Bumblebees' food preferences are jointly shaped by rapid valuation of nectar sugar concentration and viscosity

Animals are often assumed to follow a strategy of energy maximisation, and therefore should evaluate feeding options based on energy intake rates. Contrastingly, rhesus macaque's learned food preferences are based on sensory properties, e.g. sweetness and resistance, regardless of energy differences. Here, we show that nectar sugar concentration (sweetness) and nectar viscosity (resistance) drive preferences of bumblebees, 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, bumblebees 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 bumblebees simultaneously evaluated sweetness and resistance to make decisions quickly (in seconds). These results indicate that bumblebees' food preferences are jointly affected by the immediate sensation of nectar sweetness and resistance as positively and negatively reinforcing properties, respectively, irrespective of energy intake rate. From these findings we propose that subjective valuations of sensory food properties should be considered as constraints in models of foraging behaviour.

学界通常认为动物会遵循能量最大化策略，因此应基于能量摄入率（energy intake rates）评估取食选择。与之相反，恒河猴（rhesus macaque）的习得食物偏好基于感官特性，例如甜度与质地阻力，且不受能量差异影响。本研究表明，花蜜糖浓度（甜度）与花蜜粘度（阻力）会影响熊蜂（bumblebees）的取食偏好——熊蜂是经济决策与觅食行为研究的经典模式生物。我们利用无味无臭的生物聚合物（biopolymer, tylose）构建了甜度与阻力存在差异的取食选择体系，这些特性既会影响能量摄入率，又可被快速感知。当能量摄入率相同时，熊蜂会基于甜度与阻力形成取食偏好；当能量摄入率存在差异，但甜度与阻力的影响相互抵消时，熊蜂则不会形成明确偏好。训练过程中的决策动力学分析显示，熊蜂会同时评估甜度与阻力，从而在数秒内快速做出决策。上述结果表明，熊蜂的食物偏好同时受花蜜甜度与阻力的即时感知影响——二者分别作为正强化与负强化特性，且不受能量摄入率的制约。基于上述发现，我们提出：在觅食行为模型中，应将动物对食物感官特性的主观估值作为约束条件纳入考量。