The role of secondary pollinators in the evolution of complex colour signals in a bimodal pollination system

Flower colour is recognized as a key trait influencing pollinator
visitation and behaviour, evolved to match the sensory system of a
particular pollinator group. However, some flowers combine colours
associated with different pollinators, suggesting bimodal adaptation. The
perennial herb Cyrtanthus obliquus produces red corollas with yellow tips
and is pollinated by birds, but also by bees. To assess the role of the
two pollinator categories in shaping selection on the bi-coloured
corollas, we examined bee choices in arrays in which the colour signals of
plants had been manipulated, and we quantified total as well as
bird-mediated phenotypic selection on flower colour using both bird and
bee visual models. Bees preferred flowers with a yellow signal over
all-red flowers. The analysis of phenotypic selection indicated that both
birds and bees influence net selection on flower colour, resulting in
conflicting selection on the colour contrast between corolla and corolla
tip. Our findings show that the optimal phenotype should depend on the
relative importance of the two categories of pollinators in a given
population, and are consistent with bimodal adaptation. This study fills
an important gap by showing that secondary pollinators can contribute to
selection on complex colour signals in angiosperm flowers.