The cues of colony size: how honey bees sense that their colony is large enough to begin to invest in reproduction.

As organisms develop, they first invest resources in survival and growth, but after reaching a certain condition they start to also invest in reproduction. Likewise, superorganisms, such as honey bee colonies, first invest in survival and growth, and later commit resources to reproduction once the number of workers in the colony surpasses a reproductive threshold. The first form of reproductive investment for a honey bee colony is the building of beeswax comb made of special large cells used for rearing males (drones). How do the workers sense that their colony is large enough to start building this 'drone comb'? To address this question, we experimentally increased three possible cues of colony size - worker density, volatile pheromone concentration and nest temperature - and looked for effects on the bees' comb construction. Only the colonies that experienced increased worker density were stimulated to build a higher proportion of drone comb. We then monitored and quantified potential cues in small and large colonies, to determine which cues change with colony size. We found that workers in large colonies, relative to small ones, have increased contact rates, spend more time active and experience less variable worker density. Whereas unicellular and multicellular organisms use mainly chemical cues to sense their sizes, our results suggest that at least one superorganism, a honey bee colony, uses physical cues to sense its size and thus its developmental state.