Floral signal complexity as a possible adaptation to environmental variability: a test using nectar-foraging bumblebees, Bombus impatiens

Floral signals are typically emitted across multiple sensory modalities, although why they are multimodal is unclear. One possible explanation is that multimodal signalling ensures that at least one signal component will be transmitted effectively under varying environmental conditions (the ‘efficacy backup’ hypothesis). For example, by transmitting both component A and B, a signaller can communicate under environmental conditions where transmission of component A is reduced; component B ‘backs up’ A. To test this hypothesis, we determined whether a floral scent could back up a floral colour signal when light levels were low. We trained nectar-foraging bumblebees to discriminate rewarding and unrewarding targets that differed in colour, scent, or both colour and scent, and then presented the targets at different levels of illumination. We measured bees’ accuracy at distinguishing the two targets and their rate of visits to the trained target. Performance on both measures declined under low light when targets were unscented. The presence of scent reduced the loss of accuracy under low light, supporting the efficacy backup hypothesis, but this effect depended upon the colour of the previously rewarded target. In contrast, the presence of scent did not affect the overall rate of correct visits under low light (correct visits/foraging time). A backup mechanism that maintains accuracy, but not rate of nectar collection, does not necessarily benefit the pollinator. However, it most likely benefits the plant through reduced pollen wastage. In short, multimodal floral signals may benefit the plant by improving pollen transfer, while not benefiting the pollinator.