Imagine a society where there are no managers, just workers. Imagine further that all these workers know exactly what to do and they do it, as vigorously as possible. Imagine too that these workers are farmers which do not make mistakes.
One might well suppose that these farms would be highly successful, and so they are. However these are not human enterprises. The farmers to which we refer, are ants, insects with astonishing expertise in their tiny heads. Their success comes from know how, enthusiasm, suitable equipment (their bodies), and suitable crops.
A recent article in the journal Nature (September 22/05 pp. 495-6) for example, describes the little-known farming activities of some ants in the Amazon rainforest. In an ecosystem characterized by a wild diversity of tree species, there are some large clearings in the forest where all the trees, young and old, are of the same species. Local legend suggested that these single-species stands of Duroia hirsuta were created by an evil forest spirit.
Recent research has shown however that the cause is much more interesting. A local ant called Myrmelachista schumanni (not exactly a name which ripples off one’s tongue) apparently is farming the trees in these clearings. The ants live in hollow swollen stems of this tree species. By comparing the rate of tree garden growth with the size of these clearings, scientists have calculated that the oldest colony is about 800 years old. Each garden is tended by a single ant colony consisting of as many as 3 million workers and 15,000 queens.
It is all very well to talk about creating a clearing, but it is not that easy to do successfully. Ranchers in Brazil burn large tracts of forest, but the fertility of the soil declines in just a few years and their land becomes worthless. Other groups of aboriginals cut small clearings but the forest soon invades their tiny gardens and the people move elsewhere. How do the ants so successfully manage their long term gardens? That is what scientists wanted to know.
By means of studies which eliminated ants from some experimental plots, and left them in others, these scientists obtained answers. They planted saplings of a tree from the nearby rainforest into these plots. When the ants were left in the plots, the foreign saplings quickly died. When the ants were kept out, the saplings did just fine. Obviously it was not the Duroia trees which discouraged the foreign invaders, but the ants themselves. Other studies revealed that the ants are able to recognize their favourite tree, nobody knows how. To kill foreign invader trees, or simply to extend the size of their garden (by killing neighbouring rainforest trees), the ants simply bite a hole in each leaf, and then turning around, they insert the tip of the abdomen into the hole. The ant then squirts a simple molecule, formic acid into the leaf. Within hours, these leaves start to turn brown. Many ant species apparently produce this compound, but this is the first record of ants using the acid as a herbicide.
The colony is begun when a queen ant finds an isolated Duroia hirsuta tree. Her offspring eventually create a vegetation free zone around this nest tree, and eventually seedlings of the desired tree move in to fill the clearing. The ants are patient. Over time their garden develops. The expertise programmed into these tiny ant brains includes the ability to recognize the desirable trees (not apparently by obvious visual clues), and the ability to distinguish unwanted trees from the wanted one. Next they know enough to bite a hole in each leaf and insert a poison.
This whole story reminds one of other gardening ants from the same region, the tropical Americas especially Brazil. The leaf cutting ants which cultivate molds (fungi) are quite famous. There are about 200 leaf cutting ant species and they all depend upon fungus gardens for their food. The problem for the ants is that they do not have suitable enzymes to digest the tough walls of the cells which make up these leaves. Thus the nutrient content in the leaves is largely, in its fresh state, unavailable to them. That is why they cultivate a fungus on the leafy material. The mold pre-digests the plant material for the ants.
The dependence of the ants on their cultivated molds is so great that these insects lack digestive enzymes. They therefore depend on fungus manufactured proteins to carry out this function in their intestines. Apparently a number of similar fungi are suitable for these gardens, and nearby nests with the same ant species may exploit quite different fungus cultivars. Some of these fungi are also able to live in the wild and to form mushrooms, but most of these molds are known only as thread-like growths from the farms themselves.
Some leaf cutter ant species build huge underground nests, tremendous labyrinths of chambers connected by tunnels. These nests are so well engineered that warm gases rising from deep compost-filled pits, serve to draw cool, oxygen-rich air through special ventilation openings down into the nest.
Scientists initially thought that these fungus gardens were untroubled by foreign microbes. They have since discovered an invasive mold which has the potential to destroy whole colonies. The worker ants must constantly struggle to suppress this bad mold. To do this they exploit yet another microbe. Actinomycetes are special soil bacteria, famous for their production of strong antibiotics. The gardening ants somehow promote the growth of these microbes on their bodies. They thus scatter actinomycete cells as they move about the farm. How very sophisticated.
The mere act of cutting leaves also apparently requires special talents. Sharp jaws are not enough to cut a leaf. That structure needs to be held firm. But who will hold the leaf for the cutter? The answer is the cutter ant is able to do it herself by vibrating her jaw at 1000 times per second. The leaf then assumes a rigid position and a nice straight cut is possible. The ant does not merely vibrate her jaw however. She rubs two tiny abdominal organs together and the resulting vibrations radiate along her body to the jaw. Obviously this is a highly energy consuming activity and not a solution that we would immediately think of.
Some experts have tried to speculate on how leaf cutting ants could have developed their farming talents through natural processes. Suppose that the forager ants were equipped with suitable jaws but they did not know how to hold the leaves firm for cutting. Suppose they did not have the stamina to sustain the body vibrations. Suppose that the forager ants were too tired for the long trek back to the nest carrying their leaf fragments. Suppose that the worker ants did not know how to prepare the leaf material or how to build the nest to sustain the fungus garden. Suppose they did not know how to discourage the invader mold. Suppose, suppose….
The hardest thing to suppose is that such organisms could possibly have developed these talents gradually and on their own. What we see in leaf cutter ants and Duroia cultivating ants, are organisms beautifully designed for their roles in nature. There are other interesting ant societies. Why not investigate one for yourself? They are all wonderfully coordinated and wonderfully designed.
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