The latest release in the humpback whale’s haunting sound collection is a track so unusual that scientists hardly know what to make of it.

Unlike anything on the hit album Songs of the Humpback Whale (released in 1970, the wildlife recording went multi-platinum), the mysterious new noise has such a low beat it’s scarcely audible.

Near the lower limit of human hearing, the so-called “pulse trains" are deeper than any confirmed humpback vocalization, according to Jim Darling, a research biologist with the Whale Trust Maui in Hawaii.

It's “as if listening to a heartbeat with a stethoscope,” says Darling, whose study on the phenomenon was published November 5 in the Journal of the Acoustical Society of America.

In research partially funded by the National Geographic Society Committee for Research and Exploration, Darling and colleagues recorded the strange sounds near the Hawaiian island of Maui (map), where up to 10,000 humpbacks gather each winter to give birth and mate.

Humpback vocalizations, including the complex and wide-ranging “whale song” performed by males, typically have an audio frequency between 80 and 4,000 hertz (Hz). But the newly described pulse sounds were found to have a significantly lower frequency of around 40 Hz. The low limit for human hearing is 20 hz. (See "Can You Hear Me Now? What Whale Ears Have That Ours Don't.")

“We are just so used to hearing a certain type of sounds from humpbacks, and this was out of that range,” says Darling.

Background Noise

Around 2008, when Darling first picked up the whales' deep beats through his headphones, he looked for passing helicopters as a possible source, then boats, “then started wondering about submarines… the whales were way down the list.”

Easily masked by such background ocean noise, it’s a sound “it took me years to convince myself I was hearing,” he says.

Even now, Darling can’t be absolutely sure humpbacks are making the noise. (See "Humpback Whale Crittercam Video Reveals Bottom-Feeding Activity.")

“I’m trying to be cautious here as it's still possible—although very unlikely—that they are produced by something other than the whales.”

But Alison Stimpert, a vertebrate ecologist with Moss Landing Marine Laboratories in California, says it wouldn’t surprise her if humpbacks create the sounds.

“Though they are amongst the lowest frequency I have heard of for this species, humpbacks have arguably the largest acoustic repertoire” of baleen whales, she notes via email.

Listen to the new whale sound. For the best experience, wear headphones and listen for the heartbeat-like pulsing noise.

Stimpert, who wasn’t involved in the new finding, adds that blue and fin whales also produce pulses in the frequency range of these latest sounds, “but we’re still learning about the specific behavioral functions.”

Vocal Females?

What role such pulses might play in humpback communication is impossible to say at this stage.

But they were recorded when adult females had male company, suggesting the sounds are connected to the mating game.

Whether it's males or females that make the deep beats isn’t yet known, raising the intriguing possibility that supposedly quiet females use them to be heard among the loudly musical males. (Read "Do Whales Have Culture? Humpbacks Pass on Behavior.")

Video Reveals Surprising Humpback Feeding Behavior October 11, 2013—Humpback whales are known for their unique feeding behavior—called bubble—netting-at the water's surface. But researchers noticed the whales also spent a lot of time on the bottom. By mounting cameras on the animals, researchers saw for the first time that these whales were bottom-feeding.

“We still don’t know exactly the function of humpback song—whether it is directed at females or other males—and no one has definitively assigned any particular sound production to females yet," Stimpert says.

Neither do researchers know how females attract or ward off males during courtship. So “the idea that the sounds could be female communication is really exciting.”