Bumble bees’ food preferences are jointly shaped by rapid evaluation of nectar sugar concentration and viscosity

Animals are often assumed to follow a strategy of energy maximization, and therefore should evaluate feeding options based on energy intake rates. However, at the proximal level, a direct estimate of energy intake rates, if that is possible at all, might require postabsorptive senses with relatively longer processing times, whereas an indirect estimate of energy intake through proxies like pre-absorptive senses of different sensory food properties might support rapid foraging decisions. Here, we show that nectar sugar concentration (sweetness) and nectar viscosity (resistance) drive preferences of bumble bees, Bombus terrestris, classical models for economic and foraging decision making. Using a tasteless/odourless biopolymer (Tylose), we created feeding options that differed in sweetness and resistance, properties that affect energy intake rate and can be immediately sensed. When energy intake rates were similar, bumble bees developed preferences based on sweetness and resistance. When energy intake rates were different, but sweetness and resistance were balanced against each other, bees developed no preferences. Decision dynamics during training indicated that bumble bees simultaneously evaluated sweetness and resistance to make decisions quickly (in seconds). These results suggest that bumble bees’ food preferences are jointly affected by the immediate sensation of nectar sweetness and resistance as positively and negatively reinforcing properties, respectively, indicating a pre-absorptive proximate mechanism for rapid energy-sensitive decisions in bumble bees.