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ScienceWeek
ANIMAL ECOLOGY: ON ARMY ANTS
The following points are made by Stefanie M. Berghoff (Current Biology 2003 13:R676):
1) One of the most striking forms of life to be found in tropical regions are the army ants. Originally renowned for their spectacular raids, army ants have attracted scientific attention for almost two centuries. Army ants are characterized by a unique combination of mass-raiding and recurrent migrations. Associated with their unusual life style, army ants form large colonies with specialized queens -- known as dichthadiigynes -- and reproduce by colony fission. This unusual combination of traits -- mass-raiding, migrations, colony fission and dichthadiigyne queens --is commonly referred to as the "army ant adaptive syndrome" (1).
2) Mass-raiding is probably the most outstanding feature of army ants. Originating from their temporary nest -- the "bivouac" --thousands of worker ants collectively scour about the surrounding area in search for food. Depending on the army ant species, they either prey on almost any animal they can overwhelm, or are specialized to feed on colonies of other social insects, such as termites, wasps or other ants. Their impact on prey populations is considerable, rendering them top predators of leaf-litter arthropods and even of small vertebrates in some areas (2,3). In the tropics, army ants are an important factor contributing to the maintenance of biodiversity.
3) Related to their predatory efficiency, army ants move to new foraging areas at more or less regular time intervals (2). There they resume their mass-raiding until they move again. As numerous workers are needed to conduct these mass-raids, army ants have large colonies which reproduce by fission: this is a rather unusual form of colony reproduction in which a mature colony splits, with parts of the colony and brood walking off and following either their old queen or a sister queen. Army ant queens thus never leave the ground for a nuptial flight and remain flightless throughout their lives. Furthermore, they are able to produce huge amounts of eggs in the relatively short time between successive migrations.
4) The question of whether such a complex combination of behavioral and morphological traits could have arisen independently more than once in the evolutionary past has been the subject of much debate. A new genetic study by Brady (4) supports the view that army ant behavior has arisen just once in evolution, and then been conserved for more than 100 million years.
5) To date, 298 army ant species from three subfamilies of the Formicidae have been described, all from tropical or subtropical regions (5). This high diversity, and the fact that all investigated species show the entire set of army ant traits, are indicative of the ecological success of this lifestyle.
References (abridged):
1 Gotwald, W.H.Jr. (1982). Army ants. In Hermann, H.R. ed. (New York: Academic Press), pp. 157-254
2 Gotwald, W.H.Jr. (1995). Army ants: the biology of social predation. (Ithaca: Cornell University Press/Comstock Press)
3 Brosset, A. (1988). Le peuplement de mammifères insectivores des forets du nord-est du Gabon. Rev. Ecol. (Terre Vie) 43, 23-46
4 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. Proc. Natl. Acad. Sci. U.S.A. 100, 6575-6579
5 Bolton, B. (1995). A New General Catalogue of the Ants of the World. (Cambridge, London: Harvard University Press)
Current Biology http://www.current-biology.com
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ON THE CONTROL OF SEX RATIOS IN ANT COLONIES
The following points are made by L. Passera et al (Science 2001 293:1308):
1) Social insects provide some of the most striking examples of elaborate cooperative behavior, yet life within colonies also entails conflicts. In ants, the most strident conflict concerns sex ratio, with workers favoring a more female-biased sex investment ratio than queens. This conflict arises because of the hymenopteran haplodiploid system of sex determination, whereby unfertilized eggs develop into males and fertilized eggs develop into females. As a result, workers in colonies headed by a single once-mated queen are three times more related to sisters than to brothers.
2) Because of this asymmetry in relatedness, the population-wide sex allocation ratio should equilibrate at 3:1 (female:male) if workers control the colony's investment in reproductive offspring. In contrast, because queens are equally related to their daughters and sons, an equal investment in male and female reproductives is expected if the colony's allocation of resources is under the control of the queen. Because they control brood rearing and food flow in the colony, workers can bias sex allocation, for example, by selectively eliminating males and/or preferentially feeding females, and workers have indeed been demonstrated to manipulate colony sex ratios in this way.
3) The authors report a study in which they exchanged queens between male- and female-specialist colonies of the fire ant Solenopsis invicta. These exchanges quickly reversed the sex-ratio biases of adopting colonies. The sex ratio of queen-laid eggs differed strongly between male- and female-specialist colonies. The authors suggest that these findings indicate that queens can force workers to raise male sexuals by limiting the number of female brood, and that the findings help to explain why sex investment ratios lie between the queen and worker equilibria in this and many other ant species.
Science http://www.sciencemag.org
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GENETIC CONTROL OF SOCIAL ORGANIZATION IN ANTS
Biological evolution is marked by a number of major transitions, one of which is the evolution of complex social behavior. Animal social life can take a variety of forms, each distinguished by features such as group size and the reproductive roles of group members. One focus in evolutionary biology is to identify the causes of social behavior and its conspicuous variation, and to determine the extent to which social organization is under genetic control. Such information is useful for reconstructing pathways of animal social evolution. Current views on insect social evolution stress the importance of ecological and behavioral environments in molding what are largely plastic social behaviors.
The following points are made by K.G. Ross and L. Keller (Proc. Natl. Acad. Sci. 1998 95:14232):
1) The authors report evidence that major variation in the social organization of fire ant colonies is under simple genetic control, providing a demonstration of an apparent strong genetic component to complex social behavior.
2) The authors report that a single genomic element (the gene [Gp-9]) is responsible for the existence of two distinct forms of social organization in the fire ant *Solenopsis invicta. This genetic factor apparently influences the reproductive *phenotypes and behavioral strategies of ant queens and determines whether workers tolerate a single fertile queen or multiple queens per colony.
3) The authors suggest "these findings reveal how a single genetic factor can have major effects on complex social behavior and influence the nature of social organization."
Proc. Nat. Acad. Sci. http://www.pnas.org
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Notes:
Solenopsis invicta: The fire ant S. invicta is an introduced pest species in the southern US, the species existing in two distinct social forms. The "monogyne" form features colonies with a single fertile (egg-laying) queen, whereas the "polygyne" form features colonies with multiple fertile queens. The two social forms differ in other major aspects of their reproductive biology.
phenotypes: The term "phenotype" refers to the total appearance of an organism as determined by the interaction during development between its genetic constitution (genotype) and the environment.
ScienceWeek http://www.scienceweek.com
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