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Army Ants
The Army ants are a large group of mostly tropical and subterranean ants, represented in the Old World by the subfamilies Aenictinae (one genus, 15 species) and Dorylinae (one genus, 40 species), and in the New World by the Ecitoninae subfamily (4 genus, 120 species). The two main characteristics determining the “army ant adaptive syndrome” are group predation and nomadism. Although neither behavior is restricted to army ants, the combination of both is. Other characteristics of the group include an intricate division of labor among the worker caste, huge colonies that reproduce by fission, and a unique form of sexual selection.
Eciton burchellii
The New World Army Ant Eciton burchellii is by far the most studied Army Ant species. It is truly epigaeic (nesting over the ground) and has a wide distribution range, from Mexico to northern Argentina. Like the other 11 Eciton species, it exhibits a high worker polymorphism with three or four discrete worker subcastes. The combination of huge colony sizes (between 500 000 to 2 million workers) and a generalist diet (consisting of almost all kinds of arthropods, worms, and even small vertebrates), provide these Army Ants with an incredible predation capacity. Foraging is performed in swarm raids of hundred of thousands of workers simultaneously searching, killing, tearing and caring tens of thousands of prey items back to the temporal nests or Bivouacs.
Life Cycle
E. burchellii is a “phasic” species, exhibiting periodical cycles of alternating nomadic and statary phases, conditioned by brood-stimulative factors. Nomadic phases begin with the emergence of callow workers from the cocoons, and extend 11 to 16 days. During this period the queen does not lay eggs and her gaster remains contracted. The pupation of the larva, hatched from eggs laid in the immediately preceding statary phase, marks the end of the nomadic period. Than the queen starts feeding voraciously, and during the second week of the statary period, fully physogastric, lays from 100.000 to 300.000 eggs. Few days later larva hatch, and by the end of the statary phase (which can last between 19 and 22 days) callow workers emerge once more recommencing the cycle. The whole cycle repeats throughout the year, and is only interrupted by the production of sexual brood at the onset of the dry season.
Mating biology
Army ants posses a unique form of sexual selection. Newly hatched winged Eciton males stay a few days in their mother colony prior to their departure from the emigration trail. Once departed from their mother colonies, the males try to find a conspecific colony, following pheromone and visual traces. After successfully locating a foreign colony, they first have to be accepted by its workers, which literally "chose" the next fathers of the colony. Once a male is allowed to join a foreign colony, it either sheds its wings or has them torn from its thorax by the workers. It then lives within the new colony for a relative long period of time, before gaining access to the queen. After copulation, which takes place in the bivouac, the male dies.
Ongoing Research
Relying on DNA microsatellite analyses, we are trying to answer two main questions:
- Is there a genetic component involved in the worker caste determination of the army ant E. burchellii?
- Do the mating and dispersion strategies of Eciton army ants promote gene flow in natural populations?
Selected Literature
- Brady, S.G. (2003) Evolution of the army ant syndrome: The origin and long-term evolutionary stasis of a complex of behavioral and reproductive adaptations. PNAS 100(11): 6575-6579.
- Franks, N.R., Sendova-Franks, A., Anderson, C. (2001) Division of labour within teams of New World and Old World army ants. Animal Behaviour 62: 635-642.
- Gotwald, W.H. Jr. (1995) Army ants, the biology of social predation. Cornell University Press, Ithaca, N.Y.
Hölldobler B., Wilson, E.O. (1990) The Ants. Belknap/Harvard Univ Press, Cambridge, MA. 732 pp. - Kronauer, D.J.C., Boomsma, J.J. (2007) Do army ant queens re-mate later in life?. Insect Soc 54 (1): 20-28.
- Kronauer, D.J.C., Johnson, R.A., Boomsma, J.J. (2007) The evolution of multiple mating in army ants. Evolution 61, 413-422.
- Powell, S., Franks, N.R. (2005) Caste evolution and ecology, a special worker for novel prey. Proc R Soc B-Biol Sci, 272 (1577): 2173-2180.
- Roberts, D.L., Cooper, R.J., Petit, L.J. (2000) Use of premontane moist forest and shade coffee agroecosystems by Army Ants in western Panama. Conservation Biology 14(1): 192-199.
- Schneirla, T.C. (1971) Army Ants - A Study in Social Organization. San Francisco: W. H. Freeman and Company.