Nest thermoregulation of the paper wasp Polistes dominula

- P. dominula regulate actively the temperature of their nest by effective cooling.
- Passive mechanisms for regulating the nest temperature were underestimated so far.
- P. dominula nests are not as unprotected against temperature extremes as assumed.

Wasps of the genus Polistes build combs without any cover and hence are insufficiently protected against temperature fluctuations. Due to this fact, different types of thermoregulation of Polistes dominula nests were investigated using the modern method of thermography. The study of active mechanisms for nest thermoregulation revealed no brood incubation or clustering behaviour of P. dominula. Furthermore, we found out that wing fanning for cooling the nest was almost undetectable (4 documented cases). However, we could convincingly record that water evaporation is most effective for nest cooling. By the direct comparison of active (with brood and adults) and non-active (without brood and adults) nests, the start of cooling by water evaporation was detected above maximum outside temperatures of 25 °C or at nest temperatures above 35 °C. The powerful role of water in nest cooling was manifested by an average decrease of temperature of single cells of about 8 °C and a mean duration of 7 min until the cells reached again their initial temperature. The investigation of passive thermoregulatory mechanisms revealed that the architecture of the nest plays an important role. Based on the presented results, it can be assumed that the vertical orientation of cells helps maintaining the warmth of nests during the night, whereas the pedicel assists in cooling the nest during the day. Therefore, our study of nest thermoregulation has revealed that P. dominula wasps regulate the temperature of their nest actively by evaporative cooling and passively by a careful site selection and the architecture of their nests.