Phenology and causation of nest heating and thermoregulation in red wood ants of the Formica rufa group studied in cariferous forest habitats in southern Finland

Abstract

The inner nest temperature in field colonies of ants of the F. rufa group was studied with thermograph recordings and spot sampling. The observations indicate that in colonies with a worker complement exceeding 1 million, nest-warming after winter could start as an autocatalytic process and hence may not require triggering by sunning be haviour. In the colony studied with the thermograph the intranest temperature remained at a stable level, near 30°C in late spring and summer, even when the outside temperature temporarily dropped below freezing point A slight overshoot or increase in inner nest temperature, was evident, however, when the outside temperature dropped below the threshold for ant activity in spring. It was found that nests later producing sexuals main tain a significantly higher inner temperature in spring than nests later producing only worker broods. The nest-warming effect of insolation is considered more important in small or weak colonies, whereas endogenous nest heating, based on the metabolism of the ants and their clustering behaviour, is more compatible with the observations in the case of vigorous colonies. The recordings suggest that a capacity for social thermoregula tion is a cause of thermal stability in red wood ant nests.