Pinnipeds, the group of marine mammals that includes seals, sea lions, and walruses, are highly social animals that rely on sound to navigate their lives in both aquatic and terrestrial environments. While the familiar, loud “bark” is the most recognized vocalization, it represents only one component of a diverse and sophisticated auditory communication system. These marine carnivores employ a wide range of sounds, from guttural roars to high-frequency whistles, to interact with their environment and each other. Understanding why seals bark requires looking beyond the single sound to the full context of their complex vocal repertoire.
The Anatomy of Seal Sounds
The production of sound in pinnipeds is fundamentally similar to that of land mammals, relying on the movement of air through the larynx or voice box. Air is forced from the lungs across the vocal folds within the larynx, causing them to vibrate and generate sound. The resulting sound is then filtered and shaped by the pharynx, mouth, and nasal cavities, producing the species’ unique calls.
The anatomy of sound production varies subtly between the two major seal groups, the eared seals (Otariids, like sea lions) and the true seals (Phocids). Otariids typically produce sounds that are effective in the air, such as the loud bark, which is generated by the rapid, forceful expulsion of air over the vocal folds. True seals, which spend more time in the water, often rely on specialized laryngeal structures or air manipulation to produce complex underwater vocalizations.
The Communication Functions of Barking
The characteristic barking sound serves several distinct functional roles, particularly in species that congregate in large numbers on land during the breeding season. The primary function of the bark is to manage social interactions, which are often highly competitive and dense on terrestrial haul-out sites. For male sea lions, incessant barking is a form of acoustic display used to establish and maintain the boundaries of their breeding territories.
During the peak of the breeding season, the male California sea lion’s deeper bark operates as a long-range signal of dominance and territory ownership, warning rival males to stay away. This vocal posturing is a lower-cost alternative to physical confrontation, helping to establish the male’s position in the dominance hierarchy. Males also use barking and other related vocalizations in close-range courtship displays directed at females within their territory.
A crucial function of vocalization occurs in mother-pup recognition, especially on crowded rookeries. A female returning from a foraging trip will emit a loud, species-specific call, which can be a trumpeting vocalization or a high-pitched bark, to locate her offspring among hundreds of others. The pup responds immediately with a distinctive bleating or mother-response call, allowing the pair to quickly confirm identity by sound before a final check by scent.
Species Variation in Vocal Behavior
The frequency and context of barking vary dramatically across the pinniped family, reflecting differences in their social structure and environment. The California Sea Lion (an Otariid) is the quintessential “barker,” using its loud, airborne vocalizations to manage its highly social, polygynous breeding colonies on land.
In contrast, the true seals (Phocids) generally exhibit a quieter, less frequent use of aerial barking, instead focusing on an elaborate repertoire of underwater sounds. For example, the Weddell seal, an Antarctic phocid, produces a stunning array of complex underwater sounds, including chirps, whistles, and trills, with some calls reaching ultrasonic frequencies up to 50 kilohertz. These calls are used by males for territorial advertisement beneath the ice and possibly for a rudimentary form of echolocation in the dark environment.
Environmental factors like human-generated noise pollution also influence vocal behavior and call design. For species that rely on acoustic communication, such as the Weddell seal, ship noise can mask their calls, reducing the effective range of their communication. Some harbor seal pups, a phocid species, demonstrate vocal plasticity by actively lowering the fundamental frequency of their calls in response to increased background noise, a modulation that helps their sounds stand out against the ambient clamor.