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LEAF-CUTTING ANTS ON PARADE
| The display potential and educational value of leaf-cutting ants was recognized as early as 1938, when the Bronx Zoo in New York City temporarily maintained a colony for public viewing. In 1978, the Cincinnati Zoo began exhibiting leaf-cutting ants in the newly opened Insectarium, and at that time was one of very few institutions working with live colonies. In recent years there has been a dramatic increase in the number of zoos, museums and related organizations keeping leaf-cutting ants; currently over two dozen educational institutions in North America and Europe maintain colonies. |
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| Now Sonoran Arthropod Studies Institute (SASI) has a unique observation nest which allows leaf-cutting ants to forage outside under natural conditions. Leaf-cutting ants are popular educational exhibits because of their fascinating societies and astonishing mastery of fungus gardening.
ANT SOCIETIES
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| Ants are social insects living in organized colonies, that is, groups of cooperating individuals. The societies of different species may be organized in various ways, but all are matriarchal, or female-dominated families. A typical ant colony contains a single fertile queen and numerous sterile female workers. The queen is the mother of the colony and is specialized for egg production, while the workers, her daughters, construct and defend the nest, care for the queen and brood (eggs, larvae, and pupae), and forage for food. |
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| Any smoothly functioning society depends on an effective system of communication. Ant societies are coordinated largely by pheromones, glandular secretions produced by the ants that function as chemical signals. Pheromones regulate complex social behavior and broadly serve to identify members of the colony, recruit nestmates to food or nest sites, and elicit alarm and defensive behavior. For example, workers instantly distinguish nestmates from intruders based on their smell; the former might be offered food and groomed while the latter would be attacked and killed or driven from the nest. |
| Depending on the season and colony cycle, an ant society may also contain many winged reproductive forms, or males and virgin queens. Mating usually takes place in the air in spectacular nuptial flights. The males die shortly thereafter while the fertilized queens attempt to initiate new colonies. They shed their wings, quickly burrow into the ground, and rear their first brood in isolation. |
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Most of the world's 8,000 known species of ants prey on insects and other small arthropods, many collect plant nectar and the sweet secretions produced by homopterousinsects, and a few scavenge for seeds.While the majority of ants live as hunter-gatherers, one unique group has adopted a genuine agricultural existence; the extraordinary societies of fungus growing ants skillfully cultivate their own food in specialized gardens.
FUNGUS GROWING ANTS
Ants in the tribe Attini, consisting of about 190 species in 12 genera, farm gardens of fungus in protected chambers and are known as fungus growing ants, or gardening ants. They are native only to the New World and are found chiefly in Central and South America and Mexico, although a few species occur in the southern United States. Most species of fungus growing ants live in moist tropical habitats, but some are adapted for life in cool temperate or harsh desert regions.
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| Fungus growing ant gardens look like globular masses of grey, sponge-like material. The gardens are honeycombed with small chambers and are composed of interwoven hyphae, or threadlike growths of fungus. The hyphae grow on various organic matter, called fungal substrate, which is collected and processed by the ants and incorporated into their gardens. The workers are adept farmers, fertilizingtheir crop with fecal droplets and meticulously weeding out nondesirable species of fungi. |
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| As gardens mature, the hyphae develop swollen tips, called gongylidia, which the workers harvest as food. The nutrient-rich gongylidia are the principal diet of the adult ants and their larvae.
When a virgin queen leaves the nest on her nuptial flight, she carries a bit of fungus with her, tucked into a small, specialized cavity within her mouthparts. After mating and digging a burrow, she carefully nurtures the precious young garden along with her first brood. If the fungus garden fails, the colony is doomed! Soon tiny workers emerge and assume custody of the growing fungus; the queen and brood remain nestled within the garden as the colony continues to develop.
The ants obviously obtain food from the fungus, but what does the fungus get in return? Consider that the ants cater to its every need; they shelter the fungus and bring it substrate, protect it from competing fungi, and help it reproduce and disperse. For services rendered, the fungus rewards the ants with bits of food. So who is manipulating whom? Do the ants grow the fungus or has the fungus domesticated the ants? Essentially, both views are correct! The ants and fungus evolved in close association, resulting in a mutualistic relationship where both species benefit from their intimate partnership. They are now totally interdependent; neither ants nor fungus can survive in nature without the other.
Within the Attini, there is an intriguing evolutionary progression in social organization; some species form colonies that are comparatively simple, while others are extremely complex. The more primitive species are characterized by small populations of workers; colonies sometimes contain only a few hundred or thousand individuals. The workers are monomorphic, or similar in size, and thus are relatively unspecialized for the various tasks within the colony. These species often collect insect excrement, pieces of dead arthropods, and bits of rotten wood to use as fungal substrate. The small, obscure colonies of these rudimentary fungus growing ants have only minor impact on the environment.
LEAF-CUTTING ANTS
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| Fungus growing ants in the evolutionarily advanced genera Acromyrmex and Atta are known as leaf-cutting ants because they regularly cut a wide variety of leaves and other plant material for use as fungal substrate. Species in these genera are distinguished by large colony populations, while the workers are polymorphic, or of different sizes, permitting a high degree of task specialization within the colony. |
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| The genus Atta is the pinnacle of Attine evolution, with 15 species representing the premier leaf-cutting ants. The huge queens, well over an inchlong, are efficient egg-laying machines, giving rise to worker populations as large as several million individuals. The workers are strongly polymorphic, varying in length from about 1/16 to 5/8 inches, and correspondingly, a several hundred-fold increase in body weight!
Relative to their size, the workers comprise four physical castes specialized for various roles: gardeners-nurses, within-nest generalists, foragers-excavators, and defenders. While depending on their age, the workers also make up three temporal castes, reflecting that younger workers tend to remain in the nest for domestic duties while older ants are more likely to venture and work outside the nest.Thus, based on both size and age, at least seven specialized castes of workers function
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| within the colony, and these perform over twenty distinct tasks. Hence, Atta species demonstrate one of the most elaborate social organizations known for any kind of ant, and this adaptation is pivotal to their sophisticated gardening behavior.
Leaves, stems, flowers, and fruit are harvested and processed assembly-line style. Medium sized workers are primarily responsible for leaf cutting and transport.
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| A worker straddles a leaf to be cut, measures a portion with her legspan, then rotates and shears the leaf with sharp mandibles. Returning foragers typically carry small disks of plant material held above their heads, somewhat like tiny green umbrellas, and are popularly called parasol ants. |
| Once plant fragments reach the nest, they are processed by groups of progressively smaller workers. The substrate is broken apart, finely macerated, treated with enzymes, and ultimately distributed within the growing gardens. Besides tending the fungus, smaller workers also serve as nursemaids, feeding and caring for the developing brood held within garden chambers.
In sharp contrast are the largest workers which function as soldiers.
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| Their giant armored heads and sharp powerful mandibles are formidable weapons. Although they do little else, the soldiers are always ready to rush out of the nest when disturbed and aggressively defend the colony. |
| Mature Atta nests dominate the landscape, sometimes forming mounds 50 feet or more indiameter and housing hundreds of garden chambers. A large, well-established nest exhibits complex architecture designed to control the internal environment. Passageways above the gardens vent excess heat and stale air produced by fungal decomposition, while the nest is ventilated in turn with fresh air drawn in through peripheral tunnels. Another system of passages drains the garden chambers to prevent flooding of the nest during heavy rains. |
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| Colonies often maintain an extensive system of ant highways, debris-free trails that lead from their nest to distant foraging sites sometimes hundreds of yards away. The seemingly endless columns of workers returning to their nest laden with leaf fragments have always fascinated naturalists and amazed visitors to the tropics.
THE IMPORTANCE OF LEAF-CUTTING ANTS
Leaf-cutting ants are a conspicuous feature of most Neotropical habitats, and because their populous colonies are efficiently organized, have a profound effect on the environment. Depending on one's perspective, leaf-cutting ants can be considered either highly beneficial organisms or terribly destructive pests.
Acting on behalf of their fungus, leaf-cutting ants are the principal herbivores in tropical and semi-tropical regions; they have far greater effect than any other group of plant-feeding animals. Leaf-cutting ants shape plant communities and ultimately impact the grand structure of biological systems.
Many endemic species of plants have evolved defenses against attack by leaf-cutting ants; noxious chemicals and tough or otherwise modified leaves offer at least partial protection against defoliation. Even so, native plants utilized by the ants subsequently show accelerated growth, thus natural pruning by leaf-cutting ants stimulates plant productivity. In addition, leaf-cutting ant nest excavations frequently extend deep into the ground, improving the soil by introducing organic material, and increasing aeration and drainage. Biologists and ecologists respect leaf-cutting ants as an important and necessary natural component of tropical ecosystems.
Unfortunately, leaf-cutting ants show a propensity to harvest most types of agricultural crops, in part because domesticated plants lack ant deterring mechanisms. The ants are extremely difficult to control because raiding parties deploy quickly from distant nests and routinely decimate large fields overnight, causing billions of dollars in crop damage each year. Consequently, farmers and gardeners regard leaf-cutting ants as the single most destructive agricultural pest in the Neotropics.
Besides their significant ecological relationships, leaf-cutting ants are increasingly recognized by educational organizations as important exhibit animals.
Part 2
LEAF-CUTTING ANTS ON DISPLAY,
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