How Big Is a Gray Seal? Size, Weight and Sex Differences

How big are gray seal bulls and cows?

The gray seal is one of the largest seals in the North Atlantic, and the size gap between the sexes is easy to see on any breeding beach. Bulls have massive necks and shoulders, broad muzzles and long, heavy heads, while cows are slimmer, with shorter, narrower snouts.

Average weights show the difference between the two sides of the ocean. In studies of adult seals, males in Britain averaged roughly 230 kg (510 lb) and females about 155 kg (340 lb), while in Nova Scotia, Canada, males averaged about 295 kg (650 lb) and females about 225 kg (495 lb). Genetic studies suggest the eastern and western Atlantic populations have been separate for a very long time, which may help explain why they differ.

Gray seals keep growing for years. According to Animal Diversity Web, males reach their full size at around 11 years of age, while females grow more slowly and may not reach full size until about 15.

Why are male gray seals so much bigger?

The size difference comes from the way gray seals breed. On crowded beaches, the largest and strongest bulls hold positions among groups of females and try to mate with as many as possible, sometimes up to about ten in a season. Rivals threaten each other with open-mouthed displays and fight, biting at the neck and shoulders, which is why old males are heavily scarred.

Size helps in two ways. A heavy bull is more likely to win fights, and a large store of blubber lets him stay on the beach, without eating, for as long as several weeks. Smaller males can still mate, sometimes in the water or at the edges of colonies, so size is an advantage rather than a guarantee. Over many generations this competition has favored large males, a process called sexual selection.

Gray seals are a moderate example. In elephant seals, competition is even fiercer and the size gap is far larger: a bull elephant seal can weigh several times as much as a female, as explained in How big is an elephant seal?

Gray seal vs harbor seal: which is bigger?

In Europe and on the US East Coast, gray seals often share beaches with the harbor seal, also called the common seal. The gray seal is clearly the larger of the two, and male gray seals in particular dwarf harbor seals.

Size alone can mislead, because a young gray seal can be no bigger than an adult harbor seal. The head is a better clue: gray seals have a long, straight profile and nostrils that sit almost parallel, while harbor seals have a rounded forehead, a short muzzle and V-shaped nostrils.

How big is a gray seal pup?

A newborn gray seal pup weighs about 11–20 kg (24–44 lb), with NOAA giving an average of about 16 kg (35 lb). It is born in a thick, creamy-white coat and looks thin and baggy at first, because it has very little blubber.

That changes quickly. For roughly 15–21 days the pup drinks its mother’s milk, which can be up to about 60% fat, far richer than cow’s milk. It gains around 1.5–2 kg (3–4 lb) a day, and by weaning it has tripled or quadrupled its birth weight and turned into a round, fat ball of blubber.

The mother then leaves for good. The pup sheds its white coat, grows a waterproof adult coat and stays ashore for up to several weeks, living on its fat reserves before it heads to sea to learn to catch fish. During this fast it loses weight, which is why a heavy pup has a better chance of surviving its first winter.

How much does a gray seal’s weight change through the year?

Gray seal weights are only snapshots, because these seals fatten up and fast on a yearly cycle. Before the breeding season both sexes feed heavily and build thick layers of blubber.

Nursing mothers then eat nothing for two to three weeks while they produce rich milk, and they lose a large share of their body weight. Breeding males fast too, often for longer, because leaving the beach to feed would mean losing their place among the females. Both sexes also eat little during the molt, when they spend long periods hauled out while they grow a new coat.

Because of this cycle, the same seal can weigh tens of kilograms more in late summer than at the end of the breeding season. Scientists who measure seals therefore note the date and the animal’s condition, not just its weight. To learn what fuels these big seals, see What do seals eat?