of ResearchBlogging.org, where you can find thousands of blog posts on this and myriad other topics. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns ».
Originally published November 10, 2010
