The female harp seal reaches sexual maturity between four and eight years old. The male reaches sexual maturity at around the same age as the female, but does not begin to mate until it is older (1). Mating usually occurs on the pack ice, with males fighting for access to females (2). This is followed by a period of delayed implantation, in which the embryo may remain within the female’s womb for up to three months before implanting in the uterus wall, ensuring the birth of pups is synchronised between females, when there is plentiful pack ice (1) (4). The total gestation period of the harp seal is around 11.5 months (1).
The harp seal gives birth between late February and April, with the exact timing varying between populations (1). Females haul out onto the pack ice in large groups (4), where they space themselves 1.5 to 2.5 metres apart (2). A few days after hauling out, the female gives birth to a single pup (2) (4), which generally weighs around 11 to 12 kilograms (2) (4) and is roughly 1 metre long (2). The female nurses the pup for around 12 days with fatty milk. During this time, the pup gains around 2.2 kilograms per day in weight, and will develop a thick blubber layer to protect it against the cold (1) (4). Pups are usually left alone on the ice for long periods, where they typically remain sedentary, and will occasionally melt the ice beneath them, creating an ‘ice cradle’ (1). While nursing, the female will spend up to 85 percent of the day in the water hunting for food (1). Towards the end of the nursing period, the female enters oestrus and mates again (1) (4).
When it is weaned, the pup weighs around 33 kilograms (2). It will remain on the ice for a further six weeks and will fast, losing around 50 percent of its body weight before the pack ice melts and the pup can enter the water and begin to hunt for itself (1) (2) (4). During this time, the pup also begins to moult, exposing its silver-grey juvenile fur (2) (4).
The harp seal is a highly migratory species (1), undergoing an annual migration of between 6,000 and 8,000 kilometres (2) (3). The migration north begins after the breeding season, with the seals returning to the breeding grounds around late autumn (1), in large, noisy groups (3). The migration north is paused for an annual moulting season in the spring, when harp seals haul out in large groups of tens of thousands onto the pack ice (2). This annual moult sees the harp seal develop the characteristic harp-shaped colouration on the upperside of the body, which develops further with each moult. After this period, migration begins once more to the summer feeding grounds (4).
The harp seal is able to move fast on ice, as well as being a powerful swimmer (2). It uses its flippers to propel itself through the water (4), while the hind and fore flippers are also used to fight for access to females, as well as for heat regulation (4). The fore flippers are held close to the body and the hind flippers are held together, reducing the exposed surface area of the body to retain heat. A thick blubber layer also helps the harp seal to insulate itself, as well as being used as a source of energy when prey is not readily available (4).
The diet of the harp seal is extremely variable and is dependent on age, location, season and year (4). Small fish are the main constituent of the adult harp seal’s diet, while pelagic crustaceans are also eaten (1) (2) (4). As a newly-weaned pup, the harp seal forages near the surface of the water, usually taking small crustaceans (2), with juveniles eating a similar invertebrate diet, including euphausiids (Thysanoessa spp.) and amphipods (Parathemisto spp.) (1). Natural predators of the harp seal include the polar bear (Ursus maritimus), orca (Orcinus orca) and Greenland shark (Somniosus microcephalus) (1) (4).
The harp seal has very sensitive ears and eyes, which are supplemented with well-developed whiskers, thought to respond to low frequency vibrations and used to detect prey. The large lens and mobile pupil of the eye give the harp seal its skilled vision, as well as enabling it to see in dark and bright light. The lack of tear ducts in the eyes of the harp seal ensure they are constantly lubricated, protecting the eyes from the seawater (4).
The harp seal’s sense of smell is not as acute as its other senses (2). However, the female is able to use its sense of smell to identify its own offspring while nursing, and may also use it to detect predators on the ice (4).