Why do seals have oil?

Size

• Males reach about 1.4 to 2.0 m (4.6-6.6 ft.) and 70 to 170 kg (154-375 lb.).
• Females reach about 1.2 to 1.7 m (3.9-5.6 ft.) and 50 to 150 kg (110-331 lb.).

Body Shape

Harbor seals have a rounded, fusiform (rounded in the middle and tapered at the ends) body.

Coloration

Harbor seals range in color from light gray to silver with dark spots. Some are black or dark gray to brown with white rings. Spots or rings are numerous on the dorsal (back) surface and more sparse on the ventral (underside) surface. In some areas, such as San Francisco Bay, a number of harbor seals have a red or rust coloration from iron oxide deposits on their fur.

Foreflippers

• Limbs are modified into flippers. The foreflippers, or pectoral flippers, have all the major skeletal elements of the forelimbs of land mammals, but they are foreshortened and modified.
• A harbor seal's flippers are short and webbed. Each foreflipper has five digits of about equal length.
• The foreflippers have noticeable claws. The claws are blunt and measure about 2.5 to 5 cm (1-2 in.). Harbor seals use their claws for scratching, grooming, and defense.
• Foreflippers are covered with hair.
• Foreflippers are not as broad as the hind flippers and have less resistance to water flow.

Hind Flippers

• Like land mammals, seals have five bony digits in the hind, or pelvic, limbs. The first and fifth digits are long and stout; the middle digits are shorter and thinner.
• Digits of the hind flippers are webbed. When a harbor seal spreads its hind flippers, the flippers look like wide fans.
• Like the foreflippers, the hind flippers have claws and are covered with hair.
• Harbor seals move their hind flippers side-to-side to propel themselves in water. The hind flippers also function as a rudder.
• Unlike a sea lion, a seal cannot rotate its hind flippers underneath the pelvic girdle. When on land, a seal moves by bouncing in a caterpillar like motion.

Head

• A harbor seal has a rounded head with a fairly blunt snout.
• A harbor seal lacks external ear flaps. Its ear openings close when it dives.
• Harbor seals have 34 to 36 teeth. The front teeth are pointed and sharp, adapted for grasping and tearing (not chewing) their food. Harbor seals often use their back teeth for crushing shells and crustaceans.
• Vibrissae (whiskers) grow from the thick pads of a seal's upper lip and cheeks. Vibrissae are attached to muscles and are supplied with blood and nerves. Vibrissae continually grow throughout a seal's life.

Tail

A harbor seal has a short, flattened tail - tucked between its hind flippers.

Hair

• Harbor seals have thick, short hair. The coat is made of coarse guard hairs and finer, but denser underhairs. Each guard hair has three to six underhair fibers attached to the root.
• The density of a harbor seal's hair increases with age.
• Glands in the skin secrete oil which helps waterproof the hair.
• The hair provides no insulation for the harbor seal.
• Molting.
  
  • Harbor seals molt (shed their hair) each year after the breeding season. They gradually lose their hair in patches. Molting generally lasts one to two months.
  • Females molt after their pups are weaned. In fertilized females, hormonal changes at the end of the molt may trigger blastocyst implantation and embryo development.
  • Pups shed a white coat called lanugo shortly before or after birth. They do not molt again until they are a year old.
  • Based on observations in zoological environments, harbor seals generally seem to decrease their food intake during molting.

Blubber is a thick layer of fat, also called adipose tissue, directly under the skin of all marine mammals. Blubber covers the entire body of animals such as seals, whales, and walruses—except for their fins, flippers, and flukes.

Blubber is an important part of a marine mammal's anatomy. It stores energy, insulates heat, and increases buoyancy.

Storing Energy
Energy is stored in the thick, oily layer of blubber. The energy stored in blubber includes both proteins (mostly collagen) and fats (mostly lipids). The ability of blubber to use these stored nutrients means marine mammals are not forced to search for food for long periods of time. Nursing mothers, for instance, build up thick stores of blubber before giving birth. In addition to feeding offspring, mothers cannot regularly search for food. They rely on the energy stored in their blubber.

Insulation
Blubber also insulates marine mammals, or helps keep them warm in icy waters. This insulation is necessary. Mammals are warm-blooded, meaning their body temperature stays about the same no matter what the temperature outside is. Keeping a warm body temperature in cold water requires more energy than keeping a warm body temperature in warm water. Some marine mammals, such as sea otters, have a thick fur coat, as well as blubber, to insulate them.

To insulate the marine mammal, blood vessels in blubber constrict, or get smaller, in cold water. Constricted blood vessels reduce the flow of blood, thus reducing the energy required to heat the body. This conserves heat.

Buoyancy
Finally, blubber helps marine mammals stay buoyant, or float. Blubber is generally less dense than the ocean water surrounding it, so animals naturally float.

Animals with the thickest blubber, such as right whales, are found in Arctic and Antarctic regions. In these animals, blubber is more than a foot thick! The thickness of their blubber does not indicate better energy storage, insulation, or buoyancy, however. Those characteristics are determined by the chemical property of the blubber.

People and Blubber

Many ancient cultures of the Arctic relied on blubber as a staple part of their diet. Muktuk, for example, is a traditional food consumed by the Eskimo and Inuit people, native to the U.S. state of Alaska and the Canadian Arctic. Muktuk is thick slices of whale blubber and skin. Besides being an excellent source of energy and vitamin D, muktuk was often the chief source of vitamin C for these Arctic people. (Citrus trees, whose fruit is probably the most familiar source of vitamin C, do not grow in such cold temperatures.)

Today, the process of biomagnification has made consumption of muktuk and other whale meat a possible health risk. Biomagnification is the process in which the concentration of a substance increases as it passes up the food chain. Blubber's high concentration of toxic substances may be a result of marine mammals' position as top predators in the marine food web. High concentrations of PCBs, chemicals that can cause cancer, and other toxins have been detected in blubber. The concentrations may be natural, or it may be augmented by bioaccumulation of marine pollution.

Some countries, such as Japan and Norway, continue to harvest whale blubber for food. Environmental groups have expressed concern about the high concentration of PCBs in the blubber.

Whaling
Blubber was the basis of the whaling industry, one of the most lucrative businesses of the 18th and 19th centuries. Millions of whales were hunted throughout the Atlantic, Pacific, and Arctic oceans in sophisticated whaling "factory ships."

After killing a whale and stripping it of its blubber, workers rendered the blubber in enormous iron cauldrons called trypots. Rendering is the process of slowly cooking blubber or other animal fat (such as lard) over a low temperature.

As blubber renders, it turns into a waxy substance called whale oil. Whale oil was a primary ingredient in soap, margarine, and oil-burning lamps. Today, some indigenous Arctic communities, such as the Inuit, still harvest blubber and render it for use in traditional whale-oil lamps.

The whaling industry dwindled as petroleum and natural gas replaced whale oil as a major fuel source. Vegetable oils replaced whale oil in margarine and soaps. Environmental laws and hunting limits have slowly allowed whale populations to recover.