The mechanics of nectar offloading in the bumblebeeBombus terrestrisand implications for optimal concentrations during nectar foraging

Nectar is a common reward provided by plants for pollinators. More concentrated nectar is more rewarding, but also more viscous, and hence more time-consuming to drink. Consequently, theory predicts an optimum concentration for maximizing energy uptake rate, dependent on the mechanics of feeding. For social pollinators such as bumblebees, another important but little-studied aspect of foraging is nectar offloading upon return to the nest. Studying the bumblebeeBombus terrestris , we found that the relationship between viscosity (µ ) and volumetric transfer rates (Q ) of sucrose solutions differed between drinking and offloading. For drinking,Q ∝µ −0.180 , in good agreement with previous work. Although offloading was quicker than drinking, offloading rate decreased faster with viscosity, withQ ∝µ −0.502 , consistent with constraints imposed by fluid flow through a tube. The difference in mechanics between drinking and offloading nectar leads to a conflict in the optimum concentration for maximizing energy transfer rates. Building a model of foraging energetics, we show that including offloading lowers the maximum rate of energy return to the nest and reduces the concentration which maximizes this rate by around 3%. Using our model, we show that published values of preferred nectar sugar concentrations suggest that bumblebees maximize the overall energy return rather than the instantaneous energy uptake during drinking.