The coldest months of the year stir up the reproductive instincts in one of the rarest seals in the world. The last months of the year bear witness to the Grey seal breeding season. Female grey seals start to give birth during the autumn and routinely battle with the elements to raise their young.

Played out all around the British coastline, the drama is one of the most exhilarating wildlife experiences available to residents of the British Isles. The drama intensifies when the mothers stop suckling their young and start mating with their attendant males. The males compete for each other for the right to breed with groups of up to 10 females. Grey seals are sensitive to disturbance by people and dogs, particularly at this time of year. So learning about their lifecycle and behaviour will help us to better understand these marine mammals.

UK Grey Seal Population

Grey seals are the largest breeding seals found in the British Isles. 40% of the world’s population of grey seals live around British coasts, and numbers here have doubled since 1960. The population is thriving as a result of three factors. Hunting pressure decreased after the UK introduced the Grey Seals Protection Act in 1914.  Then the grey seal population numbered around 500. Human populations have declined in islands across the UK, resulting in more land becoming available for grey seals to breed. Finally, grey seals are adaptable in terms of the fish they eat.

Compared with other times of the year, grey seals spend long periods hauled out between October and January. They do so at traditional pupping sites called rookeries. Although the entire breeding season spans approximately 8 weeks, individual females will spend 18-20 days ashore.  During this time they each bear and suckle one pup. They come into oestrus towards the end of lactation (approximately 16 days after giving birth!) and mated.

These wild animals thrive in the turbulent waters of the Atlantic Ocean and the North Sea. They seem to be able to cope with natural disasters. For example, tidal surges and with a little help from us they recover quickly. How have they evolved and what are the key stages of their lifecycle?

Evolution

Seals (pinnipeds) took their own evolutionary path some 50 million years ago. They split from their nearest relatives – either bears or mustelids. The grey seal evolved into a streamlined marine mammal. With a tapered body, broad skull, long snout, flexible necks, limbs modified into flippers, and small tail, they are able to thrive in the waters around the British Isles.

One of the key evolutionary traits that seals share with both of their possible ancestors is delayed implantation. This embryonic diapause allows synchronised birthing and mating to take place sequentially when they gather at their coastal breeding colonies.

Timing for breeding coincides with severe weather on the seas and oceans of the northern hemisphere. Storms occupy the maritime history and folklore of our island nation.  We can empathise with the seals as they haul out on our beaches when the furious winds are icy cold and the tempestuous waves are crashing against rocks.

Puphood

Grey seals in UK waters breed in the autumn and winter. During the Grey seal breeding season they typically gather at remote offshore islands. Pupping starts during September in the southwest of England, Wales and Ireland. Here our grey seals favour sea caves as breeding grounds. Next to start in October are the colonies off the west and north coasts of Scotland. Here, the largest colonies are on remote uninhabited islands. For example, Causamul and North Rona. Smaller groups breed in coves and beaches. Lastly the east coast populations start to breed in November. The Isle of May and the Farne Islands are the most significant of the North Sea breeding populations. However, the most famous and popular with wildlife enthusiasts are the sandbank of Donna Nook in Lincolnshire and the North Norfolk beaches of Blakeney and Horsey.

Grey seal pups weigh up to 14 kilograms at birth. The mother’s first action is to sniff her pup in order to recognise its unique scent. The bond between them has to develop quickly to ensure successful suckling. The lactation period is brief. As their milk contains 60% fat, the pups quickly balloon. They develop the blubber layer essential for maintaining body temperature out at sea. However, in the critical first couple of weeks the pups appear clumsy on land. Furthermore, they will struggle with the cold if they enter the sea now. When they are weaned at 18 days old, the pups will have gained 30 kilograms. In this period they feed on about three litres of their mother’s milk per day.

Motherhood

There is little maternal care given by Grey seal females except suckling and protection of the pup. It is not uncommon for Grey seal mothers to adopt other pups.

As Grey seal pups gain weight the mothers lose weight dramatically because they fast during the two and a half week lactation period. They use stored fat to sustain their own needs as well as the enormous drain of energy in producing such high fat milk. While they are suckling young they lose as much as 100 kilograms in weight! It is possible to identify a female who has recently given birth to one ready to mate at the end of lactation by their differences in shape.

Breeding Male Dominance

During the pupping season, the polygynous male grey seals also come ashore in readiness to mate. The largest males, usually no less than 10 years old, compete for a position within groups of breeding females. Occasionally males fight, and may sustain deep scars on their necks as a result.

The Grey seal breeding season, which may last several weeks, is one long, continuous battle for the dominant male to keep other males away. The advantage is with the older and larger bulls that are able to stay ashore longer because they have larger energy stores. They too lose weight during this period, but at a rate approximately half of the females’. A bull cannot risk going to sea to feed, because if he does, he might not be able to re-establish himself. Instead he will patrol in the water just offshore from the nursery site, defending his breeding group from any competitors. Relations between established males are typically peaceful. It is only when unattached males try to muscle in with a breeding group that threats turn to bloody combat.

By remaining ashore, the bulls make sure that they do not overlook any cow in their area. When the cows become sexually receptive at the end of lactation, the bulls move in, aiming to mate with as many as possible. The longer the males stay ashore, the more offspring he will sire.

Mating

Copulation can last for up to 45 minutes, although most are quicker than this. The male lies to one side of the female with a flipper draped across her and he might also hold her neck in his teeth. Sometimes a female will respond aggressively to the advances of a male typically when its too soon for them to mate but also when they are in oestrous. The female Grey seal’s aggressive behaviour includes:

• Snarling: “opening mouth in the direction of intruder, often with added accompaniment of a growl, moan, or hiss”
• Flippering: “rapid, stiff flapping of the fore flipper with claws flexed outwards”
• Lunging: “extending neck in a serpentine manner; escalating to biting and scratching with claws”.

This behaviour might encourage other males to take in an interest in the female. Most female Grey seals mate several times during their breeding season.

Dispersal

Within a day or two of mating the females disperse to feeding grounds either in the vicinity of the breeding colonies or further afield. Males will typically return to the sea after three weeks ashore. The females leave their weaned pups behind to fend for themselves. The pups shed their white fur and head into the water to fish for themselves. There is some segregation of sexes and age groups, but pups tend to stay relatively close to where they were born.

Outside of the Grey seal breeding season little is known of their behaviour when they forage in the open sea. They frequently travel over 100 km, with foraging trips lasting anywhere between 1 and 30 days. When feeding, they can dive to a depth of 70 metres.

Dispersal at sea, synchronised pupping and the short mating season is enabled by delayed implantation. The blastocyst (fertilised egg) lies dormant in the uterus for 4 months, the embryo develops in the spring under the influence of hormones and gestation lasts 8 months. And the cycle continues.

Author: Tim Plowden

References:

1. Seals (British Natural History Series) by Sheila Anderson.