Experience changes pollinator responses to floral display size: from size-based to reward-based foraging

Plants that display many open flowers usually receive higher pollinator visitation rates, but it is unclear whether pollinators select plants to visit based on the size of floral display (apparent size) or the value of the floral rewards (reward size). To examine how pollinators respond to apparent size and reward size, we observed bumble bees foraging among arrays of artificial plants. We constructed two kinds of artificial flower: (i) rewarding flowers that produced nectar constantly; and (ii) unrewarding flowers that produced only water. Thus, we could construct plants that varied both in numbers of flowers (apparent size) and in numbers of rewarding flowers (reward size). At the beginning of the experiments, bees made more visits to the plants with the most flowers, irrespective of the rewards they contained. However, after several hours of foraging, bees returned selectively to plants with the greater number of rewarding flowers, irrespective of the number of flowers the plant presented. After we replaced rewarding plants with non-rewarding plants, bees continued visiting plants at formerly-profitable locations for a while. Our results demonstrate that bees initially showed preferences for plants with larger floral displays, but eventually bees were able to discriminate between rewarding and less-rewarding plants of equal display size by associating a plant's location with its reward size. Our results suggest that plants with many flowers can achieve higher visitation rates from pollinators in two ways: (i) by attracting inexperienced pollinators with large displays; and (ii) by encouraging experienced pollinators to return with the promise of greater rewards.