Bumble Bee Workers Give Up Sleep to Care for Offspring that Are Not Their Own

Sleep is ubiquitous in vertebrates and invertebrates, and its loss is typically associated with reduced performance, health, or survival, for reasons that are yet unclear [1-3]. Nevertheless, some animals can reduce sleep for increasing foraging time [4], under predation risk [5-8], during seasonal migration [9-11], or for having greater mating opportunities [12, 13]. Here, we tested the hypothesis that social bumble bee (Bombus terrestris) workers give up sleep for improving brood care. We combined video recordings, detailed behavioral analyses, sleep-deprivation experiments, and response-threshold assessments to characterize the sleep behavior of worker bees and showed that immobility bouts of ≥5 min provide a reliable proxy for sleep. We next used this index to study sleep with an automated video-based activity monitoring system. We found that isolated workers severely reduce sleep time in the presence of both larvae that need to be fed and pupae that do not. Reduced sleep was also correlated with around-the-clock activity and wax-pot building, which are typical for nest-founding mother queens. Cocoons, from which we removed the pupae, elicited a similar but transient sleep loss in tending workers, suggesting that the pupa effect on sleep is mediated by pheromonal signals. Sleep time increased following brood removal but remained lower compared to control bees, suggesting that the brood modulated sleep need. This first evidence for brood modulation of sleep in an insect suggests that plasticity in sleep can evolve as a mechanism to improve care for dependent juveniles, even in social insect workers that do not care for their own offspring.