A generalised mimicry system involving angiosperm flower colour, pollen and bumblebees’ innate colour preferences

Abstract

Flower colour is a major advertisement signal of zoophilous plants for pollinators. Bees, the main pollinators, exhibit innate colour preferences, which have often been attributed to only one single floral colour, though most flowers display a pattern of two or several colours. The existing studies of floral colour patterns are mostly qualitative studies. Using a model of bee colour vision we quantitatively investigate two questions: whether or not component colours of floral colour patterns may mimic pollen signals, and whether or not bumblebees exhibit innate preferences for distinct parameters of naturally existing floral colour patterns. We analysed the spectral reflectances of 162 plant species with multicoloured flowers and inflorescences, distiniguishing between inner and outer colours of floral colour patterns irrespective of the particular structures so coloured.

We found that:

– The inner colour of radially symmetrical flowers and inflorescences and of zygomorphic flowers appears less diverse to bees than the peripheral colour.

– The inner colour of most radial flowers and inflorescences as well as the inner colour of a large number of non-related zygomorphic flowers appears to bees to be very similar to that of pollen.

– Bumblebees (Bombus terrestris) exhibit innate preferences for two-coloured over single-coloured dummy flowers in a spontaneous choice test.

– Bumblebees exhibit innate preferences for dummy flowers with a large over those with a small centre area.

– Bumblebees exhibit innate preferences for dummy flowers with a centre colour similar to that of pollen over those with another centre colour.

Our findings support the hypotheses that the inner component of floral colour patterns could be interpreted as a generalised and little recognised form of mimicry of the colour of visually displayed pollen, that bumblebees exhibit innate preferences regarding colour and size parameters of floral colour patterns, and that these correspond to visually displayed pollen. These findings together suggest a prominent role of floral colour patterns in advertisement to and guidance of naive flower visitors.