Honeybees maximize efficiency by not filling their crop

Summary Honeybees often abandon non-depleting food sources with a partially filled crop. This behaviour does not maximise the net rate of energy extraction from the food sources, and thus contradicts predictions of some common models for central place foragers. We show that including the metabolic costs of transport of nectar leads to models that predict partial crop-loading. Furthermore, the observed crop loads of honeybees are less consistent with those predicted by maximization of delivery rate to the hive (net energetic gain/ unit time), than with those predicted by maximization of energetic efficiency (net energetic gain/unit energy expenditure). We argue that maximization of energetic efficiency may be an adaptation to a limited flight-cost budget. This constraint is to be expected because a worker's condition seems to deteriorate as a function of the amount of flight performed.