An interesting case demonstrates a breeding difference between populations. Land-breeding gray seals are polygynous, with males competing to monopolize matings with as many as 7 females. Ice-breeding seals do not appear to be polygynous. Due to the instability of the early January ice, little is known of their habits. However, initial research indicates that a more monogamous system exists.
Mating System: monogamous ; polygynous
The breeding season of the grey seal varies greatly, occurring anywhere from mid-December to October, depending upon the location of the population. Breeding rookeries are formed on various types of habitat including sandy beaches, rocky islands, coasts, caves, and ice. During the months prior to the breeding season, seals actively feed. The females do so to grow for the future developing fetus and to build the fat reserves which will sustain them and the calf for the fasting which follows the birth, usually lasting for three weeks. The males also actively feed, because they too will fast for the breeding season, however their fasting will typically last for up to six weeks. The males ordinarily enter the rookeries once the females give birth and try to gain sole access to groups of females. Territory-related fighting occurs during the breeding season, although it is relatively minor compared to other seal species. Fighting in grey seal communities differs among populations, but generally increases as does the density of females. The successful males are able to mate with up to ten females, depending upon locality and density of the females.
Sixteen percent of female grey seals are sexually mature on their third birthday and give birth to their first young one year later. This figure rises to seventy-one percent by the fourth year and eighty-nine percent by the fifth year of life. The males also become sexually mature at age three, but due to competition for females, rarely mate before they are eight years old.
Once impregnated and following a gestation period of eleven months, females usually give birth a day after coming ashore at the rookery. Grey seals are attentive mothers and defend their pups against predation and intrusion. The pup is nursed for approximately 2 weeks after it is born, gaining around 1.5 kg per day. Once the pup is weaned, the female mates with one or more males and then leaves the pup at the rookery. The pup will remain on land, living off of its blubber reserves until it has fully molted, at which point it will feed at sea. The young seals generally disperse in many different directions from the rookery and are known to wander to distances of over 1,000 km.
Breeding season: The breeding season of the grey seal varies greatly, occurring anywhere from mid-December to October.
Average number of offspring: 1.
Average gestation period: 11 months.
Average weaning age: 14 days.
Average time to independence: 14 days.
Range age at sexual or reproductive maturity (female): 3 (low) years.
Range age at sexual or reproductive maturity (male): 3 to 8 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); viviparous
Average birth mass: 14000 g.
Average gestation period: 240 days.
Average number of offspring: 1.