Residents who live near the San Francisco Bay have been serenaded by a song that is as unmistakable as it is constant. It’s been described as like a chorus of kazoos or a swarm of bees. And scientists think they have solved the riddle on why this species decides to sing its song at night.

The Porichthys, also known as the plainfin midshipman fish, has been behind the “singing.” The male suitors are the ones who sing the song looking for a mate, always at night and always ending their call before morning. And scientists think they’ve figured out why: melatonin, the same thing that makes us sleep at night.

In the past, melatonin in animals had made scientists scratch their head, as it didn’t seem to make some species sleepy, such as this fish. But new research from Cornell University indicates that melatonin isn’t a chemical that induces sleep necessarily, but rather simply something that regulates nighttime behaviors. For us, it’s sleep. For Porichthys, it’s mating time.

“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,” said Andrew Bass, professor of neurobiology and behavior in the College of Arts and Sciences, the paper’s senior author. “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.”

“Melatonin is an ancient and multifunctional molecule that is found almost ubiquitously in the animal kingdom,” said Ni Feng, Ph.D. ’16, a former graduate student in Bass’ lab who is currently a postdoctoral researcher at Yale, the paper’s first author.

“Similarly, circadian rhythms govern the daily lives of diverse lineages, from plants to animals,” Feng said. “Our study helps cement melatonin as a timing signal for social communication behaviors.”