Low temperature triggers sociality in a facultatively social bee.

The evolutionary transition to eusociality in the solitary ancestors of social insects, like ants, termites, wasps and bees, may have been initiated by an environmental induction of group living. Plasticity in social behaviour in response to the environment can be found in facultatively social insects. However, it is currently unknown which environmental cues trigger the individual decision of an insect to nest solitarily or socially. Such environmental cues could be identified by exposing individuals of a facultatively social insect to experimentally manipulated cues in a controlled environment. Here, we report such an experiment with the facultatively social allodapine bee Exoneura robusta, which we exposed to different temperature and predation treatments in climate chambers. We observed that low temperature triggered the bees to behave more socially. Furthermore, there was substantial variation in the tendency to be social among individual bees. Our study identifies a specific environmental cue that triggers a plastic response in sociality. Such plasticity could be crucial at the origin of group living and social breeding, facilitating the evolution of eusociality in insects.