Functional spatial distribution and sociometric characteristics of Formica fusca ants during winter dormancy

Abstract

1. Ant colonies passing through the period of winter dormancy are subject to the two conflicting selective pressures: (i) on escaping the coldest temperatures during the winter by hiding deep into the soil and (ii) on the reactivation after overwintering as soon as the weather conditions become favourable and the soil surface is warm enough. 2. If selective pressures associated with overwintering are affected differently by the spatial variation in the climate conditions then different overwintering strategies can be expected to occur across geographical gradients. 3. In this study, Formica fusca colonies were excavated in northeastern Poland to examine the spatial distribution of overwintering ants. It has been found that queens, accompanied by part of the workers, shelter themselves at a depth close to the colony's lower limit. However, workers were clustered at different depths along the soil profile, including a few centimetres from the soil surface. 4. Natural selection might promote to allocate individuals who are less valuable to the colony to more risky tasks (e.g. those performed outside the nest). Thus, playing the role of a forager should be associated with the occupation of more risky positions during overwintering. However, no statistically significant difference has been found between workers overwintering in the upper and lower soil strata in their propensity to explore the experimental arena. 5. The observed pattern of the spatial distribution of overwintering ants indicates the existence of a trade-off between winter mortality, costs of physiological adaptations, and the time needed for a colony to reactivate in response to temperature increase.