Brown Fur Seal

Brown Fur Seal

The Brown Fur Seal (Arctocephalus pusillus), also known as the Cape Fur Seal, South African Fur Seal and the Australian Fur Seal) is a species of fur seal.

Physical description

The Brown Fur Seal has a large broad head and a pointed snout. Males are brown to dark gray with a darker mane and a light underside. They grow up to 2.2m (7ft) and weigh around 200-360 kg (440-800lb). Females are gray to light brown with a dark underside and light throat. They grow up to 1.7m (5ft) and weigh on average 120kg (260lbs). Pups are black at birth but turn gray with a pale throat after molting.

Homologous features

The structure of the brown fur seals flipper is homologous to that of many other animals such as the structure of a bat’s wing or the arm of a human. This is because the seal’s flipper consists of the same basic bone structure as the previously listed examples. As you can see in the linked diagrams below the seals flipper is made up of a series of bones which are exactly the same as that of a bat or human. They have a humerus, which is connected to two forearm bones, the ulna and radius, which then meet a series of smaller wrist bones known as the metacarpals, which connect to the phalanges better known as fingers. This suggests that all of these animals have evolved from a common ancestor at some stage and have developed the structure of their unique limbs to suit their needs best. .

Analogous features

The Brown Fur Seals flippers are analogous to the flippers of animals such as the right whale and the blue whale because they have all independently developed the flipper which has a similar function even though they are not closely related to each other.

Phylogenetic tree

This mammal phylogenetic tree, on the link below, shows that the brown fur seal, who are in the family otariidae, are closely related to the walrus who are in the Odobenidae family. The seal and walrus diverged from a common ancestor approximately 30 million years ago. If you go back as far as 80 million years ago the brown fur seal even has a common ancestor with certain types of bats and horses.

Distribution and habitat

The Brown Fur Seal is found along the coast of Namibia and along the west coast of South Africa to the Cape of Good Hope and the Cape Province. Its Australian subspecies breeds on nine islands in the Bass Strait between Tasmania and Victoria.

Both subspecies mostly haul out and breed on rocky islands, rock ledges or reefs and pebble or boulder beaches. However South African Fur Seals have large breeding sites on sandy beaches in South Africa, and a non-breeding group regularly hauls out on a sandy beach in Cape Fria in northern Namibia.

Foraging

Brown Fur Seals feed mostly on bony fish as well as cephalopods, crustaceans and even birds. The Australian subspecies forages at the bottom of the continental shelf while the African subspecies forages in the open ocean.

Behavior and reproduction

Although Brown Fur Seals normally travel alone, large group of these seals can be seen rafting in kelp beds. Pregnant females will stay 7 weeks away from the land before the breeding season.

Brown Fur Seals breed in mid-October. Unlike many eared seal species, females are free to choose their mate and he is judged based on the value of his territory. Both males and females fight for territories with individuals of the same sex. Females have smaller territories and a male's territory may overlap that of several females. A harem may consist of 50 females for one male.

Pups are usually born between late November and early December. After they are born the females start to mate with their harem leaders. The female spends the next several months foraging at sea and nursing her pup, who is weaned at four months old. The pups begin swimming at an early age and the time they spend in the water increases as the pup learns more. By seven months the pup can swim for two to three days at a time.

The Brown Fur Seal's main predator is the Great white shark. In False Bay, the seals employ a number of anti-predatory strategies while in shark-infested waters such as:

  • Swimming in large groups
  • Low porpoising; to increase sub-surface vigilance
  • Darting in different directions to cause confusion when attacked.
  • It rides near the dorsal fin to keep out of reach of the shark's jaws when attacked.

Human interactions

The Brown Fur Seal is an inquisitive and friendly animal when in the water and will often accompany SCUBA divers. They will swim around divers for periods of several minutes at a time, even at a depth of 60m. On land they are far less relaxed and tend to panic when people come near them.

Australian fur seals were hunted greatly between 1798 and 1825 for commercial reasons. Seal hunting stopped in Australia in 1923 and their population is still recovering. Breeding and haul out sites are protected by law. However the Tasmanian government authorized the killing of "nuisance" fur seals in October 2000.

South African fur seals have a very robust and healthy population despite annual harvesting.

References

External links

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