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Insectes soc. 49 (2002) 153–1570020-1812/02/020153-05 $ 1.50+0.20/0© Birkhäuser Verlag, Basel, 2002 Insectes Sociaux
Thermal preference for fungus culturing and brood location by workers of the thatching grass-cutting ant Acromyrmex heyeri
M.Bollazzi and F.Roces
In ant colonies, the control of adequate micro climatic conditions for brood development is achieved through both the construction of a nest and the behavioral tracking of the appropriate conditions inside the nest by nurse workers that carry the motionless brood (Seeley and Heinrich, 1981; Roces and Núñez, 1995). Although the nest architecture can help to stabilize the climatic fluctuations inside the nest, brood transport by nurse workers ultimately represents a fine-tuning response to cope with excessive deviations from the adequate values.The New World fungus-growing ants, tribe Attini, live in symbiosis with a fungus they cultivate, which provides the colony members with food. Although workers cover only a portion of their energetic demands by feeding on the fungus staphyllae, the fungus represents the unique food source for the developing larvae (Quinlan and Cherrett, 1979; Bass and Cherrett, 1995). As a consequence, workers are expected to maximize the growth of their symbiotic fungus not only byselecting suitable vegetable substrate, but also by an active choice of proper locations for culturing, according to the prevailing micro climatic conditions inside the nest. For instance,workers of a laboratory colony of
Atta sexdens rubropilosa
were observed to relocate the fungus garden along a hu-miditygradient, choosing the environments with the highest humidity (Roces and Kleineidam, 2000). With regard to temperature, it is known that the isolated attine ant fungus has strict demands of temperature for proper growth in vitro (Quinlan and Cherrett, 1978; Powell and Stradling, 1986).But remarkably, there are no published studies dealing with thermal preferences of attine ant workers for fungus culturing, even though they are responsible for the relocation of the fungus gardens when temperature varies beyond suitable values.
It has been suggested that field colonies of the genus Atta move the fungus to different subterranean chambers according to their temperature (Eidmann, 1935; Weber, 1972). This appears to be a long-term response, since leaf-cutting ant species building large and complex subterranean nests are expected to achieve a more or less natural control of temperature in deep chambers buffered by surrounding soil (Kleineidam and Roces, 2000). In contrast, leaf-cutting ant species constructing more simple nests closer to the surface, particularly those inhabiting the southernmost latitudes of the attine ant distribution range (e.g., Acromyrmex lobicornis, A. ambiguus, A. lundi, A. heyeri; Farji Brenner and Ruggiero, 1994), may be more exposed to marked changes in environmental temperature. Colonies of the thatching grass-cutting ant Acromyrmex(Moellerius) heyeri occur in open habitats of Uruguay, Brazil and the Pampean grasslands of Argentina (Bonetto, 1959). They construct a thatch mound with dry grasses and soil that protects a central and single fungus garden usually located at the general soil level (Gonçalves, 1961). Mound-building in
Acromyrmex is unique among fungus-growing ants, and seems to represent an adaptation to low temperatures, since the thatch mound insulates the fungus to some extent and prevents rapid cooling (Weber, 1972). On the other hand,mounds may be warmed more rapidly by the sun than sub-terranean nests. As a consequence, behavioural control of fungus temperature is expected to be particularly developed in mound-building Acromyrmex species inhabiting areas that undergo large temperature variations. In the present study, temperature preferences by workers of the thatching grass-cutting ant Acromyrmex heyeri for the location of both brood and fungus were investigated in the laboratory along a temperature gradient. The removal of brood and fungus from either 10°C or 37°C, and the temperatures selected to relocate them, were evaluated under otherwise constant conditions. This schedule allowed us to investigate the response of workers to low and high temperature values that are experienced by colonies in the field.