A Blog by Ed Yong

Tiny water insect makes record-breaking song with his penis




There are animals that make far louder calls. The record goes to the sperm whale, which can create clicks of around 236 decibels underwater (equivalent to 170 decibels on land). Other animals, including elephants, hippos and dolphins can produce louder calls than Micronecta.

But pound for pound, there is no competition. All of these animals are very big, and it stands to reason that large objects can produce louder sounds – think about the difference between a concert amp and a set of headphones. The sperm whale, for example, grows up to 16 metres in length and weighs up to 14 tonnes. Micronecta, on the other hand, produces its phenomenal song with a body that’s no bigger than one of these letters. Sueur compared the ratio of call intensity to body size for 227 different animals, from whales to insects, and found that the water boatmen out-sang them all.

How does such a tiny insect make such a loud noise? It’s not clear. It seems to do so by rubbing its ribbed penis against ridges on its belly, playing its genitals like a miniature fiddler. But the “bow” here is just 50 micrometres long, and there are no obvious body parts to amplify the noise.

But maybe the amplifier isn’t a body part at all. Like other water boatmen, Micronecta traps a layer of air around its body using microscopic hairs. This layer helps it to breathe, but Sueur speculates that it could also act as an echo-chamber, reflecting the sound of the penis-fiddling again and again. The details, however, are a mystery. As Sueur writes, “To observe the micro-mechanics of such a small system remains a significant challenge.”

Sueur also has an idea about why the water boatman’s song is so loud. He compares the song to the complex melodies of birds or the long antlers of deer – it’s a sexual signal that indicates a strong, powerful mate. If females prefer loud males over quiet ones, the male’s song would become exaggerated over time.

There are some obvious ways of testing this. If Sueur is right, females should prefer louder males, which should be easy to test with speakers and some recordings. Sueur also thinks that M.scholtzi probably has no predators that hunt by sound – otherwise, such hunters would limit the evolution of an extreme song by snatching up the loudest males. We know nothing about what eats M.scholtzi and Sueur plans on finding out.

Reference: Sueur, Mackie and Windmill. 2011. So Small, So Loud: Extremely High Sound Pressure Level from a Pygmy Aquatic Insect (Corixidae, Micronectinae). PLoS ONE http://dx.doi.org/10.1371/journal.pone.0021089

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