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... "The ants's unless lies in the evolutionary concept of kin selection" (Hoyt 1996). This concept occurs when members of a species work for the survival of relatives - genes by descent from a common ancestor rather than for one's survival of its own offspring (which is called individual selection). This is a very complicated process in which many people have devoted their lives to its study. The gathering of food by the leaf cutting ants is a complex series of interactions and communication which I will discuss.
The foraging for food first is started from the central nest. Special ants called scouts range off of ant trails in search of food for the nest and if they find food, they start to recruit workers to collect the resource. This sounds simple but the process is actually very complicated. Using Howard, Henneman, Cronin, Fox, and Hormig's research (1996) I will explain the process. When a scout find a food source, the decision of whether or not to recruit workers is based on the location, quantity, and quality of the source.
Also the scouts examine resource consistency such as protein, lipid or simple sugar content. If a scout finds a familiar resource, it will recruit workers faster then if it found an unfamiliar resource. How much an ant is familiar with a resource was found to be in direct proportion to the number of times the ant physically touched the resource or food. The same goes with the workers. If a scout presence a worker with an unfamiliar resource, it is more likely to reject it.
If the resource is accepted, the worker ant will follow the special scent trail laid down by the scout to the location of the resource. The time delay for the ants to follow a new type of food can be a negative and a positive. This can carry costs by the delay in acceptance of a new resource such as food delay, overexploitation, and starvation. This mechanism can also protect the ants in that the ants know that the food they are feeding the fungus works but a new type of food can poison the fungus. Wirth, Beyschlag, Reel, and Holldobler (1997) studied the cycle of foraging in the leaf-cutter ant and they found some interesting results. They found that on an average, the total yearly input of collected green leaf material corresponded to 1706 to 3, 855 meters square of foliage area depending on the size of the nest.
The number of leaf fragments collected per day ranged from 9, 770 to 374, 200. During the dry season the ants harvested more leaves than the wet season due to the loss of availability of green leaves during the dry season... Also the type of material collected during the two seasons differed. During the wet season, greater quantities of green leaves were collected while during the dry season, non green material was collected (fruit parts, stipule's, and flower parts).
The ants collect resources according to their availability during that time. The amount of material collected also varies with the seasons. During the wet season, more material is collected. A possible reason for this is that since during the dry season the ants collect more flowers and fruits which contain more energy then leaves, the ants don't need as much material to feed the fungus. The rain limits the amount of foraging that can go on during the wet season by physically stopping the ants from moving on their paths due to the abundance of water on top of the soil. This may be important in regulating colony size.
The average area of a leaf fragment carried by individual ants was determined to be 0. 79 cm square and with an average weight of 5. 51 mg. The scientists discovered that fragment area and weight was determined by the type of plant harvested. Succulent leaves (which are normally thick) were cut into small pieces (< 0. 5 cm square) while fragments cut from glabrous leaves (thin leaves) were often larger than 1. 0 cm square. The daily foraging cycle of the leaf-cutter ant was also determined.
Foraging increases rapidly during daybreak and remained high during the daylight hours but returned to low levels by nightfall. In some colonies, foraging is limited to the night. During the day some types of carpenter ants (especially species Camponotus) guard the trees the leaf-cutting ants use for foraging. This prevents that Atta foragers from foraging during the day.
When nightfall comes the carpenter ants leave and the leaf-cutter ants forage during the night to make up lost foraging during the day. Maximum rates of harvest occur from 3: 00 pm to 4: 00 pm. Roces, Holldobler, Tautz, Kleineidam, and Krumme found that when leaf-cutting ants were cutting leaves, their metabolic rate was dramatically above their resting rate. It was found that cutting leaf tissue use up 31 (31 000 %) times more energy than in a resting state.
They also found that the size of the leaf cut depended on the amount of energy invested. The more energy spent in a short amount of time, the smaller the size of leaf cut. This is why when cutting at the stem of a plant, the ants cut such small pieces (personal observation). H. L. Vasconcelos (1997) found that the number of attacks on a given species was mainly determined by its abundance in that area.
Thus changes in species composition near an ant colony will result a change in the type of resources brought into the colony. His study showed that there is no short term variation in the amount of secondary compounds or leaf nutrients in response to defoliation of a plant by the ants. The reason why some plants are only partially defoliated and some are fully defoliated is that only the preferred leaves are cut by the ants. In large trees there differences in light conditions in the canopy which causes changes in the leaf structure and content. Immature trees have less variation in nutrient and content of its leaves and therefore is more likely to be totally defoliated than older and larger trees.
On the average, 85. 7 % of attacks on a given tree results in complete defoliation (Vasconcelos 1997). The ants have many defense mechanisms and behavior patterns. Some ants guard the nests entrances. In doing this they are usually still and are in the stilt-legged position. Stilt-legged position is when the ant is still and when its head is pointing upward and its mandibles are open. Another defense mechanism is capping.
This is when the ants panic because of some enemy and all sizes of ants use matter to close the nest entrances. Avoiding can lead to their not being a battle. This is when ants avoid each other and one or both nests leave the feeding site. The armadillos are the main vertebrate predator of the leaf-cutter ants (Whitehouse & Jaffe 1996).
The armadillos dig up the nests to get at the eggs and the many ants tending the nest. As soon as there is a mechanical disturbance of the nest the percentage activity of soldier ants outside of the nests is increased approximately 49 %. The number of small ants decreased by an average of 47 % on top of the nests (Whitehouse & Jaffe 1996). Thus soldier ants with their large mandibles are a better deterrent against large enemies then small ants.
Leaf-cutter ants must also defend against other ants and even against ants of its own species. Ant battles are rare but when they occur they are spectacular. Battles occur when at least 50 ants are fighting. Fighting is done by one ant biting the leg or body part of another ant with its mandibles. Fighting usually occurs in balls of ants with a ball having about 14 ants. Ants start dying usually if the fighting lasts more than one hour.
Usually when ants from other nests meet there is a short skirmish and then avoidance. The ant battle operates on the square law which predicts that is all individuals are equally vulnerable to attack then the side with the most attackers will win. Thus many smaller ants are more beneficial then a few large soldier ants. Small ants fight but they also lay down scent or territorial markers.
It is found that the better a nests territory is marked, the more likely the nest will win the battle. Thus the more small ants a nest has, the more territory will be marked and more opposition ants will be killed. Also it appears that the large soldier ants are primarily to protect the nest while the small ants and workers fight the intra specific battles. The substances the ants use to mark their territory are hexadecanoic acid heptadecane. Hexadecanoic acid is a special compound produced by the meta pleural glands and is always being released by the workers. Heptadecane is produced by the Dufours gland and is actively secreted as a territorial marker and also as am alarm pheromone.
The overlying theme of leaf-cutting ant's defense is to protect its food supply. Even though the ants use avoidance most of the time this is viewed as a type of defense so that in the case of a large confrontation, many ants can be present on a marked territory (Whitehouse & Jaffe 1996). Roces, Holldobler, Tautz, Kleineidam, and Krumme found an interesting defense mechanism in the foraging column of the leaf-cutting ants. In the foraging column the minima workers "hitchhike" on leaf fragments carried by the workers. The purpose of this is that the large workers will defend the small workers from parasitic peoria flies. The leaf-cutting ant has a profound effect on its environment.
In some areas the leaf-cutting ant is a determining factor of forest dynamics and composition (Herald and Cherrett 1997). When a leaf-cutting ant nest moves into a new environment they immediately start to chew up and move leaves and seeds. Vasconcelos and Cherrett (1997) found that the damage done by leaf-cutting ants negatively affected seedling survival and growth. They also noted that time most dangerous to a seedling was between 3 - 9 months. Thus the older the seedling becomes, the less chance it will die from being chewed on by the leaf-cutter ants. The ants have a preferential taste for certain types of plants.
This leads to selectivity and thus the ants can change forest composition. Since the ants mainly go after leaves of seedlings, populations tend to increase after clearing of mature forest (Herald and Cherrett 1997). Thus the ants can reduce tree establishment by two ways. They can act as seed predators and seedling predators.
Another way that the ants effect the environment (this one can be positive) is through facilitated succession. This research was done by Brener and Silva (1995). It was found that some trees grow preferentially on abandoned leaf-cutter ant nests (Bucher 1982). It was found that between the open grass lands and forest there was numerous ant nests. Here there are many seedlings and smaller trees as the forest starts to colonize the savanna. This is where many leaf-cutting ants are (for reasons already described).
The woodland trees changes the soil which the facilitates the establishment of shrubs which allows the further survivorship of near by ant nest between the forest and grass land. As show by Brener and Silva's (1995) results the ants benefit from direct food availability and better habitat conditions and as the nest grows, it promotes changes in soil and nutrient status improving conditions for the survival and growth of trees. Nitrogen, phosphorus, potassium, magnesium, calcium, sodium, moister, organic carbon, all increased due to ant nests. The scientists found that groves would grow larger and more quickly than those who had no ant nests. Bucher in 1982 found that the effects of the nests last for a long time when it is abandoned. The type (s) of communication leaf-cutting ants use is still in the process of research.
What we do know is that workers produce vibrational signals while they are cutting leaves (Roces, Holldobler, Tautz, Kleineidam, Krumme). The vibrations have two purposes. They act as close-range recruitment signals and it also facilitates the cutting of leaf tissue, like a vibrating knife or jigsaw. The plant-borne vibrations produced by the workers are used by other workers in locating the cutting site (s). Roces, Holldobler, Tautz, Kleineidam, and Krumme found that the vibrations mean differently to the ants depending on the social context being used.
The leaf-cutting ants also produce rapid vibrations, which are audible to humans, the ant does this by rubbing two abdominal organs -one a sharp scraper which scratches the fine ridges on the other. The vibrations radiate along the ants body all the way to her sickle, serrated mandibles. The mandibles vibrate up and down at 1, 000 Hz at the ant uses one mandible to cut the leaf (Batman 1995). Roces and company also found that the leaf-cutting ants carry several thousand chemo and mechanoreceptors but only 12 sensible that respond to carbon dioxide. These sensible act as measuring devices which constantly measure atmospheric carbon dioxide. The ants can scan spatial and temporal carbon dioxide gradient and using this mechanism they can locate the entrances to their nest.
It was found that the ant even move their antennae in such a patter which stimulates the carbon dioxide sensible optimally (Roces, Holldobler, Tautz, Kleineidam, and Krumme). Controlling leaf-cutting ants is a major problem is some countries. The structure of the ants' nests make it extremely difficult to destroy with pesticides and machines. Individual plants can be protected by using insecticides such as acetate (Orthene), diazinon or chlorpyrifos (Durban), but these treatments do not affect the colonies.
The Eradicator Fire Ant Eradicator System can be used on leaf-cutter ants too. It is designed to injects vaporized resmethrin. Methyl bromide gas (from Great Lakes Chemical Corporation's Brom-O-Gas) is very effective against the ants. But this is highly toxic to humans, plants, and animals. There is also some concerns about the toxin travelling up ecosystems. Thus you have to have a special permit to use it.
Production of the gas will be terminated at the end of the year 2000. Baits and other foods do not effect the ants because they only eat the fungus. The American Cyan amid Company has developed a unique product called Audio Leaf-Cutting Ant Bait which is a formulation of hydramethylnon. It is the most effective leaf-cutting ant killer on the market. The ants are attracted to the substance which is spread on the ground and feed it to their fungus. The product poisons the fungus and it dies along with the ants (Stewart 1982).
The leaf-cutter ant has become a marvel of cooperation and unselfishness. The ant system has become a collection of individual ant that are each specialized in their tasks as the cells of the human body and that jointly perform the task of keeping the nest alive. They do precisely the right things for their own survival. Guided by instinct, the super organism responds adaptively to the environment.
Humans depend on the ants for their very survival but the ants do not depend on us. For a while at least, they will help to hold the world in balance to our liking, and they will serve as a reminder of what a wonderful place it was when we arrived in north and central America. There is still many more answers unsolved about the Atta tribe. In time we will learn more and will be amazed. For when Solomon said "Go to the ant, thou sluggard, Consider her ways and be wise!" (Proverbs 6: 6 - 8). I am sure it was meant to be taken literally.
Works Cited Alejandro, G. , Brener, F. , and Silva, J. 1995. Leaf-cutting ants and forest groves in a tropical parkland savanna of Venezuela: facilitated succession? . Journal of Tropical Ecology 11: 651 - 669 Bass, M. , and Cherrett. J.
M. 1996. Leaf-cutting ants (Formicidae, Attini) prune their fungus to increase and direct its productivity. Functional Ecology 10: 55 - 61 Bass, M. , and Cherrett J. M. 1996. Fungus Garden Structure in the Leaf-Cutting Ant Atta sexes (Formicidae, Attini). Symbiosis 21: 9 - 24 Boyd, N.
D. & Martin, M. M. (1975) Faecal proteinases of the fungus-growing ant Atta texas: their fungal origin and ecological significance. Journal of Insect Physiology 21, 1815 - 1820 Bucher, E. H. 1982. Chaco and casting - South American arid savannas, woodlands and thickets.
Pp. 48 - 69 in Huntley, B. & Walker, B. (eds). Ecology of tropical savannas. Singer-Verlag, Berlin Batman, John. 1995. Vibrant ants. Discover 4: 12 - 13. Fogel, Robert. "Ant gardens" April 21, 1997.
Available: web gard. htm Holldobler, Bert. , and Edward O. Wilson. Journey to the Ants, A Story of Scientific Exploration. London, England: The Belknap Press of Harvard University Press Cambridge, 1994.
Howard. J. J. , Henneman. M. , Cronin. G. , Fox. J. , and Hormiga.
G. 1996. Conditioning of scouts and recruits during foraging by a leaf-cutting ant, Atta colombia. Animal Behavior 52: 299 - 306 Hoyt, Erich. The Earth Dwellers, Adventures in the Land of Ants. New York: Simon & Schuster, 1996. Quinlan, R.
J. & Cherrett, J. M. (1977) The role of substrate preparation in symbiosis between the leaf -cutting ant Acromymex octospinosus (Reich) and its food fungus. Ecological Entomology 4, 151 - 160 Roces, F. , Holldobler, B. , Tautz, J. , Kleineidam, C. , Krumme, S. "Communication, eco physiology and energetics in leaf cutting ants (Atta). " Date unknown. Available: web - wuerzburg.
de / bericht /zoo 2 /enter. htm# 2 Smith, C. "Leaf-Cutting Ants. " 23 March. 1997. Available: web >. Stewart, J. W. "Texas Leaf Cutting Ant" Sept. 1982. Available: web > Unison Services. "The Leaf Cutting Ant or Fungus-Growing Ants. " 4 Jan. 1998.
Available: web >. Vasconcelos, H. L. 1997. Foraging activity of an Amazonian leaf-cutting ant: responses to changes in the availability of woody plants and to previous plant damage. Oecologia 112: 370 - 378 Vasconcelos, H. , and Cherrett J. 1997.
Leaf-cutting ants and early forest regeneration in central Amazonia: effects of herbivore on tree seedling establishment, Journal of Tropical Ecology 13: 357 - 370 Weber, V. A. (1972) Gardening Ants, the Attini. American Philosophical Society. Philadelphia.
Whitehouse, M. , and Jaffe, K. 1996. Ant wars: combat strategies, territory and nest defense in the leaf- cutting ant Atta Laevigata. Animal Behavior 51: 1207 - 1217 Wirth, R. , Beyschlag, W. , and Reel, J. 1997. Annual foraging of the leaf-cutting ant Atta colombia in a semi deciduous rain forest in Panama. Journal of Tropical Ecology 13: 741 - 757
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