May 9, 2016

How moths trick bats with their own sonar

by Megan Cartwright

If you’re a big, delicious insect like the luna moth, how can you escape a predator like the big brown bat?

You can’t outfly bats. Your beautiful, light-green wings span up to 4.5 inches, but they can’t flap fast enough. Big brown bats can fly up to 40 mph as they chase after insects.

You can’t trick bats with eyespots. Those colored markings on the less-essential body parts of butterflies can distract birds, which use sight to locate prey. Unlike birds, bats hunt using sonar—high-pitched squeaks that echo off objects, allowing the bats to echolocate prey.

But you can still deceive bats: The trick lies in those long tails fluttering at the end of each light-green wing. According to a recent conference abstract and paper by naturalists from Boise State University, those tails are a graceful example of the bloody evolutionary arms race that has lasted millions of years between moths and bats.

Naturalist Jesse Barber and his Idaho team have been studying how large moths deceive big brown bats. Using high-speed infrared cameras and sonar recorders, the team recorded bat vs. moth encounters inside a dark, insulated room. By comparing moths with intact tails to moths with snipped-off tails, the team found that bats caught only 35% of the moths with tails, compared to 81% of the tail-less moths.

A female luna moth. (Credit: Kugamazog, Wikimedia Commons)

The difference wasn’t because the tail-less moths were clumsier. When the team analyzed their sonar recordings, they found that the tails whirled in circles that bounced back a bat’s echolocating pings. The whirling tails created a sonar pattern that resembled the pattern made by wingbeats, tricking the bats to aim for the non-essential tails instead of the moth’s body.

Interestingly, there’s another way moths can deceive bats. The Idaho team found that another moth group, the hawkmoths, can jam bat sonar by using a vibrating structure at the tip of their abdomen. The result, Barber describes in a video interview with Boise State Explorer, is that “as the bats approach the [moth’s] rear end, they blast sound out and jam their ability to localize with sonar.”

These two strategies are so effective, they’ve evolved independently in multiple moth species. By examining the fossil record, Barber found that after insect-eating bats started to become common around 20 million years ago, at least two hawkmoth species evolved the abdomen structure to jam bat sonar. When he compared the tail lengths of 113 moth species in the same group as luna moths, he figured that the tail trick had evolved at least four separate times.

But why study this evolutionary arms race between moths and bats in the first place?

For one thing, Barber loves “detailing how the world works.” But he also fears that the chance may soon be lost to describe the hawkmoths’ vibrating structure or the luna moth’s graceful, fluttering tails. “I personally feel somewhat of an obligation to describe these patterns,” he tells Boise State Explorer, “because species extinction is happening so fast, we have this narrow window to describe these transitions.”

To learn more about evolution, please check out our module on adaptation.

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Written by

Megan Cartwright is a freelance science/medical writer near Seattle. Before becoming a full-time writer, she worked as a scientist studying infectious diseases and vaccines, and earned her Ph.D. in Toxicology from the University of Washington. Megan has written for Slate and Bitesize Bio, and helps write blog posts and learning modules on chemistry for Visionlearning.

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