Researchers have known for decades that whales create elaborate songs, sometimes projecting their calls for miles underwater. A new study from the Woods Hole Oceanographic Institution (WHOI) has uncovered some unexpected information about particle motion in whale songs, and how it may play a pivotal role in locating other whales in open ocean.
In a paper published recently in the online Biology Letters journal, WHOI biologist Aran Mooney described approaching humpback whales off the coast of Maui, Hawaii. He and his team measured two components of the whales’ songs: pressure waves (the type of sound wave that pushes on human eardrums, allowing us to hear) and particle motion (the physical vibration of a substance as sound moves through it). They discovered that the particle motion in the water propagated much farther than expected.
He said, “We threw our gear over the side, and let ourselves drift away from whales while measuring both particle motion and sound pressure. We didn’t expect particle motion to be projected much at all—just a few metres away at most. But as we got progressively farther away, the particle motion stayed loud and clear.”
The group measured only as far as 200 metres from the whales, but their data showed that this particle motion, especially in lower frequencies of sound, could travel much farther than the distance recorded.
To envision the difference between these two modes of sound, imagine pulling up beside a car blasting loud music. Mooney said, “The stuff you hear is pressure waves; the stuff you feel vibrating your seat is particle motion. When it comes to whale songs, particle motion hasn’t really been studied much. It’s a lot more complex to measure than pressure waves, so we don’t have a great sense of how it propagates in water or air.”
Pressure waves are detected using a specialised underwater microphone called a “hydrophone”. Detecting particle motion requires sensitive underwater accelerometers, which until recently have not been widely available to researchers. Mooney and his team had both sensors, and they were able to record these unexpected recordings.
Mooney reiterated that they did not gather enough data to determine whether these whales could sense the particle motion present, but the anatomy of whale ear bones suggests that low-frequency vibration could be a major element of their hearing.
Humpback whale ear bones are fused to the skull, providing a direct link to any vibration in the water column. “This could mean that their hearing is influenced by the way sound conducts through their bones,” he said. “It raises the question: does a whale’s lower jaw act like a tuning fork to direct vibrations to their ears? Previous papers have shown this bone conduction might be a viable mode of hearing.”
He added that from an evolutionary standpoint, there was some precedence for this sort of vibratory hearing. Although most mammals sense sound via pressure waves, hippopotamuses are known to sense sound underwater using their bodies, even while their ears remain above the surface.
However, Mooney’s findings has raised an immediate concern. If whales can sense particle motion, similar vibrations caused by humans in the form of shipping and seismic exploration for oil and gas might interfere with the way whales communicate. Mining and construction activities are also increasing, contributing low-frequency particle motion that may propagate for miles underwater. “This could be a major concern for whales,” Mooney added.