Mystery solved: Melatonin makes these fish sing at night

There are fish that sing. Or drone like a bunch of loud kazoos, anyway. People living in houseboats in the San Francisco Bay have grown used to a low, strange hum that begins suddenly in the dark of the night and stops just as abruptly in the early morning.

The noise comes from male suitors of the species Porichthys notatus, commonly called the plainfin midshipman fish. While female midshipman only grunt when showing aggression, the males trying to mate with them use their swim bladders to create noises that have been likened to a chorus of kazoos, a formation of flying jets or a swarm of droning bees.

"They sound like an orchestra full of mournful, rasping oboes," SFGate reported in 2004.

Researchers now believe they've figured out how the midshipman keeps his crooning so punctual: melatonin. Melatonin is a hormone produced by many plants and animals, its release triggered by darkness. In animals that sleep at night – as humans do – melatonin is thought to help regulate the internal "clocks" that tell our bodies it's time for some shut-eye. Some humans even find respite from insomnia by taking melatonin supplements at bedtime.

But the role of melatonin in the lives of nocturnal vertebrates remains quite mysterious. Nocturnal animals also make melatonin when plunged into a dark night, so it clearly doesn't put them to sleep.

The new study published in Current Biology suggests that melatonin may act as more of a signal to trigger nighttime behaviors than a sleepy-time chemical. In midshipman fish, it seems to prompt males to start singing – which is what makes them midnight crooners.

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"Our results, together with those of others that also show melatonin's actions on vastly different timescales, highlight the ability of hormones in general to regulate the output of neural networks in the brain to control distinct components of behavior," Cornell University's Andrew Bass – yes, Bass – the paper's senior author, said in a statement. "In the case of melatonin, one hormone can exert similar or different effects in diurnal vs. nocturnal species depending on the timescale of action, from day-night rhythms to the duration of single calls."

To trace melatonin's role in the humming, scientists tested the singing cycles of fish kept in total darkness and constant light. In the dark, they maintained roughly the same schedule as usual – they switched to a 25-hour cycle, so their singing drifted by an hour each day, but their periods of singing and silence maintained the right pattern. But in the light, they never sang at all. Then researchers gave fish drenched in sunlight an artificial melatonin boost, and their not-so-sweet melodies began in earnest – though at random intervals.

It seems we have a lot to learn about melatonin. The next time you reach for a pill to help you drift off to sleep, be glad it doesn't make you hum like a kazoo until dawn.