The harbor seal (Phoca vitulina) is a true seal, or phocid, widely distributed across the temperate and Arctic coasts of the Northern Hemisphere. These streamlined marine mammals are commonly observed hauling out on rocky shores, sandy beaches, or ice floes in coastal waters. The question of how they manage to survive in these cold aquatic environments comes down to their unique body covering and internal structure. The simple answer is that harbor seals possess specialized hair, but it is not what primarily keeps them warm; instead, they rely on a thick layer of fat beneath their skin for insulation.
The Harbor Seal’s Unique Coat
The harbor seal’s coat (pelage) is short, stiff, and coarse to the touch. This coat is composed of bundles containing one thick guard hair and several finer underhairs, which are significantly less dense than the underfur of marine mammals like sea otters or fur seals. Unlike a sea otter’s fur, the harbor seal’s coat does not trap an insulating layer of air when submerged. Due to this structural difference, the hair provides negligible thermal insulation against cold water temperatures.
The primary functions of the hair are mechanical protection, waterproofing, and hydrodynamic streamlining. The smooth, short nature of the coat helps water flow efficiently over the body, reducing drag as the seal swims and forages underwater. On land, the hair offers protection to the skin from scrapes, mechanical damage, and UV radiation while the seal is hauled out.
The pelage also provides effective camouflage, which is important for seals resting on varied coastal substrates. Harbor seals exhibit two main color phases: a light phase with dark spots, and a dark phase with light rings. This unique spotted pattern helps them blend in with the mottled appearance of rocks and sand.
Blubber: The Primary Insulator
Blubber is a thick layer of subcutaneous fat beneath the skin that serves two distinct biological purposes: an energy reserve and the main thermal insulator. The insulating properties of blubber are so effective that they allow the seal to maintain a constant core body temperature of about 37.8°C (100°F) in water.
This fatty tissue works by creating a thermal gradient, with the outer layers of blubber matching the ambient water temperature while the inner layers near the muscle remain warm. To fine-tune their temperature control, true seals can adjust blood flow to the blubber layer; constricting blood vessels reduces heat loss, while increasing blood flow releases excess heat, particularly through poorly insulated areas like the flippers. In winter, blubber can account for up to 27% to 30% of a northern Pacific harbor seal’s total body mass, highlighting its role as both an energy store and insulation.
Annual Cycle of Molting
Harbor seals undergo molting, the annual process of shedding and replacing their entire coat of hair. This event typically occurs in the late summer or early autumn, usually a few months after the pupping season. The duration of the molt is often one to two months, during which the seal’s behavior changes significantly.
During this period, seals must spend much more time hauled out on land to complete the process successfully. The body shunts blood closer to the skin’s surface to provide energy for new hair growth. Remaining in cold water while this blood is near the surface would cause severe heat loss, forcing them to seek warmer air temperatures on shore.
Harbor seals may reduce their foraging and feeding activity while molting, which is linked to a temporary reduction in their metabolic rate. Hauling out for extended periods during the molt allows the seals to conserve energy that would otherwise be spent maintaining body heat in the water. The timing of the molt can vary depending on the seal’s age, sex, and geographic population.