Ah, summer in Winnipeg: muggy heat we can’t stand after eight months of zero humidity and freakishly cold winter, freak storms, flooding, mosquitoes, canker worms, E. coli-infested eutrophic lakes — we love it all and by George we’ll take it! And in this sweltering mid-summer issue I would like to tell you about a really cool bug. A bug you can respect: the water boatman.
The water boatman belongs to the insect order Hemiptera, the true bugs — a cold virus is not a bug, a bacterial infection is not a bug, a fly is not a bug. Whether an insect is truly a bug depends on specific details of the wings and mouthparts, which are modified for piercing and sucking, oh my!
Water boatmen are aquatic and have oar-shaped hind legs used for paddling. They typically inhabit lakes and ponds, though they do not have gills and must breathe air from the water’s surface. They will, however, frequently carry an air bubble with them during their lengthy underwater adventures and breathe oxygen from within the air bubble.
Over 500 species of water boatmen have been identified, more than 100 of which are found in North America. And amazingly, one species in particular, Micronecta scholtzi, has just been credited as being the loudest animal on earth! Despite only measuring about 2.5 millimetres long, they create sound at volumes comparable to sitting in the front row of a loud orchestra performance. This brain-boggling volume is produced during stridulation, which is a fancy term for sound produced by rubbing the differently sized ridges of two body parts together. In the case of the water boatman, stridulation is performed by rubbing the penis against the abdomen, which in a sense means the water boatman has the loudest penis in the world — hard to believe, I know.
A recent study by Sueur et al. (2011) reports a peak value of M. scholtzi calls at 100 decibels (dB) SPL (a log ratio between measured sound pressure level and a reference point defined by the threshold of audible hearing for humans). M. scholtzi thus has the highest ratio of decibels to body size ever recorded, making this water boatman the loudest animal on the planet!
Why do water boatmen need to be so loud? The song produced by stridulation is used to attract females. The males produce a three-part song and it is the third part that is the loudest, meant to drown out the songs of competing males. Sueur et al. propose that the extreme volume achieved by M. scholtzi during stridulation may be the result of runaway sexual selection via intra-male competition.
The idea is this: females localize the acoustic signals produced by loud males more easily than those of quieter males whose songs are masked by the loudest males. Typically, there is a balance between signal volume (high volume = more females) and predation (higher volume = more predators) but in this case, it is possible that the water boatmen either have no predators that use acoustics to find prey or the boatmen are well able to evade these predators so the volume of their signal is not regulated by negative selection pressures like predation. In short, the male with the loudest penis gets the most females — in the water boatmen world, at least. We at the Manitoban are going to need to see some further studies before we suggest penis-singing contests on campus. ’Nuff said.