Mirounga angustirostris

Northern Elephant Seal

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Northern Elephant Seal

 

Description

As is often the case in species where males compete to mate with as many females as possible, northern elephant seal males are much larger than females (1,800 kg versus 650 kg on average). Competitions can be battles, but more often involve mock threats and loud vocalizations. The dominant (alpha) male gets to mate with the most females, maximizing the number of offspring he produces. Fasting is part of their life cycle: males stay on the beach and fast for up to three months during the breeding season, while they are guarding a harem. Females stay on shore with their pups and fast for about a month while they are nursing. Afterward, males and females spend 8-10 mostly solitary months at sea. Males often migrate along a northern route and the females remain farther south. They may travel as far as Japan and log more than 20,000 km annually, spending much of their time underwater foraging for squid and fish. Dives can be as deep as 1,500 m and last as long as two hours, but the seals typically stay submerged for about 18-25 minutes and forage at depths of 350-650 m

Geography

Northern elephant seals are found in the eastern and north central North Pacific. Breeding takes place on offshore islands and at a few mainland localities from central Baja California to Northern California. The species breeds at over fifteen discrete locations, of which a dozen are well-known and distributed mostly in islands of southern California (USA) and Baja California (Mexico) (Condit et al. in press). A few pups are born in Oregon, Washington and southern British Columbia. Año Nuevo, in Central California, is a major breeding site.

Northern elephant seals migrate to and from their rookeries twice a year, returning once to breed from December to March and again later for several weeks to molt, at different times depending on sex and age. They also show up at additional coastal sites as far north as southern Oregon for moulting. Their post-breeding and post-moult migrations take most seals north and west to oceanic areas of the North Pacific and Gulf of Alaska twice a year. Adult males tend to travel further north and west than adult females. Wanderers have been found as far away as Japan and the Midway Islands.

 

Geographic Range

Northern elephant seals are found in the coastal waters of the Pacific Ocean from the Gulf of Alaska down to Baja California. Foraging migrations by males and females are made seperately, two times yearly. Males journey north to the Aleutian Islands and the Gulf of Alaska. Females don't travel as far north, but instead migrate further west to more open ocean. The total linear distances migrated by these animals each year has been recorded at 21,000 km. Seals can be seen on shore most often from December through March, during the mating season and again beginning in April and continuing through August as they haul out for moulting.

Biogeographic Regions: nearctic (Native ); oceanic islands (Native ); pacific ocean (Native )

Ecosystem

Habitat and Ecology
Northern elephant seals are huge and imposing animals. Significant sexual dimorphism exists in size and secondary sexual characteristics. Males are three to four times heavier, and nearly 1.5 times the length of adult females. Adult males reach 4.2 m in length and a maximum weight of 2,500 kg. Mean length for males is 3.85 m and mean weight is 1,844 kg. Maximum weight is seen when the bulls are newly arrived for the breeding season after a summer and fall largely spent feeding.

The mean length of adult females is 2.65 m, and they can reach a length of 2.82 m. Mean mass is 488 kg, with a maximum recorded mass of 710 kg, both values are from shortly after females give birth. Newborn pups are about 1.25 m long and weigh 30-40 kg. Pups are born in a long woolly black lanugo coat that is shed without the epidermis starting at about 4-5 weeks and usually lasting several weeks until after the pup is weaned. After molting, the newly weaned juvenile’s coat is made up of short hairs like those found on adults and they are counter shaded dark gray above and silver gray below.

Body weight of adults fluctuates dramatically due to the demands of fasting during the breeding season. Adult females can lose almost half their mass during lactation, when they take their single pup from birth to weaning in approximately 25 days. The situation is similar for breeding males, which also lose about half their mass while ashore for periods that can exceed three months.

Females reach sexual maturity at the age of 2 years and males at 7-9 years. Gestation lasts 11.3 months, including a delay of implantation. The annual pregnancy rate of mature females is 95 %. Longevity is above 14 years for males and 21 years for females (Reijnders et al 1993).

Northern elephant seals are highly polygynous, but not strictly territorial. Males compete for access to females by ranking themselves in a hierarchy. There is much male-male fighting, vocalizing, and displaying during the breeding season, when bulls may be ashore for months at a time. One of the most impressive displays occurs when a male rears up on his hindquarters, thrusts one half to two-thirds of his body upward and produces a distinctive clap threat vocalization as a challenge to other bulls. The sound is a rolling, resonant, metallic-sounding series of backfire like sounds punctuated with pauses.

Females give birth within a few days of coming ashore, from late December to March. Females have a throaty sputtering growl, made with the mouth wide open, which is used as threat gesture. Females and pups have a warbling scream call that they use to call to each other, and in the case of the pup, to notify its mother when it is disturbed. Mother and pup form a strong bond immediately after birth, and females aggressively bite other pups that approach them, occasionally killing them with bites to the head. However, bulls are a greater source of pup mortality as they regularly crush pups when they charge through aggregations of mothers and pups to chase off or fight encroaching males, or when approaching females for mating. Occasionally, bulls suffocate pups by stopping on top of them and not moving again soon enough.

Northern elephant seals hold the record as the deepest-diving pinniped. Time-depth recording devices have documented dives to an astounding 1580 m by an adult male. They also have extreme breath holding ability and have been recorded to dive for as long as 77 minutes. Rest intervals at the surface are usually short, lasting only several minutes between routine dives that last 20-30 minutes and reach 300 to 800 m in depth. After leaving the rookeries, most of these seals spend 80-90% of their time underwater, helping explain why they are infrequently seen at sea.

During migration, females travel further west, but males travel further north. Most of the males go all the way up to the coast of Alaska and the Aleutians, while females move into the deep waters of the Pacific.

Fifty-three species of prey have been identified in the diet of northern elephant seals. More than half of these species are squid. Other prey includes various fishes, such as Pacific whiting, several species of rock fish, and a variety of small sharks and rays. They have also been reported to feed on pelagic red crabs. Approximately 70% of their prey comes from mid and deep water zones in open ocean habitat far offshore.

Great white sharks and killer whales are predators on northern elephant seals. Recent work at the Farallon Islands off of central California has revealed that large great white sharks aggregate around the islands in the fall when juvenile elephant seals return for their annual moult. Seals that swim at or near the surface as they are approaching or departing the islands are particularly vulnerable to ambush attacks by fast-rising sharks that patrol near the bottom in waters seven to ten meters deep.

Systems
Terrestrial
Marine

Conservation

Northern elephant seals are not presently endangered. At one time, however, this species was thought to have been hunted to extinction. They were presume extinct by the 1880's, after being exploited by hunters and whalers seeking to use the animals' thick layer of blubber as an oil source. A few animals were then discovered in 1892 which were captured and killed for scientific study. Eventually, it was discovered that a population of about 20 to 100 individuals had survived. Studies have shown that all individuals of the current population, which has grown to over 175,000, are relatives of these few survivors. The population bottleneck that occurred during this time is of concern because genetic variation is reduced, creating the possibility for the population to be vulnerable to disease or reproductive failure.

 

Threats

Major Threats
The northern elephant seal was hunted to the brink of extinction in a surge of commercial exploitation in the late 1800s. Estimates are that as few as 20 animals survived the period of commercial harvesting. Fortunately, for the species, their pelagic nature and the fact that most seals spend 80% or more of their lives at sea, and that they all do all not return to their rookeries at the same time, ensured that enough seals were at sea to support continuation of the species when sealers undertook wholesale slaughters at rookery sites. Following a slow recovery in the early 1900s, northern elephant seals began to recolonize formerly used sites throughout the 1980s.

Incidental take in a variety of coastal net fisheries does occur, but at low levels. This is assumed to be because northern elephant seals move offshore rapidly after leaving colonies to minimize their risk of shark predation. Most of the prey of the northern elephant seal is either of low commercial value or minimally harvested in fisheries. If the population continues to expand there will likely be new rookeries on mainland beaches, and there will be additional challenges to keep conflicts with humans and domestic and feral animals to a minimum. The risk of transfer of diseases, such as morbillivirus from domestic animals to northern elephant seals, is unknown, but the species is considered to be one of several pinnipeds at high risk of disease outbreaks because of their rapidly expanding population and environmental changes associated with global warming (Lavigne and Schmitz 1990). Tourism at several mainland locations in the United States is extremely popular but highly regulated and is not considered a major threat to the species. Tourist access to nearly all of the islands occupied by elephant seals is controlled by law or otherwise regulated in the United States and Mexico, although a number of Mexican islands are inhabited either year round or most of the year by fishermen and their families.

As the species has now recovered from a very small number of survivors, it has likely lost a considerable amount of diversity from passing through this genetic bottleneck, and may now be at greater risk from disease outbreaks and environmental change.

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