Testing the peak-end rule in bumblebees: lack of preference for a higher-reward sequence when the final reward is disappointing

The peak-end rule describes the tendency to evaluate experiences by their most intense and final moments, rather than considering the entire experience as a whole. While this cognitive bias is well-established in humans, studies on nonhuman animals are very limited. Bumblebees make foraging decisions largely based on past experiences, but whether peak-end effects influence their subsequent flower choices is still unknown. Here, we trained individual Bombus terrestris workers on two artificial flower types, alternating blue and yellow, over 12 consecutive foraging bouts. One flower type offered a sequence of three high-quality rewards (25 L drops of 50% w/w sucrose solution: "consistent" sequence), while the other provided the same sequence but ended with an additional, lower-quality reward (25 L drop of 20% w/w sucrose solution: "poor end" sequence). We then tested the bees flower type preference in a final binary choice. Bees showed a strong preference for blue flowers, both in their initial and overall visits. Across all visits during a 1-minute period, they also favoured flowers associated with the "consistent" sequence, though this preference was significant only when these flowers were yellow. Interestingly, despite offering more sucrose per foraging bout, bees did not favour the "poor end" sequence flower. This study is, to our knowledge, the first to investigate peak-end effects in an insect. How bees evaluate sequential rewards when foraging remains largely unexplored, yet could provide valuable insights into nectar distribution and plant-pollinator co-evolution.