The courtship song of the midshipman fish is a pleasure that can only be enjoyed by night. It turns out that these long, droning hums follow a daily rhythm that is set by light and the hormone melatonin. With enough light, singing fish can be made to quiet down, a condition that is remedied with a boost of melatonin, scientists reported today in the journal Current Biology.
There are many reasons for a fish to make some noise; some use sound for hunting or staking out territory. Others, such as plainfin midshipmen (Porichthys notatus), sing to entice females to come and spawn. In spring and summer, male midshipmen set up shop close to shore, building nests sheltered among the rocks. They sing throughout the night in nocturnal choruses, belting out individual hums that can last up to nearly two hours.
To investigate what prompts the fish to sing only after nightfall, researchers at Cornell University collected male midshipmen from the shores of California and Washington State. Fish kept in total darkness continued to follow cycles of humming and silence, indicating that their tunes are set by an internal clock. This schedule fell apart, though, when midshipmen were exposed to days of constant light.
The scientists knew that too much light can suppress melatonin, a hormone secreted primarily at night by the brain’s pineal gland. Melatonin controls the wake and sleep cycle in people. It also prompts songbirds to stop singing at night.
But for the fish, an infusion of melatonin led them to renew their crooning, in spite of the light shining into their tank. The researchers also examined where melatonin was active in the midshipmen’s brains, and found that the hormone influences regions that are involved in social, mating and vocal behavior.
“Melatonin conveys the appropriate time for vocalization,” concluded the team. This means that the hormone can play opposite roles in different animals, quashing the songs of diurnal animals but telling nocturnal creatures like the fish to let loose.
Strangely, though, melatonin can also have similar effects in bird and fish songs. In this experiment, melatonin implants incited the midshipmen to make longer hums. Melatonin can also spur birds to make longer individual calls when they do sing. It’s a reminder that figuring out how a hormone affects our bodies often isn’t simple or straightforward.