The Harp Seal, Pagophilus groenlandica, is a highly migratory, medium-sized phocid, or earless seal, found across the North Atlantic and Arctic Oceans. These animals spend the majority of their lives in the water, hauling out onto the ice only for breeding, molting, and giving birth. An individual Harp Seal’s weight is not static; it changes dramatically based on age, sex, and the time of year, reflecting a life cycle dictated by periods of intense feeding and prolonged fasting.
Adult Weight and Dimensions
Mature Harp Seals typically range in length from about 1.7 to 2.0 meters (5.5 to 6.5 feet). A fully grown adult can weigh between 130 kg and 180 kg (287 to 397 pounds), depending on stored body fat. While slight sexual dimorphism exists—an average male weighs approximately 135 kg compared to 120 kg for a female—the time of year plays a larger role in an individual’s mass. The maximum weight of 180 kg is usually reached just before the breeding and molting seasons, when the seals have maximized their energy reserves.
Weight Dynamics Seasonal Changes and Blubber
Harp Seals spend the summer and fall in intensive feeding periods, building up energy stores in preparation for winter breeding and molting. During this time, they accumulate a thick layer of blubber, which is the primary form of energy storage for marine mammals. This blubber layer provides insulation against frigid Arctic waters and is metabolized for energy during periods when the seals cannot feed. Harp Seals enter a period of prolonged fasting during the breeding and molting seasons in late winter and spring, during which they can lose more than 20% of their total fat reserves. Consequently, the minimum weight for an adult can drop to as low as 85 kg at the end of the fasting and molting periods.
Weight Loss During Nursing
Females undergo significant weight loss while nursing their pups, as they typically do not feed during the lactation period. They can lose about 3 kg per day during this time. During the approximately four weeks spent molting on the ice, seals rarely feed.
The Incredible Growth of Harp Seal Pups
Newborn pups, known as “whitecoats” due to their thick, insulating fur, weigh only about 11 kg (24 pounds) and measure around 85 cm in length. They are born without a thick blubber layer, relying instead on their mother’s milk for rapid energy gain. The nursing period is short and intense, lasting only about 12 days. During this time, the mother produces milk with an extremely high-fat content, which can increase from an initial 25% to nearly 48% fat by the end of the nursing period. This rich, energy-dense milk allows the pup to gain an astonishing amount of weight, often exceeding 2.2 kg (4.9 pounds) per day. By the time they are abruptly weaned, a Harp Seal pup will have more than tripled its birth weight, reaching a mass of 35 to 45 kg (77 to 99 pounds). After the mother departs, the newly weaned pup must rely on this newly acquired blubber store, fasting for up to six weeks before it begins to hunt, during which time it can lose up to half of its body weight.