Whales can boom their songs across thousands of kilometers of ocean, and elephants' low-frequency calls can be heard by other pachyderms several kilometers away. But when body size is taken into consideration, these mammoth mammals produce but a relative whisper compared with other animals—especially one odd arthropod.

The water boatman (Micronecta scholtzi), a 2.3-millimeter-long insect, has been named the world's loudest-known animal. The males' mating call, produced underwater, can reach 100 decibels—about the same level of a motorcycle—according to new findings, being presented July 2 at the Society for Experimental Biology Annual Conference in Glasgow.

The amorous water boatman's song can even reach human ears on land. "Remarkably, even though 99 percent of sound is lost when transferring from water to air, 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," James Windmill, a collaborator on the new work, said in a prepared statement. He works at the University of Strathclyde's Center for Ultrasonic Engineering in Glasgow.

Recording of water boatman mating call:

The water boatman specimens used for the study were collected from a river in Paris and a pond in Morsang-sur-Orge, France. The researchers recorded the sound-pressure levels (at a distance of between 0.5 and 13 centimeters) generated by each of 13 males and then compared the volume with that of recorded communication calls for 227 other animal species (which included amphibians, arthropods, birds, fishes, mammals and reptiles)—17 of which had estimates for underwater levels. The next loudest for its size? The snapping shrimp (Synalpheus parneomeris). And according to the analysis, the surprisingly quiet species for its size was the praying mantis (Mantis religiosa), which is about 60 millimeters long and calls at about 43 decibels.

Scientists speculate that most animals evolve to keep their songs down to a dull roar so as to not attract attention from predators but to remain loud enough for potential mates to locate them. Windmill and his colleagues suggest that the boatman might just be quick enough to escape any "auditory predators," or it might not have any at all.

The monsieurs boatman make their song by rubbing their right paramere's pars stridens (a grasping organ used for mating) against the eighth segment in their abdomen. This romantic instrument does not seem to be magnified by any internal resonating organs, so biologists and engineers are puzzling over just how this tiny guy manages to make such big noise. Even smaller is the contact area between the pars stridens and abdomen section, which is about 50 micrometers wide. "We really don't know how they make such a loud sound using such a small area," Windmill says.

As the researchers point out, making noise under water is quite a bit easier than it is in the open air, but the little bug might be making use of the interplay between the two environments. The freshwater bug keeps a pocket of air along its front side with so-called hydrofuge hairs. "This could induce a complex micro acoustic environment with reflections and refraction due to impedance differences between air and water," Windmill and colleagues wrote in a June paper in PLoS ONE.

Finding out how this minuscule noisemaker works up his sound might help biologists listen for and record other insects to keep auditory tabs on biodiversity, which is especially helpful when tracking aquatic animals. And, Windmill added, engineers might be able to put the information to work to improve human-made acoustics and sonar.

Image of a water boatman courtesy of Jerome Sueur