marine mammals

marine mammals comprise four groups: the Mustelidae (sea otter), the Phocidae (seals and sea lions), walruses, the cetaceans (whales and porpoises), and the sirenians (dugongs and sea cows). All have been over-exploited in the past through sealing or whaling or hunting, either for their fur (sea otters and fur seals) or for their blubber and meat (seals, sea cows, whales). Within six years of the discovery of the South Shetland Islands in 1821, sealers had entirely exterminated the vast herds of elephant (Miounga leonina) and fur seals (Arctocephalus gazellae), killing over 300,000 a year. At least two species have been driven to extinction. Steller's sea cow (Hydrodamalis gigas) was discovered living around subarctic islands in the Bering Sea by a Russian expedition in 1741. These huge 7.5-metre- (24-ft)-long animals weighing about 4,000 kilograms (8,800 lb) were so easy to kill that by 1768 they had been exterminated. The last sighting of a Caribbean monk seal (Monarchus tropicalis) was in 1952 and in 1996 the species was declared extinct.

Apart from the sirenians that graze marine plants, all marine mammals are carnivorous, feeding on krill, fish, seabirds, or even other marine mammals. They show different degrees of adaptation to life in water. Cetaceans and sirenians are fully aquatic but still need to surface to breathe; sea otters, which have four well-developed legs and beat their tails from side to side to power their swimming underwater, are the least adapted and are fully mobile on land. Fur seals and sea lions have hind flippers that can hinge forwards and are used in their rather clumsy gait on land. The true seals and the walrus have hind flippers that point backwards. These are used for steering rather than to power swimming, and are useless on land. In sirenians and whales the hind flippers are fused into a tail fluke that beats vertically to power their swimming. Like walruses, sirenians have long bristly moustaches with which they sense out their food. Early sailors, presumably after many months of separation and deprivation at sea, thought they were mermaids.

Recent results of attaching satellite tracking devices to seals have provided startling information about their behaviour. Elephant seals from South Georgia have been recorded foraging for squid far down the Antarctic Peninsula, taking excursions of over 1,000 kilometres (625 mls.) and occasionally diving to depths of over 800 metres (2,625 ft). The most abundant seals in the world are crab-eater seals (Lobodon carcinophagus) in the Southern Ocean that specialize on eating krill that abound beneath the pack ice. The most predatory is the leopard seal (Hydrurga leptonyx) which supplements its diet of krill with Adelie penguins and smaller seals. It grows to 7 metres (23 ft) and weighs half a tonne, and has also been known to attack humans, on at least one occasion with fatal results.

Bibliography

Perrin, W.,, Wirsig, B.,, and and Thewissen, J. , Encyclopaedia of Marine Mammals (2002).www.pinnipeds.org/contents.htm

M. V. Angel