Luna Moths’ Gorgeous Wings Throw Off Bat Attacks

Spinning twin tails at the end of moth wings garble bats’ sonar cries, causing the winged predators to miss the tasty mark

Luna moths - arguably the most spectacular moths in North America - deflect bat attacks with their ornate wing tails. Photo: Robert Marien/Corbis

Animals have evolved countless ways to avoid being eaten, ranging from impeccable camouflage to deadly venom to fortress-like shells. Some even adopt a seemingly desperate, last-ditch method: distract predators into attacking a non-essential part of the body. Gaudy eyespots on butterflies and fish encourage predators to strike at the periphery of wings or fins, while some lizard’s bright tails can break off in a confused bird’s mouth. These tricks buy prey precious time to escape their attacker.

All of these tactics, however, rely on visual deception, so it would seem that predators using other sensory information would be immune to such strategies. Insect-eating bats, for one, rely on echolocation—sonar cries that bounce off objects—to locate and capture flying prey. Now, however, scientists have found that even echolocation can be fooled by expendable frills.

Luna moths—the grandiose fairy queens of the North American Lepidoptera ball—can use their tails to divert bats’ attention away from their juicy, delicate bodies. When luna moths fly, two long frills on the end of their chartreuse wings spin. According to research published today in Proceedings of the National Academy of Sciences, that elegant display can muddle bats' sonar and thwart a deadly attack.  

Biologists from several U.S. institutions performed experiments with around 200 unlucky luna moths. The researchers clipped off half of the moths’ wing tails but left the others intact. They also threw in a few snout moths—drab brown insects lacking anything like the luna moths’ pompous tails—to serve as controls. The team used fishing line to tether the insects within an enclosed area where they kept eight hungry brown bats. High-speed infrared cameras and ultrasonic recorders were set up to record the ensuing moth massacres.

The team found that the luna moths that kept their tails enjoyed nearly a 50 percent higher survival rate that those whose tails had been clipped. Moreover, when bats attacked the intact luna moths, they dive-bombed the insects’ wing tails 55 percent of the time. All told, bats snagged luna moths with tails only around 35 percent of the time. Luna moths without tails fell victim around 50 percent of the time, while snout moths were eaten nearly 100 percent of the time. A moth without tails, the scientists calculated, is nearly nine times as likely to get snagged by a bat than one with the appendages.

Separate experiments showed that luna moths can fly perfectly fine without their tails, eliminating the possibility that the tails actually evolved to help with aerodynamics. Taken together, these findings convinced the researchers that the luna moth’s wing tails do indeed create a sonar diversion for the bats.

This made them wonder: Shouldn’t the tactic work for other species, too? To find out, they measured the tail lengths of more than 100 other related species of moths. They analyzed the moths’ evolutionary relationships with each other and found that long wing tails likely evolved four independent times, further bolstering the conclusion that wing tails serve a distinct advantage for moths looking to avoid becoming bat dinner.

Other interesting yet unnoticed anti-predator strategies are likely awaiting discovery, the team points out. As with bat sonar, the secret to uncovering those tricks of nature could be to start with the predator’s choice of sensory input—whether tactile, auditory or olfactory—and work backwards from there.   

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