Harp seal

The harp seal (Pagophilus groenlandicus), also known as the saddleback seal or Greenland seal, is a species of earless seal, or true seal, native to the northernmost Atlantic Ocean and Arctic Ocean. Originally in the genus Phoca with a number of other species, it was reclassified into the monotypic genus Pagophilus in 1844. In Greek, its scientific name translates to "ice-lover from Greenland," and its taxonomic synonym, Phoca groenlandica translates to "Greenlandic seal." This is the only species in the genus Pagophilus.

The mature harp seal has pure black eyes. It has a silver-gray fur covering its body, with black harp- or wishbone-shaped markings dorsally, accounting for its common name. Adult harp seals grow to be 1.7 to 2.0 m (5 ft 7 in to 6 ft 7 in) long and weigh from 115 to 140 kg (254 to 309 lb). The harp seal pup has a white coat for the first 2–3 weeks until the first molt, when it is replaced by a black-dotted silver to gray coat. Pups acquire their characteristic pattern once they near sexual maturity. In males, the transition to the harp-pattern tends to be abrupt, while in females it may be gradual and span years. Some females may never lose all their spots or not fully develop the harp-pattern. Harp seals show little sexual dimorphism, with males being slightly larger.

The harp seal is a modest diver. Dive depth varies with season, time of day and location. In the Greenland Sea sub-population, the average dive rate is around 8.3 dives per hour and dives range from a depth of less than 20 m (66 ft) to over 500 m (1,600 ft). Dive duration ranges from less than two minutes to just over 20 minutes. During the spring and summer when seals forage along the pack ice in the Greenland Sea, most dives are less than 50 m (160 ft). In the late fall and winter, dive depth has been found to increase, particularly in the Denmark Strait, where the mean dive depth was found to be 141 m (463 ft).

Lactating female harp seals spend about 80% of the time in the water and 20% of the time on the fast ice, weaning or staying near their pups. However, almost half of the time spent in the water is at the surface, well beyond what is expected to recover from dives. This behavior allows the mother harp seal to conserve energy and avoid the harsh conditions of the fast-ice while remaining near her pup. As with most phocids, she requires vast amounts of energy to ensure sufficient mass transfer to her growing, weaning pup. Harp seals remain within their aerobic dive limit for 99% of dives.

Harp seal insulation changes over the course of a seal's lifetime. Young harp seals rely on a lanugo pelt from nursing all the way up to their weaning age. The insulating quality of this fur depends on its ability to keep a layer of air trapped inside or between the hairs. It takes a year for their blubber to develop and for their first-year pelage to grow. This transition from thick lanugo fur to blubber is important because lanugo fur does not insulate well in water. Adult harp seals primarily use blubber for insulation.

Harp seals combine anatomical and behavioral approaches to managing their body temperatures, instead of elevating their metabolic rate and subsequently their energy requirements. Their lower critical temperature is believed to be under −10 °C (14 °F) in air. A thick coat of blubber insulates its body and provides energy when food is scarce or during fasting. Blubber also streamlines its body for more efficient swimming. Brown fat warms blood as it returns from the body surface as well as providing energy, most importantly for newly weaned pups. This blubber insulates the harp seal's core but does not insulate the flippers to the same extent. Instead, the flippers have circulatory adaptations to help prevent heat loss. Flippers act as heat exchangers, warming or cooling the seal as needed. On ice, the seal can press its fore flippers to its body and its hind flippers together to reduce heat loss.

They can also redirect blood flow from the periphery to minimize heat loss; the nostrils and eyes of harp seals are adapted to conserve heat, possessing a countercurrent heat exchange system and retia mirabile, respectively.

The harp seal's eyes are large for its body size and contain a large spherical lens that improves focusing ability. Its mobile pupil helps it adapt to the intense glare of the Arctic ice. Its retina is rod-dominated and backed by a cat-like and reflective tapetum lucidum, enhancing its low light sensitivity. Its cornea is lubricated by lacrimal glands, to protect the eye from sea water damage.