Developmentally determined attenuation in circadian rhythms links chronobiology to social organization in bees

We investigated labor-related plasticity in the circadian clock of the bumblebee Bombus terrestris. Bumblebee workers vary enormously in body size, and we found that size, division of labor, and diurnal rhythms in activity are correlated in B. terrestris colonies. Large workers typically perform foraging activities with strong diurnal rhythms and low activity at night, whereas small bees typically care for (nurse) brood around the clock with weak or no diurnal rhythms. Under constant laboratory conditions, circadian rhythms in locomotor activity were weaker, less stable, and developed at a later age in small (nurse-size) bees compared to their larger (forager-size) sisters. Under a light:dark illumination regime, many small bees, particularly at a young age, were active during the dark phase, fewer small bees developed rhythms, and they did so later compared to large bees. Taken together these findings reveal naturally occurring attenuation or suppression in the circadian clock of small bees that is determined during pre-adult development. This deficiency in clock function, however, does not result in pathology but rather appears to be functionally significant, because it is associated with around-the-clock brood care activity and therefore apparently improves divisions of labor and colony efficiency. This in turn suggests that variation in social biology influences traits of the circadian clock.