The tiny \'water boatman\' insect is the world\'s loudest animal relative to its body size, according to a new study. Males of the Micronecta scholtzi species serenade their sweethearts with a three-part song made by rubbing their genitalia against their abdomens, but it remains a mystery how or why the creatures make such a loud mating call. \"This insect is a few millimetres in length yet can produce sound audible from the riverside,\" says the study by scientists from France and Scotland, published by the PLoS ONE scientific organisation. The song was recorded at noise levels up to 99.2 decibels, \"a significant output considering the small size of the insect.\" The water boatman outperformed marine and terrestrial mammal vocalisations, the researchers say. \"Such an extreme display may be interpreted as an exaggerated secondary sexual trait resulting from a runaway sexual selection without predation pressure,\" they said. They admitted, however: \"The mechanism behind the intense sound production of M. scholtzi is not clearly identified. There are no obvious body or external resonating systems that could amplify the sound. \"To observe the micro-mechanics of such a small system remains a significant challenge.\" Micronecta scholtzi are freshwater insects measuring just 2mm that are common across Europe. In terms of noise produced versus body size, the insect \"is clearly an extreme outlier with a decibel to 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,\" the study said. So far, the researchers have not yet managed to find out whether females of the species like their suitors\' loud courtship calls. \"Eventually, playback experiments could test female preference for loud over soft calls,\" they hope. Dr James Windmill from the University of Strathclyde, Glasgow, told the BBC: \"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 . \"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. \"If you scale the sound level they produce against their body size, Micronecta scholtzi are the loudest animals on Earth\".