Northern Elephant Seals
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Northern Elephant Seal

Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Phocidae
Genus: Mirounga
Species: M. angustirostris
Binomial name
Mirounga angustirostris
(Gill, 1866)
Distribution of the northern elephant seal (dark blue: breeding colonies; light blue: non-breeding individuals)
Distribution of the northern elephant seal (dark blue: breeding colonies; light blue: non-breeding individuals)

The Northern Elephant Seal (Mirounga angustirostris) is one of two species of elephant seal (the other is the Southern Elephant Seal). It is a member of the Phocidae ("true seals") family. Elephant seals derive their name from their great size and from the male's large proboscis, which is used in making extraordinarily loud roaring noises, especially during the mating competition. There is a great sexual dimorphism in size, with the males (bulls) reaching five meters in length, much bigger than the females (cows), who average about three meters. The males average 16 ft (5 m) and 5,000 lb (2,300 kg), while the females average 1,400 lb (640 kg).1 Correspondingly, there is a highly polygynous mating system, with a successful male able to impregnate up to 50 females in one season.

Contents

Description

Both adult and juvenile elephant seals are bare-skinned and black before molting. After molting they generally have a silver to dark gray coat that fades to brown yellow and tan. Adult males have hairless necks and chests speckled with pink, white and light brown. Pups are mostly black at birth and molt to a silver gray after weaning.

The eyes are large, round and black. The width of the eyes and a high concentration of low light pigments suggests that sight plays an important role in the capture of prey. Like all seals, elephant seals have atrophied hind limbs whose underdeveloped ends form the tail and tail fin. Each of the "feet" can deploy five long webbed fingers. This agile, dual palm is used to propel water. The pectoral fins are used little while swimming. While the hind limbs are unfit for locomotion on land, elephant seals use their fins as support to propel their bodies. They are able to propel themselves quickly (as fast as 8km/h) in this way for short-distance travel, to return to water, catch up with a female or chase an intruder.

Like other seals, elephant seals have a bloodstream adapted to the cold in which a mixture of small veins surround arteries capturing heat from them. This structure is present in extremeties such as the hind legs.

Range and habitat

Adult male Northern elephant seal in dunes at Año Nuevo State Park mainland

The Northern Elephant Seal lives in the Eastern Pacific Ocean, migrating as far north as Alaska, British Columbia, and as far south as the shores of California and Baja California, where they come ashore to breed, give birth and molt, mostly on offshore islands. While the pelagic range covers an enormous span, there are only about seven principal breeding areas, four of which are on islands off the coast of California. Recently increasing numbers have been observed in the Gulf of California. Colonies can be observed at Año Nuevo State Reserve, Piedras Blancas Lighthouse, and Morro Bay State Park.

History and status

Beginning in the 1700s Northern elephant seals were hunted extensively almost to extinction by the end of the 19th century, being prized for oil that could be made from their blubber, and the population may have fallen as low as 100 to 1000. Finding refuge in Mexican waters, by the turn of the century, there was only a sole surviving rookery, on Guadalupe Island, Mexico; and this colony was granted protection by the Mexican government. Since the early 20th century, they have been protected by law in both Mexico and in the United States. Subsequently the U.S. protection was strengthened after passage of the Marine Mammal Protection Act, and numbers have now recovered to over 100,000.

Elephant seals: female with pup and adult male

Nevertheless, there is a genetic bottleneck in the existing population, which could make it more susceptible to disease and pollution. In California, the population is continuing to grow at around 25 percent per year, and new colonies are being established; they are now probably limited mostly by the availability of haulout space. However, numbers can be adversely affected by El Niño events and the resultant weather conditions, and the 1997-98 El Niño may have caused the loss of about 80 percent of that year's pups. Presently the Northern elephant seal is protected under the Federal Marine Mammal Act and under California law has a fully protected status.

Populations of rookery sites in California have exploded during the past half-century. At Año Nuevo State Park, for example, there were no individuals observed whatsoever until the 1950s; the first pup born there was observed in the early 1960s. Currently, thousands of pups are born every year at Año Nuevo, on both the island and mainland. The growth of the site near San Simeon has proved even more spectacular; there were no animals there prior to 1990. Currently, the San Simeon site hosts more breeding animals than Año Nuevo State Park during winter season.

Feeding and behavior

Harem of females moulting

The Northern Elephant Seal feeds on a wide range of over 30 fish and cephalopods, including squid, octopus, hagfish, ratfish and small sharks. They are nocturnal deep pelagic feeders famous for the long time intervals they remain underwater (Morejohn, 1970). This species dives to great depths while feeding, typically between 300 m (1,000 ft) and 800 m (2,600 ft), and males can dive as deep as 1,500 m (4,900 ft); moreover, the Northern elephant seal will generally not feed in depths of less than 200 m (700 ft) (Condit, 1984). Average dive times are correspondingly long, around 20 minutes for males, less for females, and they require about three minutes on the surface between dives. The deepest dive records are held by female elephant seals and is currently at 1603 meters (nearly a mile) with a time of 119 minutes. Typically this species is observed singly in its pelagic environment, although on land there may be thousands in almost tangent harems. Northern elephant seals, especially juveniles, are preyed on by great white sharks and sometimes also by orcas (killer whales). While at sea from late spring to early winter, the Northern elephant seal stores vast amounts of food as blubber and also water oxidatively produced in fat stores to prepare for his long fast on land (Ortiz, 1978).

In the summer, elephant seals undergo a "catastrophic moult" that lasts about one month, during which they lose much of their fur and skin. They spend this time on beaches to preserve body heat while they wait for the new fur to grow. During this time, elephant seals can be observed at a number of preserves on the California coastline, such as the Año Nuevo State Park and the Point Reyes National Seashore. Observers must have a permit and be very cautious because over short distances bulls can move faster on land than a person can run, despite their ungainly appearance. Elephant seals have no interest in attacking humans but are oblivious to objects blocking attacks on rival males.

Life cycle

Male elephant seals fighting for mates.

The Northern elephant seal returns to its terrestrial breeding ground in December and January, with the bulls arriving first. The bulls haul out on isolated or otherwise protected beaches typically on islands or very remote mainland locations. It is important that these beach areas offer protection from the winter storms and high surf wave action (Riedman, 1982). The bulls engage in dramatic fights of supremacy to determine which few bulls will achieve a territory and harem. While fights are not usually to the death, they are brutal and often with significant bloodshed and injury; however, in many cases of mismatched opponents, the younger, less capable males are simply chased away, often to upland dunes.

After the males have secured their territorial position on the beach, the females arrive and somehow select an alpha bull for housekeeping. In this polygynous culture, a bull will typically have a harem of 30 to 100 cows depending on the size and strength of the bull. The least successful males are restricted to the fringes of a colony but will attempt to gain access to females which can result in a conflict with the harem male. In a lifetime a successful bull could easily sire over 500 pups, whereas most bulls will never mate, due to the hierarchy established by combat. The lifetime reproduction potential of a female is about ten pups.

After arrival on shore males fast for three months, and females fast for five weeks during mating and nursing of her single pup. The gestation period is approximately eleven months. Pups nurse about four weeks and are weaned abruptly approximately two months before departing on their first journey to sea.

Notes

  1. ^ "Mirounga angustirostris: Northern Elephant Seal". North American Mammals. Smithsonian Institution. Retrieved on 2007-02-03.

Further reading

External links
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