Floral humidity and other indicators of energy rewards in pollination biology

Floral traits that correlate with nectar availability or are linked functionally to nectar production carry the potential to enable remote assessment of energy rewards by pollinators. Such floral traits can be considered "honest" in the sense that they convey information about the quality or profitability of a flower to a pollinator. Recently a new sensory channel used in plant-pollinator interactions was identified. We demonstrated that evaporation of water from the nectar itself and the petals create local humidity gradients above Oenothera cespitosa (Onagraceae) flowers. Since these humidity gradients are directly linked to nectar volume, they convey reliable information about nectar rewards to hawkmoth pollinators (Sphingidae). Several studies document a variety of sensory cues that constitute honest signaling between plants and pollinators, and shed light on the central question of when the two parties should communicate honestly. In the following sections, I will comment on different honest signals mediating plant-pollinator interactions, with a special emphasis on our recent findings about floral humidity gradients.