Alert readers Diane G. and Ray P. called my attention to a new paper in PLoS ONE by Soeur et al. that reveals the loudest known creature for its size on Earth.
What is it? It’s a water boatman of the species Micronecta scholtzi. These are in the Hemiptera (“true bugs,” remember?). It’s tiny—only 2 mm long (the size of Lou Jost’s orchid)—and inhabits streams in Europe. Here’s one:
What’s the big deal? These insects produce the loudest sound per unit length of any known species on Earth. Scientists captured some in a river in Paris and recorded them underwater in the lab using a hydrophone. Only the males make noise, a clue that the song serves a sexual function.
How loud are they? The authors note: “SPL [sound pressure level] values of M. scholtzi were compared with the values reported for 227 other species (2 reptiles, 3 fishes, 24 mammals, 29 birds, 46 amphibians and 123 arthropods) collected from the literature (Table S1).
The song consists of three parts, and the third, the loudest, can be as large as 105 decibels! That’s as loud as a power mower from three feet away, and a level that, if inflicted constantly, could cause permanent hearing loss.
Is it really the loudest animal on earth? Yes, if the volume is scaled by a measure of body size (length). As the article notes:
. . . the most striking feature of the song is its intensity. The song can be heard by a human ear from the side of a pond or river, propagating across the water-air interface. Estimating the sound intensity at a distance of one metre reveals a value of ~79 dB SPL rms. When considering peak values, i.e. the loudest part of signal, the intensity can reach 100 dB SPL. Whilst these values are far below those estimated for large mammals such as dolphins, whales, elephants, hippos, or bison, when scaled to body size, M. scholtzi has the highest ratio dB/body size. Even if such comparison might need to be adjusted with corrections taking into account different recording methods and conditions, M. scholtzi is clearly an extreme outlier with a dB/body size ratio of 31.5 while the mean is at 6.9 and the second highest value is estimated at 19.63 for the snapping shrimp S. parneomeris. This water bug might be the exception that proves the rule that stipulates that the size and the intensity of a source are positively related. This departure from the rule is apparent within the group of stridulating animals. In this sub-sample, M. scholtzi is identified as an extreme outlier. No other recorded animals rival M. scholtzi. Two other arthropods were also identified as outliers; the Australian miniature cricket C. canariensis  and the Praying Mantis M. religiosa .
Well, what’s the real loudest animal on earth? If you don’t scale by body size, it is a whale and a monkey in the aquatic and terrestrial realms, respectively. The National Zoo says this:
Blue whales’ low-frequency pulses are as loud as 188 decibels—louder than a jet engine—and can be detected more than 500 miles away. On land, the loudest animals are howler monkeys, whose howl can be heard three miles away.
How and why do they make this noise? The males rub their penis against their abdomen. The authors theorize that “runaway sexual selection” due to competition by males for females has made the song so loud. But there are other forms of sexual selection not mentioned by the authors, like “sensory exploitation” (females pre-adapted to respond to such songs), that could also explain the evolution of such loud songs. There are many forms of sexual selection and it’s very hard in a given case to figure out which has operated.
You can hear the sounds of these insects by clicking on a BBC Nature piece, “‘Singing penis” sets noise reord for water insect.
Sueur J, Mackie D, Windmill JFC, 2011. So small, so loud: Extremely high sound pressure level from a pygmy aquatic insect (Corixidae, Micronectinae). PLoS ONE 6(6): e21089. doi:10.1371/journal.pone.0021089