Moths mimic each others' sounds to fool hungry bats

Impressionists are a mainstay of British comedy, with the likes of Rory Bremner and Alistair MacGowan uncannily mimicking the voices of celebrities and politicians. Now, biologists have found that tiger moths impersonate each other too, and they do so to avoid the jaws of bats.

Some creatures like starlings and lyrebirds are accomplished impersonators but until now, we only had anecdotal evidence that animals mimic each others' sounds for defence. Some harmless droneflies may sound like stinging honeybees, while burrowing owls deter predators from their burrows by mimicking the distinctive warning noises of deadly rattlesnakes.

In tiger moths, Jesse Barber and William Conner from Wake Forest University, North Carolina, have found the first hard evidence of acoustic mimicry in animals. Tiger moths are hunted by bats, which use ultrasonic clicks - echolocation - to home in for the kill.

Moths are tuned into the sounds of these clicks and respond with their own ultrasonic sounds, created by vibrating special membranes called 'tymbals' on their abdomens (see a Quicktime video of the tymbals in action). The sounds are multi-purpose - they may startle the bats, or jam their transmissions. But according to Barber and Conner, they also carry a message - they say "Don't eat me, I won't taste very nice."

The duo worked with two species of bats - red bats (Lasiurus borealis), which eat butterflies and moths, and big brown bats (Epstesicus fuscus), which dine mostly on beetles but will take the occasional moth. They raised bats in the lab and trained them to hunt live but tethered moths (right), while using high-speed video cameras to capture the split seconds of the attack (Quicktime video). They also recorded the various ultrasonic clicks used by both moths and bats.

Over five nights, Barber and Conner presented the bats with one of two species of foul-tasting tiger moths, the dogbane tiger moth (Cycnia tenera) and the polka-dot wasp moth (Syntomeida epilais). The two species look very different, but they both absorb poisons from the plants they ate as caterpillars. All the bats learned to avoid the first species, and when they were shown the other, they avoided it too even though they had never seen it before (Quicktime video).

But they had heard it, or at least, a moth that sounded very much like it. To show that this was the case, Barber and Conner silenced the second species of moth by surgically removing their tymbals. The bats eagerly attacked the now-muted moths, although they quickly spat them out again in distaste.

This was clear evidence of a type of impersonation called Mullerian mimicry, where two or more distasteful or dangerous creatures provide predators with the same warning. But the tiger moths also show Batesian mimicry, where a delicious and defenceless animal pretends to be a more noxious one.

Barber and Conner repeated their experiment and trained the bats on the unpalatable dogsbane moth. But then, they offered them the milkweed tiger moth (Euchates egle), a moth without any chemical defences. It would be a tasty treat for a bat but that didn't matter - the bats were put off by its pretense and avoided it like the others.

However, some bats did wise up. Red bats specialise in catching butterflies and moths, and three of them started to attack the vulnerable milkweed tiger. After a day, they had learned that these charlatans were in fact quite edible (Quicktime video). When they were offered the unpalatable dogsbane tiger again, they avoided it (unless its tymbals were removed). They had quickly learned to tell the difference between the sounds of the impostors and the real deals.

The moths' charades are likely to be a common strategy. There are, after all, 11,000 species of tiger moths worldwide, many of which live in overlapping geographical ranges. Other insects too, including hawkmoths and tiger beetles, respond to approaching bats with ultrasonic clicks. For all we know, the night skies around us are full of the cacophony of impostors and impressionists.

Reference: Barber & Conner. 2007. Acoustic mimicry in a predator-prey interaction. PNAS 104: 9331-9334.

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