Their ability to hear underwater is similar to that of humans in air. Large sensitive eyes are well developed for nocturnal vision and bright light, in air or water. Their sense of smell is less acute. Harp seals live up to 40 years of age. They are sexually mature at 6 years of age.
Courtship displays are elaborate, and fighting with teeth and flippers is common.
Mating is promiscuous and occurs in late March about two weeks after the pups are born. There is a delayed implantation of the embryo with a gestation period of 7 months starting in June. Females concentrate in the Gulf of St. Lawrence and give birth between late February and early March. The mother's milk is rich in fat enabling the pup to grow rapidly for the first two weeks, after which the mother abandons the pup. Restlessness and hunger lead them into the water where they begin feeding on krill and start moving northward.