Antarctic: Ultrasonic Voice Coming From Ice Found to be Weddel Seals

Antarctica is often perceived as nothing more than a lifeless barren of cold. A frozen dessert. However, not many are aware of the booming and flourishing population of Weddel seals that dominated the waters of the frozen south. There, they create a soundscape that a lot of creatures, especially humans, are unaware of.

These pinnipeds typically emit high-pitched pings that sound like laser guns in a science-fiction movie. But that is not their entire repertoire. Research reveals that a significant portion of their calls is at ultrasonic frequencies, high pitches well beyond the 20-kilohertz limit of most human hearing.

Paul Cziko, a marine biologist from the University of Oregon, installed a livestreaming audio and video system at Antarctica's McMurdo Station in 2017.

The setup allowed researchers and scientists to listen to the aquatic mammal's underwater calls. Data from the observation device yielded surprising results: The seals sometimes vocalized at extremely high, ultrasonic frequencies of more than 200 kilohertz, Cziko and his colleagues reported in the Journal of the Acoustical Society of America.

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Ultrasonic Calls

An ultrasonic call cast a narrower and more precise beam compared to low-frequency sounds.

Bats and toothed whales have specialized anatomy that lets them use such beams for echolocation, sensing their surroundings by deciphering the rebounding sound waves. Seals, however, lack the necessary anatomical structures. So why do they make these sounds?

It might not be for similar echolocating purposes like the aforementioned bats and whales, but their ultrasonic calls are still serving a purpose, albeit different.

Weddel Seals' "Echolocation"

Cziko and his colleagues suspect Weddell seals use ultrasonic calls for less specific orientation purposes.

In the early 1970s, William Schevill and William Watkins, both at the Woods Hole Oceanographic Institution, came to a similar conclusion after using the era's limited technology to record Weddell seals' ultrasonic vocalizations, likely for the first time. 

Antarctica experiences four months of 24-hour sunlight during summer, and winters have long periods of complete darkness.

"We know that seals have perfect low-light vision," says Lisa Munger, a marine mammal bioacoustician at the University of Oregon and a co-author of the new research. "But when there's no light, they've got to be using something else to find their way around."

Echoes from ultrasonic calls may provide enough necessary environmental information to help seals return to breathing holes in the ice or locate food-even if their sound-processing is not specialized enough to be called true echolocation.

High-frequency vocalizations travel shorter distances than lower-frequency ones, and this may also help Weddell seals communicate without alerting predators. "These sounds might be instrumental if you're trying to communicate with someone that's a few feet away," Munger says. "But you don't want the sound to get out into the open water, where there are killer whales."

Peter Tyack, an animal behaviorist at the Woods Hole Oceanographic Institution and the University of St. Andrews in Scotland, who was not involved with the study, notes another possible communication hypothesis.

"If you have territorial males broadcasting threats to each other and you just hear the low-frequency [calls], you might interpret that as a background threat," he says. "But if all of the sudden you hear the high-frequency ... that could mean 'I'm threatening you, not the general world.'"

The new findings may have a simpler explanation, Tyack adds. Humans define "sonic" and "ultrasonic" ranges based on the limits of our own hearing, but for Weddell and other seals, a range of about 50 to 100 kilohertz may be optimal. As he puts it: "Why would you not make sounds in the best range of your hearing?"

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