Changes in the metabolic rate of the foraging honeybee: effect of the carried weight or of the reward rate?

The metabolic rate of free flying honeybees (Apis mellifera ligustica) foraging on a multiple automatic feeder was measured in complete absence of perturbation. Each time the sucrose flow rate was doubled, the metabolic rate increased by 18.2 +/- 2.0% (14.7 microl CO2 min(-1) and final crop load by 25.1 +/- 2.4% (7.04 microl). The possibility that the heavier load carried by the bees caused the increase in the metabolic rate was analyzed in detail. It was found that, for the same weight in the crop, the metabolic rate increased with the increasing reward rate. Therefore, a factor other than the carried weight might account for this increase: a motivational drive, whose intensity may depend on the reward rate at the food source. Although at higher reward rates metabolic rate increased during the visit, at lower reward rates it remained constant, suggesting that the effect of the carried weight on the metabolic rate might be controlled by this motivational drive. The hypothesis that honeybees maximize individual efficiency by reducing their crop load loses support, as foraging costs are not determined by the carried weight. The functional meaning of the reduction in crop load would be to increase the informational exchange at the hive.