You’ve probably never heard of the greater wax moth. But, according to a paper published in last week’s issue of Biology Letters, they hear just about everything — even sounds that don’t exists…as far as we know. These rather drab-looking insects have the highest hearing frequency range, 30 kilohertz (kHz) to 300 kHz, in the animal kingdom.
Greater wax moths (Galleria mellonella. Believe it or not there’s actually a lesser wax moth out there, too) commonly mooch off bee hives, much to the chagrin of apiarists. Humans spread them around the globe by moving bee hives from one place to another, so while their native range probably encompassed Europe and parts of Asia, they’re invasive pests in Australia and North America. Adult wax moths leave hives for one reason: to mate. At night, the males emit mating calls — at 90 to 95 kHz — to attract females. Their larva — yellowish-green caterpillars called waxworms — inhabit hives around the world and feed off the beeswax, usually from the combs where bee larvae live. Obviously, this doesn’t end well for the bees, and hives can quickly die off once the moths settle in.
Greater wax moths may be a pest, but their mating calls make them a target and a scrumptious food source for bats. While stalking their prey via echolocation, bats also make higher pitched calls to communicate without drawing the attention of an unsuspecting moth. Up until now, the North American gypsy moth (Lymantria dispar, another pest) had the highest hearing frequency limit of any insect: 150 kHz. In contrast, bat echolocation reaches up to 212 kHz. Scientists have always supposed moths’ limits to be much lower, and a group of researchers at University of Strathclyde in Glasgow, Scotland, wondered, “are any moths keeping up in the evolutionary arms-race?”
Because greater wax moths live all over the world, they hear lots of different bat calls, making them an ideal test subject. Great wax moths hear through tiny tympanal membranes (and by tiny I mean smaller than a millimeter), which vibrate when they pick up sounds, and activate the four nearby receptor cells that transmit a nerve signal to the brain. The set up is simple, but comparable to how a human ear drum works. The research team ordered greater wax moth larvae from a website that specializes in “live” food for exotic animals, and waited for the larvae to cocoon, pupate, and emerge as adults from their lab incubator. Suspending 20 adult moths so they couldn’t fly off, the researchers measured how much their tympanal membranes vibrated and whether they transmitted a signal when they heard sounds from 30 kHz to 300 kHz. At 300 kHz, all the moths “ear drums” vibrated, and 15 of the 20 specimen showed neural activity.
“Such extreme auditory frequency sensitivity is unmatched in the animal kingdom,” the researchers say. By comparison, humans hear between .02 and 20 kHz, and dolphins, another animal that relies on echolocation, can hear up 160 kHz. It’s safe to say greater wax moths are worthy opponents to bats trying to swoop in and eat them for dinner.
No animal known to scientists can make a sound as high as 300 kHz. So, why did these moths evolve such ultrasonic hearing?
In New Scientist’s write-up of the study, Hannah Moir, a member of the research team who specializes in bioaccoustics, posits two possible explanations: 1. Bats are actually making higher frequency sounds than we can record (microphones have trouble detecting sounds over 150 kHz). 2. It’s an accident. They needed to pick up high frequency bat calls, and this is just a side perk of that adaptation. Either way, f bats feel selection pressure to expand their range, these moths are already one step ahead.
Evolutionary mysteries aside, the research actually has some practical implications. Moir’s collaborator James Windmill hopes to use what they learned from the greater wax moth to develop new ultrasonic tech — think itty bitty microphones. I personally see inspiration for a Saturday morning cartoon here. If comics can feature ticks and cockroaches saving the world, a moth with ultrasonic hearing at least seems more plausible. Perhaps as Spiderman’s slightly less exciting cousin?