Weddell seals can chirp, whistle, and trill at frequencies beyond the range of human hearing. This states a study by the McMurdo Oceanographic Observatory. Weddell seals are very vocal. But the ability to emit such ultrasonic calls, never has been observed before. The researchers discovered nine types of ultrasonic calls regularly used by the seals. These were in the range of 20-50kHz. Humans can only hear the tones below 20kHz.
Weddell seal (Leptonychotes weddellii)
Weddell seals live farther south than any other mammal, mostly under the ice. The Weddell seal is one of the characteristic animals of Antarctica and can be found all around the continent, at the edge of the pack ice. In winter, they use their large teeth to keep air holes open. Through this process, a hole can be held even when the surrounding ice has reached a thickness of 2 meters. To hunt prey, they can reach depths of up to 600 meters in one breath and spend up to 80 minutes at a time underwater. On such a dive they can cover up to 12 km. Weddell seals are about 2.5 meters long and weigh around 400 kg. The population of this seal species is estimated at 500,000 to 1 million animals.
Two years observed
The high-frequency sounds were recorded using a sensitive digital broadband hydrophone under the sea ice in McMurdo Sound in the two years following the observatory’s installation in 2017. The researchers regularly dive under the ice to observe the seals, one of which has been equipped with an underwater video camera.
“The calls of Weddell seals create an almost unbelievable, ghostly soundscape beneath the ice,” said Paul Cziko, a professor at the University of Oregon and lead author of the study. “It really sounds like you’re in the middle of a space battle in Star Wars.”
“We’d like to know who is producing the ultrasonic calls – males, females, juveniles or all of them together,” said Lisa Munger, co-author of the study. “And how do the seals use these sounds when they are searching for fish in deeper water? We need to record sounds in several places to compare the sounds to the behavior.”
The scientists, joined by marine biologists from the University of Oregon, have yet to determine why the seals make such sounds. “We’ve actually had a lot of somewhat heated discussions in our group about whether or how the seals are using these ultrasonic sounds for echolocation-like behaviors,” Cziko said.
It is speculated that they use the sounds as a basic form of echo-location, as used by toothed whales, dolphins or bats. This could help the seals navigate under the Antarctic ice, where there is almost total darkness in winter.
Heiner Kubny, PolarJournal
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