Flower colour and visitation rates of Costus arabicus support the 'bee avoidance' hypothesis for red-reflecting hummingbird-pollinated flowers

1. Floral colour mediates plant-pollinator interactions by often signalling floral resources. In this sense, hummingbird-pollinated flowers are frequently red-coloured, and there are two tentative hypotheses to explain this pattern: 1. hummingbirds are attracted to red due its easier detection and 2. bees are sensorially excluded from red flowers. The second hypothesis is based on bees' red colour blindness, which lead them to be less frequent and less important than hummingbirds as pollinators of red-reflecting flowers. 2. Here, we untangled the role of different flower traits mediating plant-pollinator interactions and empirically tested the above hypotheses. We chose Costus arabicus due to its synchronopatric white- and pink-flowered individuals and its bee and hummingbird pollination system. Although pink flowers are not totally achromatic as pure red ones, they show an achromaticity degree that could drive bee exclusion. Specifically, we tested whether differences on red reflectance work attracting hummingbirds or excluding bees and the consequent implications for the plant's reproduction. 3. Flower colour morphs of C. arabicus do differ neither in morphology nor in nectar sugar content. Moreover, white and pink flowers can be discriminated by the bees' and hummingbirds' colour vision system. Both groups are able to discriminate the red colour variation morph on the flower petals, the white flowers being more easily detected by bees and the pink flowers by hummingbirds. Bees preferentially visited the white flowers, whereas hummingbirds visited both colours at the same rate - both patterns corroborating the second hypothesis. 4. Pollen loads deposited on stigmas did not differ between flower colour morphs, indicating that bees and hummingbirds play a similar role in the overall pollen deposition. However, bees are more likely to self-pollinate than hummingbirds. Self-pollination limits C. arabicus reproduction, and red-reflecting flowers may be better pollinated by discouraging bee visitation. Therefore, the intraspecific colour variation is driving flowers to show colour-related different levels of generalization. 5. Our results support the 'bee avoidance' rather than the 'hummingbird preference' hypothesis. Sensory exclusion of bees seems to be the pressure for red-reflecting flowers evolution, driving specialization in hummingbird-pollinated flowers due to the costs of bee pollination on plant reproduction.