Figure 2.

Possible mechanisms for entrainment of circadian rhythms in honeybee colonies. (a) Entrainment by environmental zeitgebers. Both workers performing outside activities, such as foragers and guards (depicted by an orange-coloured bee), and nest workers (depicted by a yellow-coloured bee) are directly entrained by sunlight and temperature cycles. (b) Social entrainment by direct contact. Bees performing outside activities are entrained by environmental zeitgebers, and these individuals entrain nest workers by means of physical or close distance interactions. Entrained nest bees in turn may socially synchronize other nest bees that have not encountered foragers. (c) Entrainment by environmental cycles driven by forager activity. Foragers that are entrained by environmental zeitgebers are more active during the day, and their activity changes the colony microenvironment (pink background), which in turn entrains the clock of other bees in the colony. (d) Self-organized social synchronization. Active bees change their environment, and the sum of the activities of all individuals form weak environmental oscillations that entrain the clocks of an increasing number of bees. The amplitude of these oscillations increases as more and more bees are entrained to the same phase and is eventually strong enough to entrain all the bees in the colony.