Floral humidity as a reliable sensory cue for profitability assessment by nectar-foraging hawkmoths

Abstract

Most research on plant-pollinator communication has focused on sensory and behavioral responses to relatively static cues. Floral rewards such as nectar, however, are dynamic, and foraging animals will increase their energetic profit if they can make use of floral cues that more accurately indicate nectar availability. Here we document such a cue-transient humidity gradients-using the night blooming flowers of Oenothera cespitosa (Onagraceae). The headspace of newly opened flowers reaches levels of about 4% above ambient relative humidity due to additive evapotranspirational water loss through petals and water-saturated air from the nectar tube. Floral humidity plumes differ from ambient levels only during the first 30 min after anthesis (before nectar is depleted in wild populations), whereas other floral traits (scent, shape, and color) persist for 12-24 h. Manipulative experiments indicated that floral humidity gradients are mechanistically linked to nectar volume and therefore contain information about energy rewards to floral visitors. Behavioral assays with Hyles lineata (Sphingidae) and artificial flowers with appropriate humidity gradients suggest that these hawkmoth pollinators distinguish between subtle differences in relative humidity when other floral cues are held constant. Moths consistently approached and probed flowers with elevated humidity over those with ambient humidity levels. Because floral humidity gradients are largely produced by the evaporation of nectar itself, they represent condition-informative cues that facilitate remote sensing of floral profitability by discriminating foragers. In a xeric environment, this level of honest communication should be adaptive when plant reproductive success is pollinator limited, due to intense competition for the attention of a specialized pollinator.pollination biology | honest signaling | floral display P lant-pollinator relationships-like all mutualisms-strike a tenuous balance between the conflicting interests of foraging animals and flowering plants (1, 2). For flowering plants, pollinator attraction through floral density, form, color, pattern, and odor is necessary but insufficient to ensure effective pollination. Ideally, pollinators should transfer pollen between conspecific plants in ways that result in favorable levels of outcrossing (3) and reduced pollen loss (4). Flowers commonly manipulate pollinator movement by varying quality or quantity of floral nectar. The offer of variable amounts of nectar (5, 6), toxic nectar (7, 8), or an empty promise of nectar elicit similar results: pollinators leave plants sooner, reducing for example geitonogamous inbreeding (if selfcompatible) or pollen discounting (if self-incompatible).The availability of floral rewards often coincides with floral display and pollinator activity on a gross temporal scale (i.e., hours) (9, 10). Nevertheless, most pollinators encounter rewardless ("empty") flowers on a finer temporal scale (i.e., minutes), often due to recent visitation by another animal. From the pollin...

Intraspecific Combinations of Flower and Leaf Volatiles Act Together in Attracting Hawkmoth Pollinators

“…It has been shown that plumes emitted by flowers contain increased humidity [25] and CO 2 [26] and that both cues can be used by moths to pinpoint the flower. However, this information appears to be relevant only in specific context since behavioral responses to D. wrightii flowers can be mimicked by widely reduced synthetic blends devoid of humidity or CO 2 .…”

Honeybees prefer warmer nectar and less viscous nectar, regardless of sugar concentration

“…Floral warmth may act as a cue for pollinators (signalling improved nectar rewards) as well as a reward [39]. Other cues recently shown to be used by hawkmoths are CO 2 [40,41] and relative humidity [42]: like floral warmth, these will be effective only under certain environmental conditions.…”

Hawkmoth flight stability in turbulent vortex streets

“…Environmental flows could affect moths in other ways. Moths use visual and olfactory clues to discern between flowers of different nectar content (Raguso and Willis, 2005;von Arx et al, 2012), and this capability can also be influenced by airflow turbulence. Field observations are required to determine the importance of these factors to wild moths and in general for nectar feeders.…”