Factors Influencing Sun Basking in Red Wood Ants (Formica polyctena): a Field Experiment on Clustering and Phototaxis

Abstract

We monitored nest interior and surface temperatures together with two aspects of the sun-basking behavior of wood ants: aggregation (by regular monitoring of occurrence and density of clusters) and phototaxis (tendency to move from shade to sun or vice versa, recorded as a response to artificial shading during sunny weather), using ten nests of Formica polyctena near Vimperk Czechia. Dense clusters occurred only in spring; weak clusters rarely occur the rest of the year. Statistical analysis showed that timing of dense clusters is affected by both environmental parameters (light intensity, insulation status of the nest) and internal nest factors (inner nest temperature, ant population size, nest volume). Phototaxis changed from positive to negative during the year. In spring, ant workers performed the sun basking readily and when shaded moved into the sun. In summer, however, the ants avoided sun basking and aggregated in the shade. The shift from sun basking to sun avoidance was driven mainly by nest surface temperature. The switch occurred when the temperature of sun-exposed nest surface reached 42.8 °C, which is two degrees higher than the experimentally measured lethal temperature (LD50) for red wood ants. This shows that two basic components of sun basking, aggregation and sun exposure, are driven by a different set of environmental conditions and their interplay is likely to maintain balance between the needs of the colony to heat up the nest and risk for individual workers arising from overheating during prolonged sun exposure.