Down‐facing flowers, hummingbirds and rain

Abstract

Flower orientation, the angle between a flower’s main axis and the horizontal, is an important but understudied flower trait. I explore the relationship between flower orientation, hummingbird pollination, and rain, and review evidence for the adaptive significance of a down‐facing orientation as a mechanism to avoid flower flooding. A screening of a tropical and a temperate flora showed that hummingbird flowers more commonly face down than insect flowers. In the temperate forests of southern South America, genera pollinated by hummingbirds and genera with down‐facing flowers—irrespective of pollination mode—increase along a strong E‐W precipitation gradient across the Andes. These patterns suggest that flowers with a down‐facing orientation are associated with pollinators that, like hummingbirds, remain active during rain and that this type of flower orientation may constitute an adaptation against the deleterious effects of nectar dilution and pollen rainwash. Still scarce experimental evidence confirms that a down‐facing morphology may prevent flower flooding. However, other morphological traits, like corolla closure and a narrow tubular shape, could also be effective in hindering nectar dilution and pollen rainwash.