A singing penis makes a tiny 2mm water bug the loudest animal on earth, relative to its size.
According to research published in a recent issue of the online journal PLoS ONE, Jérôme Sueur, David Mackie and James Windmill discovered that the Pygmy water boatman (Micronecta scholzi) – a tiny freshwater insect measuring just 2mm in length and common across Europe – makes the loudest sound relative to its size in the entire animal kingdom using its penis.
The authors analysed the calling song made by male Pygmy water boatmen, doing so with individuals caught from water bodies near Paris.
Samples of five unsexed individuals were placed in a fish net breeder inside a plastic water tank with a gravel bottom and water to a depth of 8cm; a hydrophone (underwater microphone) was placed at the bottom of the net breeder. The calling songs of 13 males were recorded (females do not make any sound) and 60 seconds of signal without background were selected for each male and analysed.
The authors found that the water boatman produces a complex calling song comprising three distinct parts, with its most striking feature being its intensity. The song can be heard by human ears from the side of a pond or river, propagating across the water-air interface. At its loudest, the sound can reach close to 100 decibels (about as loud as a jackhammer). This makes the water boatman the loudest animal relative to its size.
Although the mechanism of sound production in the water boatman is not fully identified, it is known that the sound is produced by these water bugs by rubbing a series of ridges on the right lobe of the penile appendage against a ridge on the left lobe of the eighth abdominal segment. What is not known is how this sound is amplified.
The authors hypothesise that the very loud mating calls of the water boatman may be due to runaway sexual selection, in which acoustic competition among males lead to the evolution of louder mating calls. This could also be facilitated by the lack of predators that locate the water boatmen acoustically.
For more information, see the paper: Sueur J, D Mackie and JFC Windmill (2011) So small, so loud: extremely high sound pressure level from a pygmy aquatic insect (Corixidae, Micronectinae). PLoS ONE 6, e21089. doi:10.1371/journal.pone.0021089
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