During the summer feeding season in high latitudes, male blue whales tend to sing at night. But shortly before migrating south to their breeding grounds, the whales switch up the timing and sing during the day, new research suggests.
This is not the first time that scientists have observed whales singing at a particular time of day. But the finding appears to be the first instance of changes in these daily singing patterns throughout the yearly feeding and mating cycle, says William Oestreich, a biological oceanographer at Stanford University.
In the North Pacific, blue whales (Balaenoptera musculus) spend summers off North America’s coast gorging on krill before traveling to the tropics to breed in winter. Data collected by an underwater microphone dropped into Monterey Bay in California to record the region’s soundscape for five years allowed Oestreich and his colleagues to eavesdrop on whales that visited the bay. When the team separated daytime and nighttime whale songs, it stumbled upon a surprising pattern: In the summer and early fall, most songs occurred at night, but as winter breeding season approached, singing switched mostly to the daytime.
“This was a very striking signal to observe in such an enormous dataset,” says Oestreich. The instrument has been collecting audio since July 2015, relaying nearly 2 terabytes of data back to shore every month.
The researchers also tagged 15 blue whales with instruments and from 2017 to 2019, recorded the whales’ movements, diving and feeding behavior, as well as their singing — nearly 4,000 songs’ worth. Whales that were feeding and hadn’t yet started migrating to the breeding grounds sang primarily at night — crooning about 10 songs per hour on average at night compared with three songs per hour in the day, or roughly three times as often. But those that had begun their southward trip sang mostly in the day, with the day-night proportions roughly reversed, the team reports October 1 in Current Biology.
Plenty of mysteries remain around why whales sing, though it’s generally thought to be involved in mating (SN: 6/3/16). Oestreich contends that the new findings suggest that other information about blue whales’ lives — like migration status and feeding behavior — may be embedded in whale songs.
“We still know so little about the function of the song,” marvels Jenny Allen, a marine ecologist at the University of Queensland in Australia. “It’s unclear what the biological relevance would be for males to sing at night while feeding, yet during the day while migrating.”
If the switch in timing is in fact linked to behavior, this “acoustic signature of migration” could potentially provide indirect insight into whale movements, Oestreich says.
Other researchers aren’t so sure. Ana Širović, a marine biologist at Texas A&M University at Galveston, notes that there are examples of whales tagged in Southern California predominantly singing in the daytime during their feeding season. Whales singing in the day could also be swimming through a given region without starting their southward migration. “I am not fully convinced that we can use switch to daytime calling as an indication of migrations,” she says.
To find answers, Oestreich says he and his colleagues are eager to investigate how flexible the whales are in the timing of this song shift from year to year, especially given the unpredictability and speed of climate change–driven impacts in the whales’ environment. He also wants to know if blue whales listen for time shifts in the songs of distant whales as a cue to start their own migration.
“Given that these songs travel hundreds of kilometers in the ocean environment, this might be possible, which could allow individual whales to make better-informed decisions on when to migrate,” he says.
Predicting whale movements via their songs may be particularly useful for this blue whale population, which is commonly struck by shipping vessels, Oestreich says. Advance notice of a regional influx of whales could help keep the endangered animals safe and singing far into the future.