A study reveals that the males of a nocturnal species of fish called plainfin midshipman (porichthys notatus), which can sing all night are found to behave this way because of a hormone called melatonin. The hormone regulates sleep and wake cycles and maintains the body clock.
Way back in 1980s, people living in the San Francisco Bay on houseboats had heard a strange continuous humming which started late in the evening and stopped abruptly in the mornings.
After a time taking investigation regarding this issue, it was disclosed that a male plainfin midshipman fish was the one humming for attracting mates. As per the findings, this fish can attain a growth of 15 inches lengthwise.
There is still a lot to be known about the mechanism of melatonin and circadian rhythms in nocturnal vertebrates, which includes this fish. Day-active or diurnal songbirds have been analysed by the researchers and they found that the melatonin hormone repressed the singing during night, but it was triggered during the day time as increased duration of syllables was observed.
Melatonin signalled the fish to sing at night and it signalled the bird to sing during the day, but its function remained the same - to extend their singing while affecting their behaviour.
"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 behaviour," stated Andrew Bass, professor of neurobiology and behaviour 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," Bass added.
Melatonin hormone is found in all animals in the animal kingdom and is a multifunctional, ancient molecule, explained Ni Feng, a postdoctoral researcher at Yale, the paper's first author.
"Similarly, circadian rhythms govern the daily lives of diverse lineages, from plants to animals. Our study helps cement melatonin as a timing signal for social communication behaviours," Feng said.
The researchers brought a wild-caught midshipman fish to the lab and kept it in an environment with controlled lighting. They tried finding if the internally generated circadian rhythm was responsible for control the nocturnal singing. They placed the fish in complete darkness for around a week, without any light and the fish was found singing and humming on a schedule of 25 hours, the researchers started conducting this test an hour later every night.
"You can think of it as a clock that can help the fish predict the best time to be vocally active," Feng said.
"We found that this clock ran with a delay of about an hour, causing a drift in vocal activity with respect to the 24 hour light-dark cycle, highlighting the importance of internal clocks to be recalibrated by environmental cues on a daily basis," he added.
Next, the scientists tried understanding melatonin's impact on the behaviour of the fish, they exposed the fish to light for a span of 10 days and found that the humming of the fish had reduced. It happened because melatonin hormone is secreted in vertebrates only when it's dark. The fishes were then given melatonin substitute, they were found humming, but not in a rhythm like they used to.
"Melatonin acts as a go signal for the nocturnal call of the midshipman fish," Feng said. "Surprisingly, at the single call timescale, constant light also decreased hum duration, but melatonin maintained hum duration at normal levels, a finding also found in diurnal birds," he added.
Bass and Feng then traced specific sites where melatonin impacts the brain. It was found that the action was triggered in the regions of the brain which managed social and reproductive behaviours along with including vocal initiation centres, which is similar in birds as well as other vertebrates.