Thursday, March 15, 2012

Pinniped Mammalia and Predators

Pinnipeds are flesh eating mammals living in water and on land. They represent over one quarter of the marine mammals and are found in nutrient rich coastal waters throughout the world. The majority of species are found in cooler areas with the largest populations in Antarctica. Although some spend many months at sea searching for food, they all return to land to breed and moult.

The order Pinnipedia, "pinni" meaning "winged" and "ped" meaning "foot" can be divided into three families. These are the Otariidae (O-tar-i-day), Odobenidae (O-do-ben-i-day) and Phocidae (Foe-sid-ay):
(a) Odobenidae Walrus
(b) Otariidae Eared seals, Sea lions Fur seals
(c) Phocidae True seals

Pinniped General 
As with other marine mammals, pinnipeds have developed streamlined bodies. Streamlining reduces resistance in the water, thus increasing agility and speed. Protruding body parts such as ear lobes are reduced and reproductive organs are modified and held entirely within the body. Each species has four limbs or flippers. These assist in steering, braking and grooming. Whiskers are prominent in each class and their eyes are large, enabling them to see in darker waters. All pinnipeds, with the exception of the walrus, are covered with fur which helps maintain their body temperature.

Pinniped Skeleton
 The vertebral column of pinnipeds is very similar to that of other mammals. Otariids have enlarged chest and neck vertebrae to support the large muscles used for swimming or locomotion on land. In the true seals, it is the lumbar (lower back) vertebrae that are enlarged. The vertebrae of true seals are loosely joined and the spine very flexible.

Pinniped Feeding
Pinnipeds are near the top of the ocean food chain with a diet of mainly fish and squid. Some fur seals eat sea birds and others, for example the Leopard seal, also include penguins and seals in their diet. The Crabeater seal and the walrus eat crabs and molluscs. Seals have similar teeth to dogs although they do not have the flat, grinding molars. The teeth are all sharp and designed for grasping whole fish, rather than for chewing. Walrus teeth have adapted to a shell fish diet where the upper canines grow to form ivory tusks
up to 80 cms long. These help to crack the shells and can be used as defence weapons.

Pinniped Thermoregulation
Like all mammals, pinnipeds are endothermic ('Endo' = 'inside') or warm blooded, which means that they are able to maintain a constant body temperature of approximately 38.5 degrees. Body heat is produced from muscular activity and metabolic processes such as digestion. A layer of fat called blubber, is found under the skin. This acts to insulate the animal which prevents heat being lost. The fur seal gains additional insulation with two layers of fur. The inner layer of fur traps and warms air next to the skin while the outer layer keeps them waterproof. On land, seals may huddle close together which helps to conserve
body heat.

To keep cool, pinnipeds fan themselves with their flippers or dip the front flippers into the ocean allowing the cold water to lower the blood temperature. A behaviour known as 'sailing', exposes their flippers to the breeze, cooling the blood vessels close to the surface. Cool blood then travels through the body.

Diving and Locomotion
True seals hold their back flippers together and move them from side to side to propel themselves through the water. The forearms assist in changing directions. The Eared seals, however, use their large front flippers to move through the water and their rear flippers to assist in steering. Special muscles close their nostrils and ears to stop water from entering. These muscles also close automatically when they sleep. While young, they surface to breathe every ten minutes and as they grow this extends to about thirty minutes. Times do vary between species.

Pinnipeds are able to control heart beat and respiration rate. They have the ability to draw blood from outside areas such as the limbs, while maintaining blood circulation to the heart, lungs and brain. Over fifty litres of blood supplies enough oxygen to the vital organs during a long dive. Research has shown that eared seals can reach 28 km per hour for short periods and phocids l9 km per hour. Walruses rarely cruise above 6 km per hour although they can reach short bursts of 30 km per hour.

Pinniped Breeding
Adult males are called bulls and the females are cows. The young seals are known as pups while the baby walrus is called a calf. Mating and pupping always occur on land. Environmental conditions determine the
length of time of the pupping (breeding) season. Where the conditions are more stable, the season is longer (up to eight months) compared to areas where pack ice is unstable, resulting in a season as short as three days. Moulting, which follows the mating season, occurs on land for all pinnipeds. Again, the
length of time to complete the moult varies between species.

Pinniped Migration
Most pinniped species migrate back to their colonies to breed. This coincides with increased food sources, ice drifts and breeding cycles and can be as far as 5,000 kms for some species.

Pinniped Predation
Killer whales, Great White sharks and other large sharks, Leopard seals and Hooker sea lions are the natural predators of pinnipeds in the water. Polar bears cause deaths amongst these animals on land. However, it is human's influence that has caused the most dramatic decrease in population sizes. Blubber and fur were particularly valuable in the 18th and 19th century when mass slaughter of seals and sea lions led to many facing extinction. Fishing nets also cause many deaths through strangulation and drowning. Most populations are increasing again, although the Monk seals, Gualalupe and San Fernandez fur seals are still in danger of extinction.


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