Red fruits exhibit lower colour diversity than red flowers as perceived by birds

Pollination and seed dispersal are crucial processes for plant
reproduction, sharing ecological relevance and similarities, yet they have
rarely been considered together. Flowers appear to express greater
phenotypic diversity than fruits due to multiple confounding factors,
which pose challenges for comparative analyses. The colours of flowers and
fruits are important visual signal traits in pollination and seed
dispersal, evolving under different selective pressures from their
respective pollinators and seed dispersers. Birds constitute a unique
plant-interacting group that participates in both pollination and seed
dispersal events. In this study, we focus on red flowers and red fruits
associated with avian mutualists to gain insight into the intrinsic
differences between flowers/pollination and fruits/seed dispersal. We
conducted comparisons of colouration between 94 red flowers pollinated by
birds and 99 red fruits dispersed by birds. The colour diversity was
compared in both the spectral space and the avian colour vision spaces.
Colour conspicuousness was analyzed using avian colour vision models, as
well as bee models. Pigeon colour preference was tested by controlled
experiments utilizing red stimuli with and without secondary peaks at
short wavelengths. Red fruits had lower colour diversity than red flowers,
with redder hues and fewer secondary reflectance peaks. Avian colour
vision models illustrated that fruits were more conspicuous than flowers
achromatically, but not chromatically. Pigeons did not show preference for
red with or without a secondary peak. Although both are red, there are
significant differences between flowers and fruits in terms of colour
diversity, spectral properties and colour perceptions. As we exclusively
considered avian mutualists, these differences cannot be attributed to the
differences in interacting animal groups or to their colour vision
properties. This implies that the differences in evolutionary history
between flowers and fruits may deserve further attention to understand the
colour evolution.