Floral humidity as a signal – not a cue – in a nocturnal pollination system

Abstract

AbstractAlthough visual and olfactory floral signals attract pollinators from a distance, at the flower’s threshold, pollinators can use floral humidity as an index cue for nectar presence. We evaluate the role of floral humidity in theDatura wrightii-Manduca sextanocturnal pollination system. In addition to our finding thatM. sextashows strong innate attraction toward humid flowers, we identify the hygrosensing sensillum on their antennae, demonstrate its extreme sensitivity to minute changes in RH, and observe the elimination of moths’ behavioral preference towards humid flowers following experimental occlusion of the sensilla. Despite Manduca’s attraction toward humid flowers, we find that floral humidity is not a reliable cue for nectar presence in this system. While Datura floral headspace sustains an enormous humidity gradient, it is not a consequence of nectar evaporation, but an outcome of gas exchange through floral stomata and is decoupled from nectar presence. Using interdisciplinary tools, we demonstrate the function of floral humidity as an attractive signal, not a cue, in this pollination system, thus showcasing an underappreciated modality by which flowers may manipulate their visitors.