These insects produce smelly tusks that must be heard to be believed


Karen hopkin: It’s Science in 60 Seconds from Scientific American. I am Karen Hopkin.

Hopkin: Some sounds are scary [ghostly yowl?]. Some are rude [screechy blackboard? Vinyl album scratch?]. And some are totally unsettling. [creaking door? Scream? Discordant psycho-shower-scene type music?].

But some sounds… some sounds… are unlike anything you’ve ever heard before… and nothing you would never want to hear again.

[Sound of 2-hydroxybenzaldehyde]

It was the sound of 2-hydroxybenzaldehyde. And if you think about it, wait, the molecules don’t make a sound… well, you’re right. But that jarring nightmare was an audible soundscape that represents the chemical properties of 2-hydroxybenzaldehyde.

What’s even stranger is that while that sound might have made you want to crawl out of your skin and head for the door, the chemical itself has a similar effect on ants. It depends a study in the journal Patterns.

Jean-Luc Boevé: The insect world is full of chemical compounds.

Hopkin: Jean-Luc Boeve of the Royal Belgian Institute of Natural Sciences in Brussels.

Boevé: Thomas eisner… Who created the field of chemical ecology… said that insects are… the best chemists on earth. And he said this as a kind of joke, but he was quite true in saying this because insects produce large amounts of different chemicals for different purposes.

Hopkin: Including ensuring their safety and that of their families.

Boevé: Many other groups of insects produce volatiles and other compounds to defend themselves against predators.

Hopkin: Like an irresistible scent, these volatile secretions float in the air and irritate creatures who might think of snooping on the insects that produced them. Boevé, in particular, studies the larvae of sawfly species, which produce different cocktails of chemicals that act as repellents, especially against ants.

But Boevé is not only an entomologist. He is also an amateur musician. And he started to think, well, smells send a signal drifting through the air… and so do sounds.

Boevé: I thought it would be quite interesting to explore this parallel between perception via two different sensory systems, namely smell and hearing. The idea was therefore to convert these birds into sounds. Then… to compare, on the one hand, the predators reacting against the birds with, on the other hand, the humans who hear the sounds which represent these birds.

Hopkin: The first step was to transform the aroma into audio. To do this, Boevé and his colleague Rudi giot of the Institut Supérieur Industriel de Bruxelles turned to a process called sonification, which translates chemical parameters into sound.

Boevé: The chemical parameters that we used for example were the molecular weight of the compound, or whether or not the compound has functional groups. By functional group I mean an alcohol group or a ketone group or an aldehyde group or an acid group.

Hopkin: These molecular properties were then associated with musical qualities, such as pitch and tone, duration and timbre, and even reverberation.

Boevé: In doing so, we have built a library of sounds of molecules obtained by converting each molecule into a single sound.

Hopkin: So acetic acid… basically a concentrated vinegar… it looks like:

[Acetic acid sounds]

… while geranial, an isomer of citral, which is a major component of the oil in the peel of a citrus fruit, looks more like:

[Geranial sounds]

Hopkin: Boevé is not the first to use sonication to convert chemical data into audio waves. As early as the 1970s, geneticists were turning the four letters of DNA sequences into melodies that, well, weren’t exactly at the top of the charts.

Boevé: The sounds were… not very beautiful, not very rich. Because if you only have four tones, then the music or sound you hear is… very monotonous.

Hopkin: The irritating insects were much more interesting… because each species produces its own characteristic chemical mixture. Boevé and Giot mimicked these molecular mixtures by taking the individual chemical sounds and mixing them on a soundboard… using different volumes to represent the concentrations of compounds in each species’ toxic concoction.

[Locust sawfly larva sounds]

Hopkin: This is the chemical style of Nematus tibialis, a locust sawfly larva. Which is heavy on dolichodial, an essential oil that some plants use as an insect repellant.

[Dolichodiol sounds]

And that pungent melody helps keep Hoplocampa testudinea, the European apple sawfly, from being eaten.

[European apple sawfly sounds]

Hopkin: But that’s just the configuration. Then came the experience. Boevé exposed the ants to actual chemicals… individually or in mixtures… and recorded how repelled the predators were by each. And for sonicated sounds… volunteers played clips… of single molecules or mixtures… and listened to sounds from a pair of loudspeakers.

Boevé: Then we asked them to back up, to walk backwards until they were in a comfort zone. And I measured, I noted the distance they had traveled backwards.

Hopkin: And he found that the most troublesome molecules and mixtures for ants were the same ones that, when sonicated, made volunteers recoil.

Boevé: And many told me that some of the sounds were quite scary and that’s why they backed off.

Hopkin: But Boevé is not there to scare people. The correlation between the effect of a chemical on ants… and the effect of a sound on humans… means that it can use sonication to study the defensive odors of new species… or species for which it might be difficult to scare off a specimen.

Hopkin: To scare? To find?

[Blood-curdling scream]

Hopkin: For Science in 60 Seconds from Scientific American, I’m Karen Hopkin.

[Mix of insect defense sounds]

[The above text is a transcript of this podcast.]


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