Lighthouse keepers had heard the sound at night: a low, eerie rumble, sometimes felt through the structures of their buildings.
Fishermen had heard it too. Some thought it sounded like distant machinery. Others compared it to thunder.
Only now, though, have scientists working in Alaska formally traced the noises to some of the largest animals on Earth: the region’s humpback whales.
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Researchers led by Fred Sharpe, a biologist affiliated with the SETI (Search for Extraterrestrial Intelligence) Institute, will report at the Astrobiology Science Conference in Wisconsin this week that humpback whales produce a sound never before described in scientific literature: a deep thrum that can travel at least five miles through the air.
Why the whales, which can weigh more than 40 tonnes, make the sound is a mystery. It may be a kind of self-soothing sigh, or could serve a mechanical function, like a sneeze. “We simply don’t know,” Sharpe said.
The findings come from fieldwork around the Five Finger lighthouse in Frederick Sound, southeast Alaska, where researchers deployed microphones on land and hydrophones underwater.
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The thrum changes character with distance, Sharpe said. Close to the whale, it can seem almost comic, like a low-pitched raspberry.
Further afield, the higher frequencies fall away. Asked whether it was more like an elephant rumble, distant thunder or machinery, Sharpe said: “All of the above.”
The animals sound “more mechanical” from further away, he said. This may help explain why the noises have been overlooked, or misidentified, for so long.
The sounds are mostly between 30 and 300 hertz, within the range of human hearing, yet at distance they acquire a peculiar quality. When the whales are really far away, Sharpe said, it almost sounds like the noises are “coming from inside” the listener.
His team thinks the thrums are generated by fatty plugs inside the whales’ nasal passages, which act as a seal against seawater when the animals dive.
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As the whales return to the surface and exhale with force, Sharpe suspects those plugs begin to vibrate like pistons, producing the sound.
The thrums are not the only humpback vocalisations that appear to have escaped scientific attention. Sharpe’s team has catalogued several others, giving them names including pizzle, chuff, boil, howl and hoot. He and his colleagues are now working to link each sound to specific behaviours.
The discoveries may have practical implications. Whale sounds that are detectable from shore without the need for boats or underwater equipment could potentially be used to track the animals and help avoid collisions with ships.
There is also a more philosophical dimension — one that helps explain why the research is being presented at an astrobiology conference, where the main focus will be on the hunt for extraterrestrial life.
To Sharpe, part of the lesson lies in what the thrums reveal about human perception, especially our tendency to miss signals that do not match what we expect to hear. “If conspicuous sounds from our beloved whales are falling on deaf ears,” he said, “goodness knows what cosmic signals might fall under the radar.”