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. 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. An interesting case demonstrates a breeding difference between populations.