Until about 140 million years ago, dinosaurs had been munching their way through a uniformly green plant world. What happened then is one of evolution's greatest success stories, heralding a new kind of ecological relationship that would transform the planet: The first flowers appeared, competing for the attention of animals to visit them and distribute their pollen to other flowers to ensure the plant's propagation.

The myriad of ways in which flowers attract pollinators have been studied since the beginning of biology, and few ecological relationships between organisms are as well understood as those between plants and their pollinators.

Despite decades of research, a team led by Martin von Arx, a postdoctoral fellow in the lab of Goggy Davidowitz in the University of Arizona department of entomology, now has discovered a previously unknown sensory channel that is used in plant-animal interactions.

The white-lined sphinx (Hyles lineata), the most common species of hawkmoth in North America, can detect minuscule differences in humidity when hovering near a flower that tells it if there is enough nectar inside to warrant a visit. 

The findings constitute the first documented case of a pollinator using humidity as a direct cue in its foraging behavior and are published in the journal Proceedings of the National Academy of Sciences.

The study, "Floral humidity as a reliable sensory cue for profitability assessment by nectar-foraging hawkmoths," is co-authored by Davidowitz and Joaquín Goyret and Robert Raguso at the department of neurobiology and behavior at Cornell University in Ithaca, New York, where the work was carried out.

"Traditionally, most research on plant-pollinator interactions has focused on static cues like floral scent, color or shape," von Arx said. "All this time, evaporation from nectar was right under our noses, but few people ever looked. We were able to show that the insects actually perceive this cue, and it allows them to directly assess the reward that they might get from the flower."

Unlike previously recognized cues used by pollinators such as flower size, shape or color, which don't necessarily reveal anything about the actual nectar levels waiting inside, the humidity evaporating from the flower's nectar provides an "honest" signal to a potential visitor. Scent, for example, is independent of nectar, which is odorless in most plants, whereas the fragrance usually is produced by the petals.

"We were always intrigued by this question," von Arx said. "Given that the known cues like flower shape and color are independent of the abundance of nectar, we were wondering if there is some other cue the insects might use. You would expect natural selection to favor an ability to sense a cue that is directly linked to the nectar reward."

To a hawkmoth setting out at dusk to search for nectar-bearing flowers of one of its favorite plants, the tufted evening primrose (Oenothera cespitosa), being able to quickly tell whether a flower is worth visiting, can make the difference between life and death.

Hovering in front of a flower while probing it with its long proboscis – the moth's "tongue" – is one of the most energetically costly modes of flight, von Arx explained. And once the insect plunges its head deep inside to reach all of the nectar, it is very vulnerable to predators such as bats.

"The metabolic cost of hovering in hawkmoths is more than 100 times that of a moth at rest," said Davidowitz. "This is the most costly mode of locomotion ever measured. An individual hawkmoth may spend 5-10 seconds evaluating whether a flower has nectar, multiply that by hundreds of flowers visited a night, and the moth is expending a huge amount of energy searching for nectar that may not be there. The energy saved by avoiding such behavior can go into making more eggs. For a moth that lives only about a week, that is a very big deal."