Thermal preference for fungus culturing and brood location by workers of the thatching grass-cutting ant Acromyrmex heyeri

Thermal preferences for location of brood and fungus by workers of the thatching grass-cutting ant Acromyrmex heyeri were investigated in the laboratory, using a temperature gradient between 10degreesC and 37degreesC. Workers from a large laboratory colony were motivated to pick up exposed brood and fungus by removing a small part of the protective nest thatching. Single workers carrying pieces of fungus or brood items were then gently removed from the colony and placed at one end of the temperature gradient, either at 10 or 37degreesC. They were allowed to relocate the fungus or brood following their temperature preference. Both the probability of relocating an item and the temperatures selected were recorded after 45 minutes. The probability of relocation largely depended on the starting temperature, being higher for workers starting at 37degreesC than at 10degreesC, for both fungus and brood-carrying workers. For those relocated items, the selected temperatures averaged 25degreesC and 24degreesC for fungus and brood, respectively, when workers started from the hot end of the gradient. When workers started from the cold end of the gradient, the selected values were slightly lower, 22degreesC and 21 degreesC for fungus and brood, respectively. These values match closely the temperature range in which maximal growth of the isolated attine ant fungus is achieved, between 20 and 25degreesC, as reported in the literature, indicating that the thermoregulatory behavioral responses of workers maximize fungal growth. Thermal preferences and the construction of thatch mounds are discussed as regulatory responses for the control of fungus growth in leaf-cutting ants inhabiting temperate regions.