Gray seals have a characteristically long head with a curved nose. Adult female gray seals are about 7.5 ft long when mature and weigh about 550 lbs; adult males can reach up to 10 ft in body length and weigh about 880 lbs. Underwater sounds described as low frequency clicks, growls, knocks, and roars have been recorded for the species. Image credit: NOAA Fisheries.
Gray seals (Halichoerus grypus) are sexually dimorphic belonging to the “true seal” family (Family Phocidae). Their large, long head with a curved nose is characteristic for the species. They are often referred to as “horse-heads” and their scientific name means “hook-nosed sea pig” in Latin. Male gray seals have a more pronounced, longer nose than females, are larger in size, and tend to be more uniformly dark in color when mature.
The seals have a coastal distribution that extends across the North Atlantic Ocean, from the U.S. mid-Atlantic, through Northeast Canada, to the Baltic Sea in northern Europe. They gather in large groups along sandy beaches, islands, and/or icy shores during the mating/pupping season (September through March) and during their annual spring molt. These seals are opportunistic foragers, feeding on a variety of demersal and benthic fishes and cephalopods.
Gray seals produce sounds in air and underwater. In captivity, seals have been recorded underwater making rapid, pulsed sounds (or “clicks”) emitted at rates of 70-80 pulses/sec (similar to the buzz produced by some odontocetes). Other underwater sounds produced by gray seals in captivity have been described as humming, moaning, and yodeling. These sounds, and the pulsed calls, were only produced when the seals interacted with each other. Scientists hypothesized the underwater sounds have a social communication function.
Underwater vocalizations have also been recorded in the field for gray seals. These low frequency calls, described as growls and throaty “rups” and “rupes” were recorded during the breeding season in January. Low frequency growls occur from 100 to 500 Hz, whereas the throaty sounds have a broad frequency range of 100-3000 Hz. Low frequency clicks (3000 Hz), knocks (similar to that produced by walrus), and roars have also been recorded for wild gray seals. Vocalization rates and intensity increased as social activity, especially agonistic interactions between seals, increased during the breeding season.
Sounds described as “knocks” for gray seals may be non-vocal, percussive sounds produced by the seals. Underwater video footage recorded during the breeding season in the waters of northeast England, show a male gray seal “clapping” its front flippers together. The clapping motion produces an underwater sound less than 0.01 s in duration with maximum frequencies greater than 10 kHz. More observations are needed to fully understand this behavior.