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 wee
|Contact: Daniel Stolte|
University of Arizona