Weddell seals are chirping, whistling and shaking under the ice of Antarctica at frequencies of sound that humans cannot hear, the researchers report.
Two-year recordings at the live underwater observatory at McMurdo Sound recorded nine types of tonal ultrasonic vocalizations reaching up to 50 kilohertz. People hear in the sound range of 20 to 20,000 hertz or 20 kilohertz.
“It really sounds like you’re in the middle of a space battle Star Warslaser beams and everything else. “
Weddell seals (Leptonychotes weddelii), the southernmost mammal in the world, thrives under the continent’s sea ice, using its large teeth to create air holes. They can dive to 600 meters (1,969 feet) in search of prey and remain submerged for 80 minutes. Researchers first identified 34 types of calls to print on sound frequencies in 1982, linking sounds to social interactions.
Lead author of the study Paul Cziko, a visiting professor of research at the Institute of Ecology and Evolution at the University of Oregon, began recording the sound sounds of the seal in 2017 after the installation of the McMurdo Oceanographic Observatory was completed. Workers at McMurdo station, he says, often fell asleep listening to the sound of seals coming from below.
“The calls of Weddell seals create an almost amazing, otherworldly sonic landscape under the ice,” Cziko says. “It really sounds like you’re in the middle of a space battle Star Warslaser beams and everything else. “
Over the next two years, the observatory’s broadband digital hydrophone – more sensitive than the equipment used in earlier recordings – picked up high-frequency vocalizations during passive seal surveillance.
“We’ve constantly encountered these ultrasound types of calls in the data,” says co-author Lisa Munger, a marine biologist who studies marine mammal acoustics and a career instructor at Clark Honors College University. “Finally, it dawned on us that the stamps were actually used regularly.”
Nine new types of calls consisted of one or more vocal elements that have ultrasonic fundamental frequencies. Eleven elements, including chirping, whistling, and friction, were above 20 kHz. Two exceeded 30 kHz, and six were always above 21 kHz. One whistle reached 44.2 kHz, and the downward chirping of the second type of call began at about 49.8 kHz. The harmonics or overtones of some vocalizations exceeded 200 kHz.
“It was really surprising that other researchers actually missed part of the conversation,” says Cziko.
It remains unknown how Weddell seals navigate and find prey during months of almost absolute darkness in Antarctic winter.
What ultrasonic vocalization means in the repertoire of Weddell seals is not known. Seals are among 33 species of mammals with fine legs, classified as coarse-legged. Until now, feathers, which also include sea lions and walruses, were believed to be vocalized only at the sound level.
It could be, Cziko says, that the seals produce sounds simply to “stand out above all the lower frequencies, like a change to another communication channel.”
Or, the researchers note, ultrasonic vocalizations can be used for echolocation, a biological sonar by which dolphins, whales and bats move in limited visibility to avoid obstacles and find friends or prey.
“The possibility of seals using some kind of echolocation has really collapsed over the years,” Cziko says. “We’ve actually had a lot of a bit of a heated discussion in our group about whether or how seals use these ultrasonic sounds for behaviors like echolocation.”
It remains unknown how Weddell seals navigate and find prey during months of almost absolute darkness in Antarctic winter. The study provides no evidence for echolocation.
“We’d like to know who produces the ultrasound calls – men, women, minors, or all of the above,” Munger says. “And how do seals use these sounds when they’re out in deeper water looking for fish? We need to record in multiple places so we can relate sounds to behavior. “
The discovery appears in Journal of the American Acoustic Society. Additional co-authors are from the University of California, Merced and the University of New Brunswick in Saint John, Canada.
The National Science Foundation primarily supported the research.
Source: University of Oregon