Male gray seals have a distinctively arched Roman nose, which gives them their other common name — horsehead seal. They have massive shoulders and necks with large folds of skin. Females lack the distinctive arched nose and are more delicately featured.

A gray seal's coat color varies from gray to brown, but most have darker backs than bellies. Bulls are dark with lighter, irregular spotting seen mostly on the belly. Cows are lighter with darker patches and spots. These true seals lack the extremely dense underfur that eared seals possess; gray seals only have a few secondary hairs as opposed to the 50 or so found in fur seals.

Seals have many adaptations for their aquatic lifestyle. Their limbs are short with elongated digits encased in cartilage and connective tissue to form flippers. They have strong, bulky shoulders, and their streamlined, torpedo-shaped bodies taper to the tail. Gray seals have relatively short front flippers with five prominent claws. They can curl these flippers to tear food or grasp terrain.

Compared with  sea lions, seals generally have bulkier torsos, partially due to their method of swimming propulsion. Seals use their rear flippers and move the back half of their bodies in a sculling motion. Sea lions rely mostly on their front flippers.

On land, seals move in caterpillar fashion alternately shifting weight from their chest to their pelvic region, or they roll to their destination. They appear quite clumsy and slow but can overtake a running human over a short distance.

In the water, they make up for their clumsy appearance on land. A flexible body allows them to twist and turn easily while pursuing prey. Their front flippers act as rudders, but are kept tucked away close to the body when not in use. They are able to obtain speeds of 14-23 mph (23-37 kph), but normally cruise at 6 mph (10 kph) or less.

They tend to exhibit deliberate and sluggish behavior to conserve energy, and are usually either traveling between haul-out spots, making feeding trips of short duration or resting. They often use an upright stance, called bottling, while resting in the water.

A layer of blubber, or fat, insulates the seal's body, and the seal's circulation system can shunt the blood supply to the extremities and the outer surface of the body. As a result, gray seals can haul out on ice without melting it. Their dense fur gives them some protection from the cold, as does trapping a layer of water next to the skin that warms to body temperature.

Their relatively small body surface area in proportion to their volume reduces the amount of heat lost to their surroundings. They thermoregulate by behavioral means, such as changing position in the sun, moving to damp places or shallow pools, and altering activity levels.

Adult males are about 7.5 feet (2.3 meters) in length and can weigh more than 750 pounds (339 kilograms). Females are smaller with lengths about 6.5 feet (2 meters) and weigh up to about 575 pounds (170 kilograms).

Gray seals are found on both sides of the North Atlantic in temperate and subarctic waters. They prefer remote rocky coasts with small islands and reefs. The beaches they inhabit may be rocky, sandy or ice pack. Gray seals migrate far from their rookeries when they are not breeding or molting.

There are three populations of gray seals. The Eastern Atlantic population, which includes the British Isles, Iceland and Norwegian coast, is the most numerous, followed by the Western Atlantic group from Newfoundland to Massachusetts. They Baltic Sea population was greatly reduced in the early 20th century. The world population of gray seals is estimated at about 300,000 animals.

Gray seals live an average of 35 years in human care while, wild bulls average 25 and wild cows average 35. The oldest recorded gray seal in the wild was 46 years old.

Gray seals have numerous vocalizations, many of which are used during their highly social breeding season. Males jackhammer, hiss and growl to display dominance and aggression. They also exhibit nonvocal communication, such as neck darts and open-mouthed threats.

Females that are unreceptive to breeding caterwaul or hoot at persistent males. Flipper slapping and scratching are common signs of irritation.

Little is known about gray seals' sense of smell, but bulls produce strong odors during the breeding season, and mothers identify their pups by scent. Olfactory, visual and acoustic cues are all part of the mother-pup recognition system. Tactile communication occurs as well, demonstrated through whisker greetings.

Seals have excellent senses that help them hunt. Their eyesight is particularly well developed, because they spend a lot of time underwater with reduced light levels. On the surface, their pupils contract to tiny pinholes, protecting the retinas from any intense glare.

Gray seals see and hear better underwater. They can perceive sound at a greater range of frequencies than humans. Their lack of external ear flaps and a heavy, wax coating in their auditory canal reduces their ability to perceive sound in the air. Their ears are valvular and close when in the water. Their nostrils are also valvular and are closed at rest.

The vibrissae, or whiskers, located on the head are extremely sensitive and help seals hunt in total darkness by allowing them to feel the currents created by their moving prey.

Gray seals have been documented eating 29 different species of fish and invertebrates, including herring, cod, mackerel and squid. They sometimes eat commercially valuable species, which makes them unpopular with fishermen. They regularly dive 230 feet (70 meters) for food but are capable of diving up to 1,000 feet.

At the Smithsonian's National Zoo, gray seals eat herring, capelin, butterfish, squid and mackerel twice a day. They receive vitamin supplements daily to replace any nutrients lost during the fish freezing process.

Gray seals are gregarious social animals that haul out close to one another and form large aggregations during the pupping, breeding and molting seasons.

Breeding season differs among the populations, with the Northeast Atlantic group pupping mostly on land in September through November, the Northwest Atlantic group pupping on land and ice from late December to early February, and the Baltic group pupping on ice from February to March.

About a month before pupping, large numbers of cows and bulls congregate offshore of the rookery. The birth of the first pups signals the males to come ashore. Instead of holding territories, the males compete for proximity to the females and their pups.

Bulls display, vocalize and fight, often trying to interrupt each other during copulation. Their large size and massive necks provide them with some protection from each other's shoving and biting. During the initial stages of the breeding season, bulls control large areas with many females. Once mating begins, the bulls become distracted and younger, smaller bulls can establish themselves on the fringes of the group. They mate mainly on land, though they can sometimes mate in the water.

The gray seal's gestation period lasts about nine months with a three-month delayed implantation. The fertilized egg develops into a blastocyst after which it remains dormant for two to four months before attaching to the uterine wall. This delayed implantation ensures that birthing occurs at the right time each year.

Cows birth a single pup at a time; twins are extremely rare. Females are very protective of their pups. At birth, pups are about 3-3.5 feet (90-105 centimeters) long from nose to tail and weigh about 33 pounds (15 kilograms). They have a cream-colored, natal lanugo coat that molts at 2-3 weeks old.

Lactation lasts roughly three weeks, during which the mother spends most of her time in close proximity to her pup. Pups feed for about five minutes every five hours and gain weight rapidly on the high-fat milk. Pups gain about 5 pounds (2.3 kilograms) per day from their mother's rich milk but gain little, if any, length.

Once the pup is weaned, the female cow comes into estrus and solicits attention from the male bull. At this point, pups are completely abandoned by their mothers and fast from one to four weeks before they start feeding on their own at sea. Neither bulls nor cows eat during the breeding season and both experience significant losses in weight and physical condition. 

Following the breeding season, all gray seals return to the sea and begin their migrations. Young animals may travel as far as 30 miles (48 kilometers) in a day. Gray seals regain their physical condition before their annual molt. Adults probably return to their birthplace once sexually mature, at 3-5 years old for females and 6 years old for males.

In addition to their status on the International Union for Conservation of Nature's Red List, gray seals are also protected under the Marine Mammal Protection Act.

Sharks and orcas, sometimes called killer whales, are natural predators of the gray seal. Disease and parasites are also natural threats. Although gray seals are not hunted for their fur and there is no longer a market for seal oil, people continue to be the greatest threat to these animals.

Entanglement in and ingestion of plastics results in unknown numbers of gray seal deaths. They are inadvertently caught in fishing nets, particularly drift nets, resulting in their deaths. The impact of overfishing in gray seals' natural hunting grounds has not been determined. They are unpopular with fishermen as these seals are known to consume commercially valuable fish.