Our study reveals remarkable plasticity in circadian rhythms of bumble-bee queens. Gynes typically emerge from the pupae with no circadian rhythms in locomotor activity. They develop robust circadian rhythms during the first days following emergence that persist after mating and diapause. However, queens with their first batch of brood are again active around the clock with no circadian rhythms. We further show that the queens can switch between activity with and without circadian rhythms in accordance with the presence of brood in their nest, and that this plasticity is not influenced by the ovaries.
To the best of our knowledge, this is the first study showing an ontogeny of circadian rhythms in queens of any social insect. The ontogeny of circadian rhythms is endogenous because it occurs under constant conditions and rhythms free-run with a period of about, but not exactly, 24 h. Bumble-bee and honeybee workers show a similar development of rhythm [7
]. Solitary insects, by contrast, typically rely on their circadian clock to time their emergence from the pupa, and shortly after eclosion show circadian rhythms in locomotor activity [28
]. A possible explanation for the apparent association between the ontogeny of circadian rhythms and sociality is that social insects emerge into a protected environment and therefore do not need to instantly adjust their behaviour and physiology to the day/night fluctuation in ambient conditions [8
]. Later in life however, virgin queens probably need to rely on the circadian clock for timing their mating activity. Species-specific mating rhythms have been reported for many species of vertebrates and invertebrates and are thought to be adaptive because they coordinate the activities of males and females, reduce risky exposures to predators and save time and energy (e.g. [31
In contrast to virgin gynes and mated queens before and after diapause, colony-founding queens with brood are active around the clock, with weak or no circadian rhythms. Colony-founding queens can nonetheless show robust circadian rhythms again if their brood is removed or lost. This close association between brood care and around-the-clock activity in queens is similar to that of bumble-bee workers that care for the brood around the clock, but forage with strong circadian rhythms [7
]. In honeybees, it was recently shown that direct contact with the brood is necessary for nurses to be active around the clock with no circadian rhythms [5
What is the mechanism linking the presence of brood and around-the-clock activity in queens? One possibility is that interactions with the brood, such as brood care, modulate the circadian system of the mother queen. According to this hypothesis, the brood requires continuous care and the mother bee is highly sensitive and responsive to this need. Signals from the brood repeatedly reset the circadian clock of the queen or alter it in other ways.
Another, not mutually exclusive hypothesis is that circadian rhythmicity is modulated by the reproductive physiology of the queen. This hypothesis can account for the observations that queens with eggs but no larvae, and even queens just before egg-laying are active around the clock with no circadian rhythms (f and electronic supplementary material, figure S2c; and electronic supplementary material, table S2). Thus, begging signals from the larvae or the act of food provision are not compulsory for inducing around-the-clock activity in mother queens.
The ovaries are obvious candidates for this influence because in both mammals and insects, ovariectomy has diverse and significant effects on circadian rhythms. Moreover, in many studies, the modifications caused by ovariectomy were at least partially reversed by treatment with ovarian steroid hormones [20
]. In cockroaches, there is evidence suggesting that active ovaries may mask circadian rhythms; mated females show low levels of activity with no circadian rhythms but start to exhibit strong circadian rhythms following ovary removal [21
]. In addition, bumble-bee nurses that are active around the clock typically have more developed ovaries compared with foragers [36
]. Even in honeybee workers, in which the ovaries are typically at a basal state, there is evidence that the division of labour is influenced by a reproductive regulatory network that includes the ovaries [39
]. Our overiectomy experiments, however, indicate that in bumble-bee queens the ovaries, or the endocrine signals they release (e.g. ecdysteroids, [41
]), are not necessary for the queen to be active around the clock. Ovariectomized queens were indistinguishable from sham-operated and intact queens; queens from all these groups had no, or only weak, circadian rhythms in the presence of brood (). Additional studies are needed for identifying the physiological factors regulating plasticity in circadian rhythms in queens. Nevertheless, our study suggests that multiple factors including the presence of brood and the behaviour or physiology associated with egg cup construction, can induce queens to be active around the clock; both queens with no ovaries but with brood (‘O− B+’ queens in experiment 3) and queens before egg-laying (, f
and electronic supplementary material, figure S2c
) were active around the clock with no circadian rhythms.
Our findings that mother bees are capable of activity with no circadian rhythms indicate that brood-related plasticity in circadian rhythms is not a derived trait limited to the worker caste. Rather, around-the-clock activity in nurse bees caring for sibling brood can stem from the same ancient pathways linking maternal physiology and the circadian system and allowing mother bees to care for their young around the clock. This explanation is consistent with the hypothesis that nursing behaviour in workers evolved from maternal traits of solitary insects [42
]. Interestingly, an association between maternal care and around-the-clock activity was also reported for killer-whales and bottlenose-dolphins in which neonates and their mothers are active around the clock for the first postpartum month [9
]. Thus, the interplay between maternal behaviour or physiology and the circadian system may be more common than previously thought. The adaptive value of this association is perhaps that around-the-clock activity enables better care during crucial stages of offspring development. In social insects, around-the-clock activity of queens and workers may be further selected to allow for fast population growth despite the fact that only a single or a few females produce all the workers, which are necessary for colony growth and reproduction.