The social clock: circadian rhythms in the common eastern bumble bee Bombus impatiens

I present preliminary data that supports the hypothesis of a colony endogenous clock in a temperate primitively social insect. It has been known that individuals within a social colony show differences in the expression of their endogenous circadian rhythms. For example, in honey bees young workers have less developed circadian rhythms compared to older workers (Moore et al 1998). In bumble bees these differences in circadian rhythmicity are present at a body-size level where large individuals exhibit robust circadian locomotor patterns of activity (Yerushalmi et al 2006). Here, using an electronic bee counter and serial photography analysis I show that there is a colony expression of activity and rest that resembles an endogenous circadian rhythm. Circadian analysis show that colonies exposed to constant darkness free-run with a period of ~23.5 hours. Furthermore, there is a body size difference in the rest patterns of workers within the colony, where larger bees being more inactive further from the brood than smaller workers. I also present preliminary data on the daily expression patterns, in the brain of workers of different body sizes, of two proteins (PER and PDF) that are part of the molecular machinery of the central clock in insects. In conclusion, this data suggest the existence of a colony level endogenous clock for activity and rest. This colony clock could be responsible for maintaining an internal synchrony within the colony with the external photoperiod.