'Singing penis' sets noise record for water insect

Micronecta scholtzi (c)  Jerome Sueur Tiny bugs make huge sounds with a surprising organ

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A tiny water boatman is the loudest animal on Earth relative to its body size, a study has revealed.

Scientists from France and Scotland recorded the aquatic animal "singing" at up to 99.2 decibels, the equivalent of listening to a loud orchestra play while sitting in the front row.

The insect makes the sound by rubbing its penis against its abdomen in a process known as "stridulation".

Researchers say the song is a courtship display performed to attract a mate.

Micronecta scholtzi are freshwater insects measuring just 2mm that are common across Europe.

In a study published in the journal PLoS One, the scientists discovered that the small animals make a mighty sound.

The team of biologists and engineering experts recorded the insects using specialist underwater microphones.

On average, the songs of M. scholtzi reached 78.9 decibels, comparable to a passing freight train.

"We were very surprised. We first thought that the sound was coming from larger aquatic species such as a Sigara species [of] lesser water boatmen," said engineering expert Dr James Windmill from the University of Strathclyde, Glasgow.

"When we identified without any doubt the sound source, we spent a lot of time making absolutely sure that our recordings of the sounds were calibrated correctly."

STRIDULATION FACTS

  • Stridulation is the act of rubbing two body parts together to produce a sound
  • Resonating sounds are made when a ridge is rubbed across a finely ridged surface
  • It is most commonly associated with grasshoppers and crickets but some beetles, bugs and even spiders are known to chirp, chirrup and hiss in this way
  • Only one species of mammal, the streaked tenrec, is known to stridulate by rubbing its quills together

Dr Windmill explained that the reason the insects don't deafen us is down to the bug's underwater lifestyle.

Although 99% of the sound is lost when transferring from water to air, the songs were still loud enough to be audible to the human ear.

"The song is so loud that a person walking along the bank can actually hear these tiny creatures singing from the bottom of the river," said Dr Windmill.

The majority of the loudest animals on Earth are also the biggest, with blue whale songs reaching 188 dB and elephants' rumbling calls measuring 117 dB.

Although remarkable acoustic signals are made by a range of invertebrates, including the miniature cricket and preying mantis, and by large mammals, none compare to M. scholtzi once body size is taken into account.

"If you scale the sound level they produce against their body size, Micronecta scholtzi are the loudest animals on Earth," said Dr Windmill.

Researchers believe that sexual selection could be the reason why the insects' songs reach such high amplitude.

"We assume that this could be the result of a runaway selection," biologist and co-author Dr Jerome Sueur from the Museum of Natural History, Paris, told the BBC.

"Males try to compete to have access to females and then try to produce a song as loud as possible potentially scrambling the song of competitors."

Dr Sueur explained that the competition could have exaggerated the volume of males' songs over time.

In many insects, the song volume is limited because predators would hear them, but observations suggest that M. scholtzi lack auditory predators.

Modifications

To produce the intense sound, the water boatmen "stridulate" by rubbing a ridge on their penis across the ridged surface of their abdomen.

"There is at least another one insect producing sound with its genitalia. This is a pyrallid moth, Syntonarcha iriastis, that uses highly modified genitalia to produce ultrasonic signals," explained Dr Sueur.

Micronecta scholtzi (c)  Jerome Sueur The tiny bugs belong to a family known in the UK as lesser water boatmen

"Insects seem to be able to use any part of their body to generate sound. Some of them use their wings, others their legs, abdomen, head, wings, thorax etcetera."

What makes M. scholtzi extraordinary is that the area they use to create sound only measures about 50 micrometres across, roughly the width of a human hair.

"We really don't know how they make such a loud sound using such a small area," said Dr Windmill.

Without any obvious adaptations to amplify the sound, the question of how the animals physically make such a loud call remains a mystery.

"These very small bugs create sound at very high level, and it could be very useful for future ultrasonic systems to learn how they do that," said Dr Windmill.

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