Sunning clusters of ants contribute significantly, but weakly to spring heating in the nests of the red wood ants, Formica polyctena

Red wood ants perform sun-basking behavior in the spring. This very conspicuous behaviour consists from densely packed bodies of ants, forming sunning clusters, which expose themselves to sun radiation. This led to rapid increase in ant body temperature. The expectation was that the ants so heated that return to the nest may bring heat from outside, which heats the nest. Although this was postulated a long time ago, the effect of sun-basking behaviour on nest heating has not been tested. Filling this gap is the subject of this study. To do this the presence of sunning clusters on 10 Formica polyctena nests was recorded using camera-traps, which recorded presence of sunning clusters on the nest surface daily from March 10 to June 3, 2016 in the south of the Czech Republic. Camera traps also recorded external air temperature. At the same time, internal nest temperatures were measured by thermometers located inside the nests, which continually recorded nest temperature over the same period. Comparison of consecutive days without and with sunning clusters of ants revealed an increase in internal nest temperature occurred when clustering was recorded. This, however, was only recorded for nests in which the temperature did not go above 20 °C. Frequency of daily occurrence of sunning clusters (proportion of days on which clusters were observed) was significantly positively correlated with daily increase in the internal temperature of the nests. This holds even when the increase in the internal temperature of the nests is corrected for increase that occurs at the same external temperature, but in the absence of clustering. This statistical evidence clearly indicates that sun-basking contributes to the heating of nests in spring, but it is very low. This is supported by the fact that nests that differ substantially in the frequency of occurrence of sunning clusters do not differ significantly in internal nest temperature in spring.