Evolution

The plainfin midshipman may not look like much, but it's not looks that have earned these fish the interest of scientists - it's their impressive voices.

When mating season comes around, large male plainfin midshipman (Porichthys notatus) attract females by humming. They're very good at it too; they can be so loud they've elicited complaints from California houseboat residents that they couldn't sleep or hold conversations over the din. Some claim to have felt their homes vibrating.

This is especially interesting when you consider fish don't have vocal chords. They've evolved another method - the midshipman creates its hum by vibrating muscles against the sides of its swim bladder (a gas-filled organ used for buoyancy). They use the same method to make aggressive grunts towards intruding males. Strangely, males in the same area seem to actively match the pitch of their hum against each other. Males of one "community" tend to sound different to males in another.

But a constant loud, low hum can't be good for your hearing. To avoid damaging their hearing, these fish can actually regulate their hearing and can listen for females and predators over the hum. Both sound and hearing are controlled by the same part of the fish's brain - hearing sensitivity is reduced the moment humming starts. It's hoped learning more about how the midshipman regulates its hearing could teach us more about our own.

Oh, and remember when we said large male midshipman hum? That's because there's actually two types of males, and the smaller males don't bother. Once the humming male attracts the female to his nest in the mud, she lays her eggs for him to fertilize. But sometimes he's beaten to it by a smaller male, who sneaks into his nest and fertilizes the eggs (presumably because they fertilize their eggs in the nests of large males, sneaky males don't have nests). The differences between male types is substantial: humming males can be eight times larger than sneaky males, but sneaky males have testicles about seven times larger than the humming male's.

Photo: Plainfin midshipman hiding in the sand. They spend the day this way as they are nocturnal. Credit to Terrance J. Fidler.

http://www.livescience.com/27237-fish-sings-for-mates.html

http://phys.org/news157224812.html

http://www.nature.com/news/2005/050711/full/news050711-1.html

http://beheco.oxfordjournals.org/content/17/4/670.full.pdf

http://bit.ly/10pK34u