Marine mammals have developed remarkable adaptations to thrive in diverse aquatic environments. Seals, in particular, possess a unique sensory system that allows them to navigate and hunt with precision. Unlike many land animals that rely on prominent external ears, seals present a different auditory picture. Their hearing apparatus is a testament to evolutionary specialization, enabling them to perceive sounds both above and below the water’s surface.
External Appearance of Seal Ears
True seals, belonging to the Phocidae family, lack the visible external ear flaps, or pinnae, found in many other mammals, including their relatives, the eared seals (Otariidae), which encompass sea lions and fur seals. Instead, true seals have small, inconspicuous ear openings located on the sides of their heads. These simple openings lead directly into their ear canals.
The absence of external ear structures in true seals is a direct adaptation to their aquatic lifestyle. This streamlined design helps reduce drag and turbulence as they move through water, enhancing their hydrodynamic efficiency.
The Science of Seal Hearing
Without external ear flaps, seals primarily rely on bone conduction for underwater hearing. Sound waves in water are transmitted directly through the dense bones of their skull to the inner ear, bypassing the need for an external ear canal or eardrum to capture vibrations. This allows them to efficiently process acoustic information in a dense medium like water. Their middle and inner ear structures are adapted for this function, with features like massive middle ear ossicles and a dense bulla, which is the bony capsule housing the inner ear.
The middle ear cavity in seals also contains cavernous tissue and blood sinuses that can inflate during diving, aiding in pressure equalization and sound transmission. Seals also possess sensitive vibrissae, or whiskers, on their snouts. These vibrissae detect vibrations and movement in the water, complementing their auditory sense by providing tactile information about their surroundings and aiding in prey detection, especially in low-visibility conditions.
Hearing in Different Environments
Seals demonstrate versatility in their hearing capabilities, adapting to both aquatic and terrestrial environments. Underwater, their hearing is developed and often surpasses their aerial hearing. For instance, harbor seals detect sounds from 1 to 180 kHz underwater, with peak sensitivity around 32 kHz. In contrast, their hearing range in air is reduced, typically from 1 to 22.5 kHz, with peak sensitivity at 12 kHz.
Their ears are equipped with muscles and a cartilaginous mechanism that can tightly close the external ear canal when submerged, protecting the inner ear from water and pressure changes. This closure mechanism is part of their amphibious adaptation, allowing them to switch between modes of hearing. While their underwater hearing is acute, their hearing on land is still functional and comparable to many terrestrial carnivores, although less sensitive than their aquatic performance.