San Francisco State University
Department of Geography

Geography 316:  Biogeography 

The Biogeography of Leaf-Cutter Ants (Atta cephalotes)

                                 by Airlangga Djajadi, student in Geography 316, Fall 1999
 

                                                                                                          source: Newman 1967
 

Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Genus: Atta
Species: Atta cephalotes
 

Description of Species

    There are three castes of leaf cutter ants; workers, females, and males (Weber 1972).  The workers are females and cannot reproduce eggs that become other workers but can make eggs that produce males (Weber 1972).  The role of the female in a leaf cutter ant colony is very important, queens are essential and are part of the female caste structure.  Males on the other hand do not work in or out of the colony and are winged to inseminate the virgin queens (Weber 1972).
     The worker ants are covered with small spines and have long legs and appear reddish brown in color (Weber 1972).  These tiny workers have large mandibles with fine sharp teeth (Weber 1972).  Two antennae appear at the face of the worker and are waved around as they move (Weber 1972).  The females or queens have larger eyes with and are much coarser than the other castes.  Queens and females are significantly larger than males and their wings extend past their rear  (Weber 1972).  Males are darker than workers and queens and are usually smaller (Weber 1972).  The wings are similar to the females, extending past the body (Weber 1972).
 

Habitat

    The home range for leafcutter ants is very difficult to describe.  Typically a range for the leafcutters extends across the tropical rainforest floor.  From our point of view, several small turrets make up the “front door” (Weber 1972).  Trails that lead to the opening of the nest are easily visible to the human eye, and are as wide as 30 cm. (Weber 1972).  Sometimes a range can span from 200 yards to a 20-foot area (Hoyt 1996).  The nest of the leafcutter ant is tremendous, occupying up to eight square meters below the surface (Weber 1972).  The Atta cephalotes usually have the largest nest sites of all the Attines (Jaffe & Vilela 1989).  The internal part of the nest plunges deep into the earth and comes to an end near the water table, about 15 feet or more below the surface (Wilson 1984).  From the inside there are likely hundreds of channels and rooms that create a subterranean city (Newman 1967).
 

                                                                                                        source: Weber 1972
 

Natural History
   

Diet
     The diet of a leafcutter ant may sound self-explanatory, leaves right?  However, it must first take the leaves and turn them into something edible.  This is why these tiny creatures are so specialized.  Actually most of the leaves that leafcutter ants gather contain poisons (Hoyt 1996).  They feed on a fungus that is cultivated in their nest from the leaves that the workers forage (Quinlan & Cherrett 1979).  When an ant reaches the nest it will drop a piece of leaf down the chamber where it is then gathered by a worker ant that’s smaller in size (Wilson 1984).  This worker ant chops the leaf into smaller pieces that are about a millimeter in length (Wilson 1984).  In a short while yet another smaller ant will take the pieces of leaf and form them into small pellets and then proceed to place them with similar material (Wilson).  The harvester ants, or minimas, are the smallest of the family and are responsible for feeding the colony; by far these ants work the hardest (Hoyt 1996).   This garden inside the nest contains the symbiotic fungus with leaf sap that makes up the only nourishment for these ants (Berish 1986).  Leafcutters show a preference to younger leaves, which tend to be easier to clip (Howard 1988).  They work like a well-managed production line with each individual doing something, and in the end a finished product is made.
 

                                                                   source: Hoyt 1996

Life History
     Many insects, especially ants, are social organisms that need to communicate with each other.  For instance, how do they know whom to follow when searching for food?  E.O. Wilson has termed the phrase “tandem running” which describes the way one ant follows behind another and occasionally touches the abdomen of the leader (Jaffe & Howse 1979).  Pheromones are released and the following ants pick them up with their antennae (Wilson 1984).  Most ants are chemically functional in areas where we are auditory and visually oriented (Wilson 1984).
      The caste system of the leafcutter ant is very complex and contain seven different workers (Weber 1972).  They can range from 2mm to 15mm in length (Weber 1972). (Figure 4)   Each ant has a specific role in maintaining the nest.  Soldiers, the largest, go on trails and watch for predators or often stand guard at the entrance of the nest (Weber 1972).  Media workers do most of the cutting and transporting and also help guard the nest (Weber 1972).  The smallest and probably the most important are the minimas.  These ants distribute the food and harvest the fungus garden.  Some minima ants go on the trails by riding on top of a larger worker (Moffett 1995).  These hitchhikers notify the workers if a phorid fly is spotted, which is a parasitic fly that drops its maggot onto passing ants (Lofgren & Vander Meer 1986).
 

                                                                      source: Weber 1972

    The process of mating in ants requires a nuptial flight, which involves the mating of winged ant queens and males (Hoyt 1996).  When a queen is ready to fly she takes off into the sky to be impregnated.  After a male inseminates her, his job is done and so is his life (Wilson 1984).  The next step is to find a place to begin a colony.  When she reaches the ground her wings will fall off and she will wonder around until she finds the perfect spot (Wilson 1984).  She will now begin to excavate into the earth and settle down to release from her mouth a portion of the symbiotic fungus which leafcutters cannot survive without (Weber 1972).   Before a queen leaves the nest for the nuptial flight, they gather some fungus and hide it in a pouch inside their mouth (Wilson 1984).  This will begin the entire process of a complex underground city.
 

                                                                                  source: Hoyt 1996

    Of the two generas,  Atta and Acromyrmex, morphological differences are present (Lofgren & Vander Meer 1986).  Grasscutters often harvest vegetation differently from leafcutters and tend to have shorter legs (Lofgren & Vander Meer 1986).  Leafcutters usually have longer mandibles than those of  grasscutters.
 

Evolution

    When we think of evolution we think of change and development.  It is hard to imagine that ants have evolved.  They are not walking upright and their physical appearance probably has not changed much.   Leafcutter ants are very specialized little creatures in that they have evolved with another organism (Jolivet 1996).  The process in which leafcutter ants and plants have evolved is called symbiosis (Hoyt 1996).  It took millions of years for this process to occur, about 50 million years ago these ants began this relationship with plants (Hoyt 1996).  The fungus lost the ability to produce spores and the ants became dependent upon it for food (Weber 1972).  About 65 million years ago South America was isolated from other landmasses, gardening ants started their relationship with a fungus (Hoyt 1996). It has been proposed that leafcutter ants propagated the same fungal lineage for 25 million years, which means they are causing the fungus to reproduce itself (Hoyt 1996).  The leafcutter ants are different from other ants by their growing of fungi underground, they have not been thought to be derived from another ant, but they resemble the harvester ant, Pheidole.
 
 

Distribution

    There are about 38 different species of leaf cutter ants, which belong to the Attines or fungus-growing group of ants (Wilson 1984).  The Attines can be found in every New World country except Canada and Chile (Hoyt 1996).  The Atta cephalotes is native only to the Americas and is primarily found in the Southern Hemisphere (Weber 1972).  Specifically these ants are detected from southernmost Mexico to Ecuador and Brazil, in the Lesser Antilles, and as far north as Barbados (Lofgren & Vander Meer 1986).   Since these ants are mainly found in South America, researchers have discussed why there are these boundaries.  Weber (1972), who studied leaf cutters in the wild for more than forty years, put forth the idea that they originated in the moist lowland tropics of South America where the climate has been stable since the Tertiary period.  Apparently there have been opportunities for migration to the south where they have been found in Argentina, but the north toward North America has put a greater resistance because of drier conditions (Weber 1972).  The country of Chile proves a great example of their restrictive boundaries.  There are no ants of the Attini tribe in Chile because the Andes block the east, the Atacama Desert blocks the north, and the south is just too cold (Weber 1972).  The tropics create an ideal environment for the tiny gardeners.

     The elevation range for the leaf cutter ants can reach from sea level up to 1,900 meters (Weber 1972).  The number of ants at higher elevations is substantially lower, nonetheless one could occasionally spot some “walking leaves” on the ground in these areas.  The leaf cutter ants are confined between 33 N to 44 S latitudes (Lofgren & Vander Meer 1986).  This range gives the ants perfect weather conditions and a vast amount of flora use.  Today, the leaf cutter ants still inhabits these areas and is not facing decreasing numbers.
 

Map of Distribution:

                                                                              source: Weber 1972
 
 

Bibliography
 

Berish, Cory W.  1986.  Leaf-cutting ants (Atta cephalotes) Select Nitrogen-rich Forage.
American Midland Naturalist  115 (2).  268-276.

Howard, Jerome J. 1988. Leafcutting Ant Diet Selection: Relative Influence of Leaf Chemistry and Physical Features.  Ecology  69 (1).  250-260.

Hoyt, Erich.  1996.  The Earth Dwellers.  Simon & Schuster.  New York.

Jaffe, Klaus.  Vilela, Evaldo. 1989. On Nest Densities of the Leaf-cutting Ant Atta cephalotes in Tropical Primary Forest.  Biotropica  21 (3).  234-236.

Jaffe, Klaus. and Howse, P.E. 1979. The Mass Recruitment System of the Leaf-cutting Ant, Atta cephalotes.  Animal Behavior  27 (2).  930-939.

Jolivet, Pierre.  1996.  Ants and Plants an Example of Coevolution.  Backhuys Publishers
Leiden, The Netherlands.

Lofgren, Clifford S. and Vander Meer, Robert K. 1986. Fire Ants and Leaf-cutting Ants.
Westview Press, Inc., Boulder, Colorado.

Moffett, Mark W. 1995.  Leafcutters Gardeners of the Ant World.  National Geographic
Washington, D.C. 188 (1).  98-111.

Newman, Hugh L. 1967.  Ants From Close Up.  Thomas Y. Crowell Company.  New
York.

Quinlan, R.J. and Cherrett, J.M.  1979.  The Role of Fungus in the Diet of the Leafcutting
Ant Atta cephalotes.  Ecological Entomology  4 (2).  151-160.

Weber, Neal A.  1972.  Gardening Ants the Attines.  The American Philosophical
Society.  Philadelphia.

Wilson, E.O.  1984.  Clockwork Lives of the Amazonian Leafcutter Army.  Smithsonian
15 (7).  92-101.
 
 

send comments to bholzman@sfsu.edu
 

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