The sealís life cycle
At the onset of the breeding season, the hormone levels of male seals change. They become aggressive towards other males, and begin to seek out females. Meanwhile, in females, the uterus has developed a fluid-filled sack containing an egg. Hormonal changes begin to occur in the female, making her receptive to the advances made by males.
The success of the species depends upon the female being mated by the biggest and strongest male. If she becomes pregnant by a weakling, it is more likely her offspring will also be weak, and its chances of survival will be reduced. The large, powerful males arrive at the breeding grounds and begin to fight other males to station themselves on the beach. The breeding season, which may last several weeks, is one long, continuous battle to keep other males away. A bull cannot risk going to sea to feed, because if he does, he might not be able to re-establish himself.
When the cows are receptive, the bulls move in, aiming to mate with as many as possible. By remaining on shore, the bulls make sure that they do not overlook any cow in their area. Copulation can last for up to 45 minutes, although most are quicker than this. Most cows are mated several times. A grey seal bull lies to one side of the cow with a flipper draped across her. He might also hold her neck in his teeth. Common seal mating takes place in the water, and it is difficult to see how many times a female might be mated.
All seals need to moult their old skin and hair once a year. Hormone changes prompt the seal to go to the moulting grounds. Grey seals gather together in large, noisy groups at this time. Juvenile seals usually moult first, followed by females, and finally by adult males, although there is overlap in the coming and going. The moult might take as long as six weeks to complete, during which time the seals are often irritable and listless. Once their new hair has grown, they head out to sea to resume feeding.
Once the egg is fertilised, there is a delay for about three months before the foetus starts to develop. The foetus grows for about nine months, at which point the female instinctively returns to the breeding grounds. When she arrives, she spends some time choosing a suitable place. Birth is normally quick, but despite the pup being long and spindle-shaped breech births are quite common. As soon as the pup is born, the female spins around to sniff it and to call to it. The pup and the mother bond, learning each otherís smell and voices within a few minutes of the birth.
Pups are relatively helpless, and rely totally on their motherís milk for the first few weeks. The milk is more than 50 per cent fat, and the pups grow very quickly, depositing a thick layer of blubber that will protect them from the cold and sustain them as they are learning to hunt for themselves. Grey seal mothers feed their pups with milk for 16 to 21 days, during which time the pup gains an average of 30 kg (66 lbs). During this period of intensive care, the mother might lose 65 kg (143 lbs) of her own body weight. When the female is forced to return to sea to feed, the pup might remain at the breeding ground for another 14 days or so before heading out to sea to forage for itself.
Common seals are different. The pups are less helpless than grey seal pups, and can swim with their mothers within a few hours. This may be an adaptation to avoid dangerous predators, like humans. Common seal pups are suckled for up to six weeks, twice as long as grey seal pups. This is because there is no need for the mothers to fast and remain with their pups all the time. They can return to sea to feed themselves between suckling bouts and so do not lose weight to the same extent as grey seal mothers.
Not all pups survive. Some get separated from their mothers before they have a chance to learn their smell and call. If the mother does not recognise her pup, she will not allow it to suckle from her. Others get trampled by bulls, or are orphaned. In a crowded breeding area, 15 per cent of the pups might die. But some pups are resourceful, and not only take the milk from their own mother, but from the mothers of other pups too.
The life of a seal
A female seal becomes pregnant for the first time when she is three to five years old (sometimes earlier in common seals), and gives birth to a pup a year later. Although twins are sometimes found in the wild, they are rare because it is difficult for a female to provide enough milk for two pups. After her pup is born and she has finished suckling it, she is mated by one or more of the dominant bulls present at the breeding site. She then returns to sea to feed and make up for the weight loss she has suffered while caring for her pup. Later she hauls out to shed her old coat and grow a new one, which usually takes around four weeks. When her new coat is ready, she returns to sea, hauling out occasionally near her feeding grounds. She heads back towards the breeding grounds when the pup that was conceived during the previous breeding season is ready to be born.
A grey seal male becomes sexually mature (that is, he is ready to mate) at about six years of age, although he is usually far too small to successfully compete with the dominant bulls, forcing him to prowl in the shallows, or fight his way onto a patch of land that no other bull wants until he is big and strong enough to be able to win his battles for a breeding territory. Most grey seals are more than 10 years of age before they can maintain a position on the breeding grounds long enough to mate.
Common seal males reach sexual maturity earlier than grey seals, and are ready to mate between three to six years of age. They are usually strong enough to mate when they are about six or seven. Once mating is over, the bulls return to sea to feed, hauling out a few weeks later to moult. Like females, they haul out occasionally to rest between fishing forays.
If they survive the dangers of being a pup, seals are relatively long-lived animals. Both species often live longer than 30 years and one female grey seal in the Shetland Islands was known to be 46 years old. Since many seals die at sea, it is difficult to know the major causes of death. Diseases caused by parasites, pollution, and drowning in fishing nets are some of the main reasons.