Leaf cutter ants, a 50 Million year old tradition.
About thirty years ago Diane and the kids lived in an apartment in San Jose and I traveled back and forth between there and Hacienda Barú each week. The guy who lived in the apartment upstairs was a mycologist. That's a fancy word for someone who knows everything about fungus. Jesse was his name and he grew mushrooms for a living. He used to bring home bags of fat, juicy mushrooms and share them with all the neighbors. Since I love mushrooms about any way you can prepare them, I have always retained a pleasant memory of Jesse. Before making his acquaintance it never occurred to me that it might be difficult to grow them. After all, mushrooms are just a fungus and this is a hot humid climate where fungus goes wild. It grows really well in places where you don't want it, like on bread, fruit, binocular and camera lenses, VHS tapes, and between my toes. You can't get rid of the stuff. Growing it is easy; keeping it under control is the problem. Anyway, that's what I thought. But Jesse told a different story.
I can't remember all the details because it was a long time ago and the process was very complex. The mushrooms were grown in hot houses where temperature, humidity and ventilation were carefully controlled.
This was in the 1970s before the computer age, but for that time, the equipment he used to control the environment around the mushrooms was very sophisticated. The medium where they were grown was cow manure that was specially prepared and tested to make sure that the pH was correct and that it wasn't contaminated with bacteria or other types of fungus. Minerals and other nutrients were added. Contamination was the most serious problem. I remember a time when Jesse's mushrooms had become infected with something or other. He was a nervous wreck, getting up in the middle of the night to go out and check on the mushrooms and generally driving his wife crazy. He was like a mother with a sick baby, but his baby was a fungus. Growing mushrooms was far from being the piece of cake I had imagined.
Living near the rain forest and learning its secrets tends to change the way you look at things. One of my major learning experiences with regard to nature's wonders was when I discovered ants that cultivate fungus for food. You may know them as parasol ants or leaf-cutting ants. They have been doing what Jesse does for around 50 million years and they do it with only the tools provided by mother nature.
Atta cephalotes is the most common species of fungus growing ants in this part of the tropics, but there are close to 200 species throughout the Americas. With a very elaborate social structure and communication system the Atta ants are considered to be one of the most highly evolved of the fungus growers. Daily they accomplish feats that are almost beyond human comprehension. So, let's have a closer look.
Leaf-cutting ants harvest more foliage than any other group animals in the Neotropics. They utilize around 15% of all the green matter produced by the rain forest. A large colony of seven or eight million ants will consume as much green matter as a cow, but much of this foliage is not eaten directly by the ants. They do consume some of the plant juices, but the more fibrous parts of the leaves are fed to a fungus, which breaks them down into more basic nutrients. The ants then eat the fungus, which is their primary food source. Only one kind of fungus will work for them, and DNA testing has determined that the fungus the ants grow today is genetically identical to the one their ancestors cultivated 50 million years ago when these early agriculturists began developing their skills. They have kept it pure by not allowing it to fruit and propagating it only by cloning.
Most of us have seen green columns of leaf bearing ants marching through the forest, across our lawns, or through our gardens. Some of us have followed those trails and found that it is not unusual for the leaf-cutters to walk as far as two city blocks, climb to the top of a tree as tall as a ten story building, cut pieces of leaves and flowers and carry them all the way back to their colony. Unfortunately, some of us have also learned that leaf-cutters have a special affinity for anything new that we plant in our gardens. If we plant something that is not native to Costa Rica they will usually strip it of all foliage in a very short time. A few of us who simply must investigate further have noticed that, although most of the ants appear to be about the same size and shape, there are a few that are much larger than the leaf carriers, while others are much smaller and often ride on the leaf crescents being carried by their sisters. To learn more about these fascinating creatures, we must have a look into the writings of those who have delved deeper into the world of Atta cephalotes. I owe much of my knowledge about the fungus growers and the information I share with you here to the extensive writings of E. O. Wilson, Bert Hölldobler, Eric Hoyt and G.C. Stevens.
In order to deal with the problems of cultivating their particular kind of fungus, Atta cephalotes have evolved their own complex system. You could say that the ants bring the fungus into their homes. They build an underground colony which, on the average, consists of about 1500 chambers connected to one another by tunnels. The smallest chambers are the size of your big toe and the largest the size of your head. Some are used for living and resting quarters, some for waste disposal, some for brood production, one is for the queen and others are for fungus production. Ideal levels of temperature and humidity must exist in the fungus production chambers. To accomplish these objectives and fulfill their needs, the ants build their city in a very specific way.
An average leaf-cutter colony with approximately five million ants is from three to six meters deep. Digging and moving the amount of earth necessary to create such a colony is a colossal task comparable to the building of the pyramids in ancient Egypt. One researcher measured and weighed the dirt that the ants had excavated and piled into a mound on top of their average sized colony. The leaf-cutter ants had, over a period of about five years, carried to the surface 22 cubic meters of earth weighing over 30 tons, about two large dump truck loads.
The shaft exits in the mound are always situated so that rain will drain away from the openings. The principal ant entrances are located away from the mound of dirt, usually lower. In order to create an air flow and control the humidity and temperature, the ants pile all their refuse, including dead ant bodies, unused leaves and other useless organic matter, in large chambers near the bottom of the city. As it decomposes, this material heats up and raises the temperature of the surrounding air which then rises through the labyrinth of tunnels leading to the top of the mound. The rising air creates a draft which draws in fresh air from over a 1000 vents. The temperature and humidity of the chambers will vary according to their distance from the hot air flow. The fungus is cultivated in chambers which are located where optimum conditions exist.
The basic growth medium used in Jesse's operation was specially prepared cow manure. In an Atta cephalotes colony leaf matter is the basic medium for fungal growth. Its selection, preparation and care is a meticulous process which if done properly will result in lots of good healthy fungus. Finding the proper leaves is the first step. The selection process is poorly understood by anyone but the ants. Nevertheless it is known that the colony sends out scouts to search for suitable leaf material. Any plant species new to the colony must be tested before any quantity of its leaves are brought into the fungus cultivation areas. A single scout discovers a plant that is unknown to the colony.
She cuts a piece of leaf and returns with the fragment leaving chemical markers along her path. She delivers the fragment to another worker within the colony. That worker prepares the test leaf being careful to keep it separate from the rest of the fungus growing medium. She then places some fungus on it and monitors the results. If the fungus thrives on the new material a crew of several thousand leaf carriers are dispatched out into the jungle to the exact location where the original leaf fragment was collected. There they cut and transport substantial quantities of the new foliage to the nest.
Think about the magnitude of this undertaking and the communication skills that come into play. It could be compared to a person going into a city with a population in excess of five million and delivering a single document to a government office. Imagine that this messenger leaves the document with one employee out of the many thousands who work there. That employee immediately begins a study of the document in order to verify its validity. After two days, it is determined that the document is authentic, and several thousand other employees are dispatched to a distant location which is encoded into the document. They go directly to the correct place located some ten to fifteen kilometers from the government office. Then they perform some action that will be beneficial to everyone who lives in the city. I have trouble imagining this scenario happening with any human government, but the leaf-cutters do it all the time.
All the members of the colony contribute directly or indirectly to the cultivation of the fungus. The most obvious of the seven distinct castes of Atta cephalotes are the leaf carriers marching through the forest with their green parasol-like cargo. When they arrive at the colony and deposit their leaf crescents, another caste takes over -- the cleaners. Each leaf fragment is meticulously scraped and licked until clean. Later it is cut into smaller pieces, chewed, mixed with saliva and formed into a soft wad. The ants then place some fungus starter material, called mycelia on the medium and place it beside other newly planted fungus in a suitable chamber.
From that point another caste, the fungus caretakers, step in and take over the process. These ants are responsible for keeping the fungus clean and free from impurities and infection. They do this partially by physically removing any foreign life form that tries to grow on either the medium or the bread-like fungus. But they have a few other tricks up their sleeves. A recent article in the New York Times entitled "Ants, Mushrooms and Mold: An Evolutionary Arms Race" by Nicholas Wade, tells us that the leaf-cutter fungus has long been plagued by a mold that is capable of wiping out the entire food supply of the colony in just a couple of days. However, the ants have an ally who helps them combat this enemy.
A bacterium that lives in a patch on the ant's skin produces an antibiotic that controls the mold. In Wade's words, referring to the leaf-cutters: "They developed two remarkable inventions -- agriculture and antibiotics -- some 50 million years before people did. Beyond that, they have learned how to handle technologies more skillfully than the bumbling civilization above their heads. They can grow a monoculture -- a genetically homogeneous crop, something that in human hands generally leads to disasters like the Irish potato famine -- and they have also learned how to deploy an antibiotic without the target pest's becoming resistant to it."
So next time you have a delicious mushroom sauce or salad, with your dinner spare a moment to pay silent tribute to Atta cephalotes, the leaf-cutting ants. In these short pages I have only scratched the surface in describing the lives of these fascinating creatures. Volumes have been written about them. One such book that is very readable for nonscientists like you and me is: The Earth Dwellers by Eric Hoyt. It reads like a best-selling novel.
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