Some Moths Turn Their Genitals Into Ultrasound Weapons Against Bats
Male hawkmoths rub scales on their genitals against their abdomens to make ultrasound at bats, and the same trick has evolved again and again.
Some male hawkmoths have turned their genitals into instruments that fire bursts of sound at hunting bats. The surprising part is that this trick was not invented once: it has appeared again and again, in separate branches of the moth family, as many as seven times over the past 30 million years.
No voice box, so they rub two parts together
Moths do not have larynxes, the voice box that people use to speak, so they cannot simply shout. Instead they use a trick called stridulation: making sound by rubbing two body parts together. Crickets do it with their wings, and some spiders do it by rubbing their legs against each other. Male hawkmoths do it with their genitals.
The sound comes from special scales, flat structures a little like tiny tiles, sitting on a pair of genital parts called valvae. By moving those parts against modified surfaces on the abdomen, the moth produces short bursts of ultrasound: sound pitched too high for human ears to hear. Slowed down until they become audible, the bursts sound like fast clicks, buzzes or trills.
Three different instruments, three different sounds
Paul Masonick of the Florida Museum of Natural History and his colleagues took detailed images of the genital structures of male hawkmoths across the family and looked for the surfaces doing the work. They found at least three separate mechanisms.
- In some species, the scales scrape against other scales on the abdomen.
- In others, they rub against tiny spines.
- In a third group, they scrape across small bumps called nodules.
Each version makes its own kind of noise. Scale-on-scale moths tend to produce shorter, more broken-up bursts. Scale-on-spine moths produce longer, more sustained emissions. The scale-on-nodule group has the most varied repertoire of all: even closely related species sound noticeably different from one another.
Seven separate inventions, and not all of them work
When the researchers mapped the sound-making structures and behaviors onto the hawkmoth family tree, they found the ability had not been inherited from one acoustically gifted ancestor. Instead it appears to have evolved independently as many as seven times.
Why so often? Bats hunt flying insects in the dark using echolocation, sending out calls and listening for the echoes that bounce back off their prey. A burst of ultrasound from a moth has the potential to confuse that system, which is a strong reason for the trick to keep reappearing. Predator and prey pushing each other like this is one of the most powerful drivers of evolution, and the relationship between bats and moths is a clear example.
But the team also measured the recordings, noting how long each burst lasted, its frequency, and how much time the moth spent making noise, then compared those measurements with what earlier experiments had shown about the kinds of sound that interfere with bat echolocation. Not every moth song was up to the job. The longer, more sustained sounds were the ones most likely to have the qualities that disrupt bat sonar. The shorter bursts may still have their uses: some hawkmoths have hearing organs that pick up the calls of an approaching bat, giving them time to answer with a barrage of noise of their own.
The moths in the study all belong to the hawkmoth family, a group that includes familiar species such as the oleander hawkmoth. Whatever the exact purpose of each version of the sound, the pattern is striking: a single awkward-looking solution to a life-or-death problem, arrived at over and over, in different ways, in different places on the same family tree.