Flower signal variability overwhelms receptor-noise and requires plastic color learning in bees

Color discrimination thresholds proposed by receptor-noise type models are frequently used in animal vision studies to predict a precise limit on the capacity of an animal to discriminate between stimuli. Honeybees and bumblebees are 2 closely related hymenopteran species for which precise data on photoreceptor sensitivities and receptor noise exist, enabling accurate testing on how their vision conforms to model predictions. Color vision has been proved in these species, and they are known to predominantly visit flowers using visual signals to collect nutrition. Surprisingly, however, the natural variability of flower signals has been rarely considered, and recent work also suggests bees may tune color vision through experience. We initially measured the spectral variability of flowers from 2 species: Goodenia ovata and Rosemarinus officinalis where free-flying honeybees were observed constantly foraging from conspecific flowers. We empirically determined honeybee color discrimination thresholds for color stimuli considering either absolute- or differential-conditioning discrimination functions. Secondly, we analyzed greenhouse grown wild-type Antirrhinum majus flower petal spectra as well as spectra from mixta and nivea strains of this species, and empirically determined bumblebee color discrimination considering conditioning experience. In all measured cases, within-flower type spectral variability exceeded a 1.0 Receptor Noise threshold, often by several units. Observed behavioral color discrimination functions considering the respective conditioning procedures closely matched the range of signal variability for both honeybees and bumblebees, showing that color vision in bees cannot be described by a single fixed value, and plasticity is a key component of bee foraging behavior in natural environments.