Marine biologists from the University of Oregon and the University of New Brunswick have recorded common, previously unknown, ultrasonic vocalizations produced by Weddell seals (Leptonychotes weddellii), the world’s southernmost-ranging mammal. They’ve identified nine recurrent call types in more than one year (2017-2018) of broadband acoustic data obtained by a continuously recording underwater observatory in McMurdo Sound, Antarctica.

Weddell seals (Leptonychotes weddellii). Image credit: Elliott Devries.

Weddell seals (Leptonychotes weddellii). Image credit: Elliott Devries.

Weddell seals are a species of large and relatively abundant true seals in the family Phocidae.

They have a circumpolar distribution around Antarctica, including the highest-latitude coastal regions.

In contrast to the other Antarctic seals, they prefer expanses of heavy pack ice or thick shore-fast sea ice, using their teeth to maintain access holes in the ice.

They dive to at least 600 m (2,000 feet) and for up to 82 min in search of fish and invertebrate prey year round.

Scientists had first identified 34 seal call types at sonic frequencies in 1982, tying the sounds to social interactions.

“The Weddell seals’ calls create an almost unbelievable, otherworldly soundscape under the ice,” said lead author Professor Paul Cziko, a researcher in the Institute of Ecology and Evolution at the University of Oregon.

“It really sounds like you’re in the middle of a space battle in ‘Star Wars,’ laser beams and all.”

Professor Cziko and colleagues began recording the seals’ sonic-ranged vocalizations in 2017 after completing the installation of the McMurdo Oceanographic Observatory.

Over the next two years, the observatory’s broadband digital hydrophone picked up the higher-frequency vocalizations during passive monitoring of the seals.

“We kept coming across these ultrasonic call types in the data,” said co-author Dr. Lisa Munger, also from the Institute of Ecology and Evolution at the University of Oregon.

“Finally, it dawned on us that the seals were actually using them quite regularly.”

The nine new call types identified by the team were composed of single or multiple vocal elements having ultrasonic fundamental frequencies.

Eleven elements, including chirps, whistles and trills, were above 20 kHz.

Two exceeded 30 kHz and six were always above 21 kHz.

One whistle reached 44.2 kHz and descending chirps in another call type began at about 49.8 kHz.

Harmonics, or the overtones, of some vocalizations exceeded 200 kHz.

What the ultrasonic vocalizations mean in the Weddell seals’ repertoire is currently unknown.

“It was really surprising that other researchers previously had, in effect, missed a part of the conversation,” Professor Cziko said.

The study was published in the Journal of the Acoustical Society of America.

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Paul A. Cziko et al. 2020. Weddell seals produce ultrasonic vocalizations. Journal of the Acoustical Society of America 148, 3784; doi: 10.1121/10.0002867