Inaudible Melodies - Weddell seals produce ultrasonic vocalizations!
We discovered that Weddell seals regularly vocalize at ultrasonic frequencies! That is, they produce “high-pitched” chirps, whistles and trills at frequencies that humans can’t hear. The findings are summarized in this fun video and was covered in University of Oregon news.
Our research was published today the Journal of the Acoustical Society of America (JASA). Paul Cziko, Lisa Munger, Nick Santos and John (“Jack”) Terhune authored the study. (Paper PDF)
Using the MOO’s unprecedented underwater acoustic dataset, we discovered nine recurrent ultrasonic call types that occurred throughout the year. The calls were composed of vocal elements having ultrasonic fundamental frequencies (20kHz to 50kHz). Humans can only hear sounds in the “sonic” range, less than 20kHz.
The seals were recorded making the ultrasonic sounds under the sea ice in McMurdo Sound, Antarctica. The MOO’s underwater camera also recorded a video of a Weddell seal producing the high-frequency sounds.
No other pinnipeds (seals, sea lions/eared seals and walrus) are known to use sounds originating at ultrasonic frequencies.
Weddell seals were discovered to regularly vocalize at ultrasonic frequencies (>20 kHz), above the range of human hearing. A visual representation (spectrogram) of one of its nine ultrasonic call types is shown. The U-shaped features in the upper half of the plot are part of call type U101
The ultrasonic calls of Weddell seals were common in the dataset. Several of the ultrasonic call types are labeled in this spectrogram of a 10-minute recording from the MOO.
We’re not yet sure why the seals vocalize at these high frequencies. We speculate that one possible use for some of the sounds may be in a rudimentary form of echolocation, akin to the “biosonar” of dolphins and other toothed whales.
Listening to the echoes of their high-frequency sounds could help the seals find their way around under the sea ice, especially during the months of near-absolute darkness in Antarctic winter.
Or, the ultrasonic calls could just be for communicating with other seals - like switching to another communication channel when the lower frequencies are cluttered with other sounds.
Weddell seals are perhaps the most vocal of the seal species. The Antarctic under-ice soundscape is dominated by their sonic vocalizations (those that humans can hear) in areas where the seals are abundant.
Our results demonstrate how even small, relatively-inexpensive underwater observatories, like MOO, can contribute to our understanding of the marine realm.