Colour as a backup for scent in the presence of olfactory noise: testing the efficacy backup hypothesis using bumblebees ( Bombus terrestris )

The majority of floral displays simultaneously broadcast signals from multiple sensory modalities, but these multimodal displays come at both a metabolic cost and an increased conspicuousness to floral antagonists. Why then do plants invest in these costly multimodal displays? The efficacy backup hypothesis suggests that individual signal components act as a backup for others in the presence of environmental variability. Here, we test the efficacy backup hypothesis by investigating the ability of bumblebees to differentiate between sets of artificial flowers in the presence of either chemical interference or high wind speeds, both of which have the potential to impede the transmission of olfactory signals. We found that both chemical interference and high wind speeds negatively affected forager learning times, but these effects were mitigated in the presence of a visual signal component. Our results suggest that visual signals can act as a backup for olfactory signals in the presence of chemical interference and high wind speeds, and support the efficacy backup hypothesis as an explanation for the evolution of multimodal floral displays.