In many plant species, production of floral rewards such as nectar and pollen varies markedly across flowers, both between and within plants. Often, plants provide floral cues, such as variation in flower size, that enable pollinators to discriminate between more- and less-rewarding flowers. Some evidence suggests that providing these informative cues is beneficial to plants, but the mechanisms by which they influence plant fitness are not known.
We tested whether bumblebees’ responses to reward variability and informative floral cues, in the form of flower size variation, could allow a plant to escape the plants’ dilemma. That is, would those traits allow the plant to increase its attractiveness to pollinators while at the same time keeping the rate of inbreeding low by limiting the number of flowers each pollinator visited before moving to a new plant? After training bumblebees to associate large flower size with sucrose rewards, we allowed them to forage, individually, on an artificial meadow composed of plants using four different strategies: (A) two large, rewarding flowers, (B) four large, rewarding flowers, (C) four large flowers, two rewarding and two unrewarding, and (D) two large, rewarding flowers augmented by two or four small, unrewarding flowers.
Preliminary results suggest that neither reward variability nor informative flower size cues allow plants to escape the plants’ dilemma. In the absence of informative cues, the length of pollinator visits did not differ significantly between plants with and without reward variability (i.e., strategies B and C), and both types experienced significantly longer visits than plants with fewer flowers (strategy A). Plants using strategy D, variable rewards and informative cues, did experience shorter visits than comparably-sized plants with entirely rewarding flowers (strategy B), but they were also less attractive. In fact, adding small, unrewarding flowers to a plant did not significantly increase its attractiveness.
These results contrast with those of similar studies using color, rather than flower size, as the cue identifying rewarding flowers, raising interesting questions about how the type of cue changes the consequences of providing information to pollinators. Our results suggest that flower size can influence the attractiveness of entire plants as well as individual flowers. Providing flower size cues that help pollinators identify low-quality flowers may, therefore, be more costly than we previously thought, making their widespread occurrence puzzling.