Bumble bees balance colony demand and floral feedback when structuring foraging bouts

Background/Question/Methods

Nutritional complexity in both available resources and dietary requirements can impact foraging economics. Resources may offer some, none, or all of the macronutrients an organism requires, while nutritional requirements may vary with phenotype, physiology, and life history stage.  When foraging for offspring with different nutritional requirements on nutritionally suboptimal resources, animals are faced with the question of how to structure their bouts, i.e. what food items to select (bout composition) and in what order to select them (bout order). While both the composition and temporal ordering of foraging bouts can impact trophic interactions, these components of foraging bout structure are often examined in isolation.  We investigated how the dietary needs of the colony and composition of floral rewards impacts foraging bout structure in the Eastern Bumble Bee, Bombus impatiens Cresson, which consume both nectar(carbohydrates) and pollen (protein) as larvae but primarily nectar as adults (N_colonies=6, N_foragers=101).  We manipulated colony demand for pollen, and availability of nectar resources on monotypic arrays of artificial flowers that offered both pollen and nectar.  To assess effects of treatment on foraging bout structure we used simulation models and hierarchical Bayesian analyses of transition preferences. 

Results/Conclusions

Colony nutritional state and available floral resources interacted to affect foraging bout structure.  When colony demand for pollen was high, foragers showed a strong bias towards pollen-pollen transfers; they were nearly twice as likely to make a pollen-pollen transfer as a nectar-nectar transfer. When colony demand for pollen was low, bees foraging on flowers that offered low and medium amounts of nectar showed no bias towards pollen-pollen transfers, whereas bees that foraged on high nectar flowers were indistinguishable from pollen-limited foragers. A detailed examination of the components of structure (composition and order) showed that colony demand primarily shaped composition.  Specifically, foragers from pollen limited collected larger pollen loads than their pollen-satiated counterparts.  By comparison, floral resources shaped bout order; bees on low nectar flowers tended to have longer runs of collection of one reward or the other than did foragers on high nectar flowers.  These results highlight how resource needs and identity interact to impact the structure of foraging bouts, and provide insight into the functional ecology of floral rewards.