Flashing neon diner signs often attract the attention of famished human drivers at night, but in the wild, the message sent by flashing fireflies is more like "eat me and die."
Fireflies are often toxic to bats, which see the nighttime flashing and steer clear of the insects, according to a new study accepted for publication in the journal Animal Behavior.
The research presents some of the first evidence that fireflies flash not only for courtship, but also to deter would-be predators.
"I believe it is quite possible that bats would attempt to eat fireflies, especially if the firefly was not flashing," lead author Paul Moosman, Jr., told Discovery News.
Moosman, an assistant professor of biology at the Virginia Military Institute, explained that bat echolocation is an impressive navigating and foraging system, but it "is an imperfect mechanism for identifying prey." Firefly flashes therefore benefit bats, which may, in turn, help to maintain the bug's bioluminescence.
For the study, which was funded by the National Science Foundation, Moosman and his team collected insect-eating bats, including the common bat Eptesicus fuscus, from New Hampshire and Massachusetts sites. Removal of the bats' fecal pellets revealed most ate very few fireflies. Their favorite food was beetles, comprising 80 percent of the common bat's diet.
The researchers then fed the bats mealworms, some of which were coated with homogenized fireflies. The majority of the bats "coughed" and "showed head shaking and snout wiping" when they made oral contact with the firefly-coated worms. Other studies suggest bats will gag and vomit if they accidentally swallow a firefly. At least one species of lizard has been documented as dying after eating fireflies.
"Photinus fireflies possess toxic compounds called lucibufagins," explained Moosman, who added that these compounds aren't involved in flash production. "A genus of larger fireflies, Photuris, actually attempt to eat Photinus, probably to acquire their lucibufagins and impart them into their eggs."
He believes bioluminescence may have evolved in firefly larvae as a warning signal. Bats then "may have led to, or helped to maintain, bioluminescent courtship," in adult fireflies, he said, because it is now known that their flashing also likely functions as a warning signal.
In another phase of their study, the scientists presented caged bats with moving, sometimes flashing, lures. Whenever the common bat test subjects saw flashes, they usually did not attack the lures.
Field observations suggest fireflies will emit "a burst of flashes" when a predator comes near.
Moosman explained, "Bats don't always catch insects directly in their mouths — they often catch them with their wing or tail membrane, then scoop them into their mouth."
"They also sometimes have to make more than one pass to catch an insect," he added. "So there may be plenty of opportunity for fireflies to respond to a bat once they've felt a physical disturbance, but before being chewed up."
Raphael De Cock, a firefly expert at the University of Antwerp in Belgium, told Discovery News that he agrees with the new findings.
De Cock said it's possible that other airborne potential firefly predators, such as nightjars, might benefit from firefly flash warnings.
He added, "When the fireflies land on vegetation, other nocturnal predators (could benefit) as well, such as geckos, amphibians and insectivorous mammals."