Biologists have long known that humpback whales use different sounds -- including clicks, whistles and pulsed calls -- to communicate, navigate, search for food and socialize.
But one researcher has heard a set of mysterious, low-frequency sounds near various humpbacks off the coast of the Hawaiian island of Maui, and they're not like anything he's ever heard humpbacks do before.
"Imagine a heartbeat," Jim Darling, a research biologist at Whale Trust Maui, said in a release Monday, describing the sounds.
To be clear, no one knows for sure that the previously unheard sounds are actually being made by humpback whales. But on various occasions since 2005, the sounds have been heard while humpbacks were in the vicinity of recording equipment. In the most recent case, the release says, "any fluctuation in volume appeared to match the whales’ approach to the boat, and no other species of whales were known to be in the area at the time."
One theory, National Geographic reports, is that the sounds could be related to mating.
Typically, humpback vocalizations range in frequency from 80 to 4,000 hertz, according to Whale Trust. The new sounds, however, were measured at 40 hertz -- just above the threshold of what humans can hear.
Darling describes the sounds as "ethereal" and difficult to hear except during exceptionally quiet, calm ocean conditions.
"The first time I heard them, or realized I heard them, was in 2005 when recording social sounds from an active group of eight whales," Darling said in the release. "Although I have recorded samples since, it took a long and particularly good recording of a male-female pair in 2013 to convince me they were real."
The unusual sounds are the subject of a new paper, published last month in The Journal of the Acoustical Society of America and the result of studies funded by Whale Trust and the National Geographic Society.
Darling hopes that by describing the sounds to a wider readership, he'll encourage further exploration and assessment of the phenomenon.
"I’m trying to be cautious here," he told NatGeo, "as it's still possible -- although very unlikely -- that they are produced by something other than the whales."
Ultimately, if the sounds can be conclusively linked to the humpbacks, Whale Trust says it would "add a whole new dimension to the already complex repertoire found in this species' communication."
"We have a long way to go on understanding this -- but it does remind us of how very, very little we know about these animals," Darling said in the release.
Every year from about November to May, as many as 10,000 humpback whales return from their feeding grounds in Alaska to the warmer waters of Hawaii to mate and give birth.
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