Pinnipeds. PDF created with pdffactory Pro trial version

Transcription

1

2 1 MARINE MAMMALS - PINNIPEDIA 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 THREE FAMILIES 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 Figure 1. Examples of pinniped families: (a) Walrus - Odobenidae; (b) Fur seal - Otariid and (c) True seal - Phocid.

3 2 FEATURES 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. 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. a. b. Figure 2: Pinniped skeletons showing: (a) Phocids and; (b) Otariid. Note the position of the limbs for locomotion on land and sea. Feeding 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.

4 3 Senses a) Touch are very tactile, continually touching each other for security and reassurance. Many species pack closely together on land to maintain body contact and heat. Whiskers, which have many sensitive nerve fibres, are used to detect the vibrations of their prey. The walrus would rely on touch more than any other sense to locate its food. b) Sight, except the walrus, see extremely well both in and out of water. Large eyes enable the absorption of more light in deep water. Their eyes are protected by eyelids and a nictitating membrane. The eyes continuously excrete an oily liquid which reduces friction under water and keeps them well lubricated. c) Hearing have extremely acute hearing to help locate prey. The ears close tightly whilst diving. d) Smell The nostrils or nares close tightly whilst under the water, reducing their sense of smell which forces them to rely on their sight and touch. Taste senses are also minimal. On land they have an acute sense of smell which is important for social interaction. e) Echolocation There is evidence that some seals and seal lions use sonar, or echolocation, to navigate through the water and to find food. 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. Figure 3: 'Sailing' lowers the body temperature

5 4 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. a. b. Figure 4: Comparison of hind flippers and swimming styles; a. Eared seal; b. True seal 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. 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.

6 5 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. Figure 5: a. 'Amber' is one of the Californian seal lion pups born at Sea World and b. a Sub Antarctic fur seal pup 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. 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. 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. Figure 6: Great white sharks and Leopard seals are both natural predators of pinnipeds