Читать книгу Nature’s Top 40: Britain’s Best Wildlife - Chris Packham - Страница 12

The autumnal months of September to November are not usually considered the traditional or best time to spend a day down at the beach or on an island-hopping sojourn. But this is exactly the time to visit one of a few special coastal locations if you want to catch up with grey seals as they pup – a tremendous wildlife spectacle that is one of our best-kept secrets and of which Britain as an internationally important player should be justifiably proud.

Seal pups

WHEN

From early September to November depending on the location, with southern and western sites pupping first

WHERE

Monach Isles, Outer Hebrides; Orkney Islands; Donna Nook, Lincolnshire

A pup starts to moult into its adult coat in preparation for a chilly life at sea.

The grey seal is only one of two species of seal that breed along the British coastline, the other being the common seal, which, ironically, is the rarer of the two in Britain’s waters. The wildlife novice can encounter difficulty in identifying these two superficially similar species, but it helps if you know that the grey seal’s wonderful Latin name Halichoerus grypus translates as ‘Hook-nosed sea-pig’! While hardly flattering, this translation goes some way to describing the looks of the greys: their elongated muzzle contrasts with the ‘snub-nosed’ appearance of the more diminutive common seals. Another vital distinguishing feature is the nostrils: in the grey seals the closed nostrils appear like parallel slits; in the common they are splayed to form a V shape.

The size and shape of the grey seal differs between the sexes too. The adult males or bulls are much larger and heavier than the females, with a massive pair of shoulders where the skin over this region and the chest consists of heavily scarred folds and wrinkles. Bulls have a distinctive and convex snout, giving them a ‘Roman nose’ profile, while the females, or cows, look more slender and seal-like by comparison. Although ‘grey’ is a reasonably accurate colour rendition of most of the seals, there is much variation, with some bulls being virtually black, while some cows are creamy-white with a few dark blotches.

It is thought that the grey seal’s historic distribution was in a broad swathe across the entire northern Atlantic, but the advance of the ice during the last Ice Age 20,000 years ago, split the seals into a western and eastern stock. The western Atlantic population can be found along the coast and islands of Canada’s eastern maritime provinces, and, apart from a small relict population in the Baltic Sea, the convoluted British coastline is thought to hold the vast majority of the eastern stock, with current numbers estimated at around 124,000 seals or 40 per cent of the world’s population.

The breeding and mating of grey seals is very much an autumnal phenomenon, and the species is unique in being the only seal that lives in a seasonal environment yet produces its young at a time when the newly independent pups will have to contend with winter storms. It is possible that, being a species that is very vulnerable on the breeding ground, the grey seals have changed their breeding season comparatively recently as a response to prehistoric predation. The peak of breeding also occurs in different months in different parts of the range: pups are born in early September in southwest England and south Wales; from late September to early October in western Scotland; and November at sites on the east coast.

A picture of harmony as mothers and young grey seal pups bond on the beach.

Laurie Campbell

The breeding season begins with the arrival of the sleek, fat females that have spent the summer fattening up on a large variety of fish, of which sand eel, cod and Dover sole are thought to feature heavily. Favoured breeding locations tend to be isolated and uninhabited islands with smooth, sandy beaches; a few choice mainland beaches are also regularly used. In areas with a lower breeding density of grey seals, such as in the southwest of England, caves are often used for giving birth and mating. Close to the time of giving birth, the cows will haul themselves to favoured pupping spots usually about a day before giving birth. These spots vary from the beach just above the high-tide line to a location several hundred metres from the water, such as grassy dune slacks.

Grey seal labour is not very obvious; the birth of the 14-kilogram pup can often occur rapidly, with the first sign being the sight and sound of a host of wheeling gulls as they squabble among themselves for the membranes that enclosed the pup and the afterbirth. Immediately after the birth the cow will sniff and touch the pup a number of times to learn its smell.

It’s Operation Weight Gain as the creamy white pups do their best to treble in weight in less than three weeks.

Andy Rouse

The most startling thing about the newborn pup is its creamy-white fur coat, which is thought to reveal the British grey seal’s ice-breeding ancestry. Further north in the greys’ range, many of the cows still give birth on the ice, meaning the pups are perfectly camouflaged, but, since the retreat of the ice from the British Isles after the last Ice Age, the pups have not been pressurised by predators to change colour and so stand out like a sore thumbs. In a technique still designed to offset predation by swamping the predators, the vast majority of females give birth with a remarkable synchrony, with the result that, on a dense grey seal pupping beach, the white pups can be seen regularly studded along the entire length of the beach with their mothers in attendance.

Initially upon birth, the pups are very poorly coordinated and would suffer quickly from the cold if they were to enter the water inadvertently. The cows stay close to the pups and react aggressively towards any other seals or gulls that come too close, and within a few minutes the pup will begin trying to suckle. Sometimes it takes a while for the newborn pup to locate its mother’s nipples, as they are set towards the tail-end of the body and are usually inverted to aid the seal’s streamlining in the water. However, some gentle prodding from the pup’s muzzle causes the nipples to pop up and it then latches on with its specially indented tongue and begins to suck. Feeding bouts last no longer than about ten minutes on average, and the pup is fed at five- to six-hourly intervals on the incredibly rich milk, which is 60 per cent fat and resembles mayonnaise in consistency. The growth of the pup is slow for the first day but then increases at a phenomenal rate so that the pup will have doubled its weight in the first week. By the end of the lactation period, which lasts between 16 and 21 days, the pup will have increased by an incredible 3 times its birth weight. As the cows feed their voracious pups, they will begin to lose weight in a manner that would have many human mothers green with envy; they lose their rotund shape by shedding four kilograms a day during this intense period.

The bulls generally come ashore when the first pups are born and spend the lactating period competing with the other males for sole access in among groups of breeding females. Like the females, the males do not feed during the breeding season and live off the stores of fat that have been laid down at sea over the previous ten months, meaning that they can devote their entire attention to garnering as many females as possible. For the successful grey seal bulls, size matters, with the largest males, or ‘beach-masters’, often retaining as many as ten cows in their harems. Relations between neighbouring males are often amicable, but they can also result in brutal fights when two evenly matched individuals clash over ownership of the females.

Towards the end of the lactation period, the cows come into season and are then mated with their respective bull a number of times prior to the females leaving the beach and abandoning the pups to their fate. The gestation period of grey seals is only seven months, yet the cows will not be giving birth until they come ashore the following breeding season a full ten months later. They use a combination of suspended development of the fertilised egg and delayed implantation to ensure that the birth will be synchronised to take place at the same pupping beach the following year. Very little is known about where the seals spend their time once away from the breeding ground and they may roam far and wide in order to find the five kilograms of food a day they need to get through the winter.

At around three weeks and just as their mothers begin to leave them, the pups will shed their white fur to reveal the sleek dark coat needed to survive in the sea. It is no surprise that the first few months of life are the most dangerous for grey seal pups, with mortality reaching as high as 40 per cent on some crowded beaches where they can easily starve or be squashed by a careless bull. The process of learning to feed out at sea without any parental help inflicts further casualties, meaning that two-thirds of grey seal pups never reach their first birthday. After this first year, however, their chances of survival become much better, with the cows reaching sexual maturity at around four and living anywhere up to the grand old age of 25 years old. Male seals often do not reach sexual maturity until six, but it is thought that they are not able to hold a position in a breeding group until at least ten, and, even then, will only have a few years at the top before their physical decline leads to them becoming marginalised by the new generation of younger and stronger bulls.

Prior to the Grey Seal Act of 1914, seals were so heavily hunted that numbers in the British Isles were thought to be as low as 500 individuals, but thanks to this and subsequent laws, the population has boomed to a current all-time high of around 124,000 grey seals. While an unmitigated success story for conservationists, grey seals are a contentious issue for fisherman, who see them as competition for fish stocks.