Behavioral Sensitivity In A Dynamic Social Context

The worker population of a honeybee colony normally segregates into a temporal hive bee caste that performs a series of tasks inside the nest (cell cleaning, brood rearing and cell building), and a temporal forager caste that specializes in collecting nectar, pollen, water and propolis (reviewed by Winston, 1987). A worker starts out as a hive bee and subsequently switches to foraging— typically after 18-28 days. The timing of this shift, however, is elastic (7-135 days: Haydak, 1963; Free, 1965), and this plasticity is, in part, controlled by colony demography. Specifically, bees in the foraging population produce a pheromonal cue that inhibits hive bees from initiating foraging (Pankiw, 2004; Toth and Robinson, 2005). Consequently, as foragers progressively are lost from colonies they are replaced by hive bees that are not efficiently inhibited (Huang and Robinson, 1996) (but see Regulation of Aging through Social Control of Behavioral Plasticity as well for an expansion of the concept of inhibition of foraging behavior). The regulatory mechanisms responsible for the hive bee to go on to the forager transition deserve attention in a gerontological context because the timing of the switch largely determines the overall lifespan of the worker. This is because the age at onset of foraging is normally much more variable than the length of the foraging period (for a review see Omholt and Amdam, 2004), which suggests that the duration of the hive bee phase is a dynamic entity mainly determined by the intracolonial conditions, while the duration of the forager phase is more dependent on the extracolonial conditions.

Reversion of Task-Associated Trajectories Worker bees can, moreover, revert from foraging duties to hive activities (Robinson et al., 1992). This phenomenon constitutes an inversion of the above-mentioned behavioral progression and appears to require a substantial perturbation of colony demography: behavioral reversion can be triggered experimentally through removal of the entire hive bee population. Under this condition, a proportion of the remaining foragers initiate hive tasks such as brood rearing (Huang and Robinson, 1996). It is likely that this reversion, which withdraws bees from risky foraging tasks and causes them to engage in labor within the sheltered nest, is associated with an extension of lifespan.

Temporal Cessation of Behavioral Development As foraging opportunities decline at the onset of the unfavorable season (the winter or drought period), a third and exceedingly long-lived temporal caste emerges. This group constitutes the diutinus stage (formerly referred to as the ''winter bee'' phenotype; Omholt and Amdam, 2004), and the appearance of these workers is intimately linked to the reduction of brood rearing that precedes the onset of the unfavorable season in honeybee colonies (Maurizio, 1950). The diutinus workers, like hive bees, reside within the protected nest. They thermoregulate the colony and take care of the queen, and do not initiate brood rearing or foraging until the next favorable period. Shorter intervals of reduced brood rearing can, nonetheless, arise for various reasons throughout the season (e.g., when swarming, replacement, or loss of the reproductive queen results in a reduction of the number of eggs, larvae and pupae in a colony). Therefore, diutinus bees can emerge during the favorable period also, when workers—as a rule—segregate into the hive- and forager populations, exclusively.

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