Northern elephant seal

February 1, 2013, 2:27 pm
Content Cover Image

Northern Elephant Seal, Piedras Blancas, San Simeon, CA 2008. Source: Mike L. Baird

The Northern elephant seal, (scientific name: Mirounga angustirostris), is one of nineteen species of marine mammals in the family of True seals. Together with the families of Eared seals and Walruses, True seals form the group of marine mammals known as pinnipeds.

Physical Description

 

Northern elephant seals are characteristically brown in pelage color; however, there are many subtle variations among individuals. Males are usually a darker brown, while females are a light tan color. (Warburton. 2008) Hair is reduced on adult males and females and is completely absent for a short time after moulting. A newborn's fur is black in color until the pup is successfully weaned, at which time it sheds its black coat in favor of a lighter colored one. Countershading is a feature to all adults and newly weaned juveniles, displaying a darker color dorsally and a lighter color ventrally.

caption Northern elephant seal. Source: Fred Sorenson/BioLib/Encyclopedia of Life
Conservation Status

Scientific Classification

Kingdom: Animalia (Animals)
Phylum:--- Chordata
Class:------ Mammalia (Mammals)
Order:-------- Carnivora (Carnivores)
Family:-------- Phocidae (True Seals)
Genus:--------- Mirounga (Elephant Seals)
Species:--------Mirounga angustirostris (Gill, 1866)

caption Northern elephant seal. Source: Darrin O'Brien/Encyclopedia of Life

The name of this species is derived from its massive size and the male's prominent snout. Elephant seals are the largest seals in the world, though Southern elephant seals are slightly larger than Northern elephant seal.

The Northern elephant seal possess two lobed hind flippers. External ear flaps/lobes (pinnae) are absent, giving the ear the appearance of being flush with the skin. Teeth are dimorphic in the sexes with males having considerably enlarged canines that are used in fighting.

The species exhibits pronounced sexual dimorphism, the most conspicuous feature of males being the inflated snout/trunk (proboscis). (MarineBio.org. 2010) This feature is absent in females and is larger than that of their slightly larger close relatives, southern elephant seals Mirounga leonina. Young males begin development of the proboscis at two years of age, but it is not fully developed until the animal reaches its eighth year of maturity.

These mammals are among the largest of the aquatic carnivores in the Northern Hemisphere. Females typically weigh 600 to 900 kg and males, which outweigh females by a factor of three to ten, and can top out at 2300 kg. Females reach a length of 3.1 m on average and males usually extend from 4.0 m to 5.0 m. Newborns typically weigh about 47 kg at birth. They weigh about 147 kg and measure about 1.5 m between 24 and 28 days old, when they are weaned.

Reproduction

There is a definitive hierarchy structure to the mating system of these animals because they are polygynous, with the seals aggregating in colonies on land during the breeding season. Each dominant (alpha) male controls access to mating opportunities with a group of females. This mating system has been called "female defence polygyny".

Less dominant males are restricted to the fringes of a colony and continually try to gain access to females, resulting in battles between males and aggressive charges by the dominant male. Sub-dominant males usually run away when challenged by the alpha male; however, occasionally a male will challenge the dominant male in an attempt to accede to the dominant post and take on the rulership of the harem. Sometimes a subordinate male may actually assist the alpha male in patrolling the harem perimeter and maintaining effective control over the harem. Females release an audible bawling sound when a non-dominant male tries to mate with her. This results in a defense attempt by the dominant bull, who chases the less dominant male away.

Occasionally the less dominant male becomes defiant and this can result in spectacular displays of threats and sometimes violent fighting. When a male wants to mate, he throws a flipper over the side of a female, grips her neck in his teeth and begins copulation. Resistance by a female only results in the male moving his large and heavy body on top of the female so she is unable to move. Aggressive interactions among males often result in elephant seal pups being killed by trampling.

Northern elephant seals haul out for birthing and breeding from December to March. The females come into heat only 19 days after giving birth. They remain receptive for about four days, during which mating occurs. Females become sexually mature at two years of age, but usually begin giving birth in the 4th year of life. Males are sexually mature at the age of Six or Seven, but only occasionally are allowed to mate before they reach the age of nine or ten because of the hierarchy system of mating exhibited by these animals.

These animals display a phenomenon in their development cycle called delayed implantation. Delayed implantation lasts for about three months, resulting in a total gestation time of nearly one year. This allows both birthing and mating to occur in the same time frame, during the short period of the year when these animals are aggregated in terrestrial colonies. Interestingly, the embryo is never actually implanted, by definition of most mammals. Instead, it attaches only outwardly to the uterine wall throughout its development.

The pregnancy will ultimately last just under one year, as a result of delayed implantation. Parturition, which results in one offspring per year (although there have been occurrences of twins), will occur the following winter and lactation will follow for about 27 days before the pup is weaned. Pup weight gain during the period of lactation is phenomenal, the milk is extremely high in fat. During weaning the pup remains close to the mother until such a time that the mother leaves the pup behind to return to sea.

Young pups left alone form groups or pods, which remain on shore for up to 12 weeks without parental care. They learn to swim in the surf and eventually swim further out to sea for a short time to feed. An interesting phenomenon displayed by young male pups left to fend for themselves is that of "milk stealing". An attempt to nurse from lactating females still on the beach raising their young can give the successful pup a significant advantage in survival and higher ranking later on in his life by increasing his weight and overall health.

Lifespan/Longevity

Because these animals spend such an extraordinary amount of time in the water, there are many gaps in how much is known about them. It is difficult to discern lifespans of these animals and what might be defined as a natural cause of death. Estimates of survival of reproductive females is represented in percentages with the probability of survival decreasing with each year of life. In the first year of life, a female's chances of survival are 35%, at two years, 30%, and at three years, 20%.

Adult males live an average of 11 to 13 years old. Young pups are quite vulnerable to death, particularly by predation and trampling. Trampling usually happens as a result of a large male defending its females, sometimes crushing an intervening pup under his weight, as he charges quickly toward an intruder. By some estimates as many as 10% of the young pup population may perish this fashion annually.

Behavior

Northern elephant seals are probably solitary in nature while in the water, but aggregate during mating season on the shore. They are on the move for most of their lives, migrating while foraging for food. A social hierarchy exists during mating season, but the males are less aggressive toward each other when they haul out for moulting.

One of the most extraordinary features o this speciess is dive behavior during foraging migrations. The extended period of time in which these animals stay under water is not brought about by their ability to hold their breath. Air is dispelled from the lungs before these seals dive, and throughout the 20 to 70 minutes the animal is under water, the oxygen required is obtained from their blood and tissues. Northern elephant seals possess blood that is rich with hemoglobin and tissues rich in myoglobin, thus increasing their oxygen storing capability.

Another feature of this diving behavior that has perplexed researchers is the lack of rest or sleep for such an extended period of time. Recordings have indicated that these animals conduct repeated dives, 24 hours a day, sometimes for months at a time. It is thought that the dive activity may be a form of sleeping for the seals because their metabolic rate is very low.

caption Distribution of Northern elephant seal. (dark blue: breeding colonies; light blue: non-breeding individuals) Source: Mirko Thiessen

Distribution

Northern elephant seals are found in the coastal waters of the Pacific Ocean from the Gulf of Alaska southward to Baja California. Foraging migrations by males and females are made separately, 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 annual migration distance for this species has been recorded at up to  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.

Habitat

Northern elephant seals reside terrestrially on the sandy, rocky or muddy shores of the coastline, particularly on offshore islands. They typically aggregate in large groups while on land. These animals spend only 10% of their time on land, during reproduction and moulting. The other 90% is spent in the water, diving and foraging for food, and only 11 percent of aquatic time is spent at the surface. This means that an extraordinary 85-90% of their time is spent at sea and under water. These mammals can dive exceptionally deep, to  1600 meters (almost a mile) and for extended periods of time (20 to 70 minutes).

Predators

Northern elephant seals try to feed by diving deep when in the water because the animals that view them as prey typically feed near the surface. Females migrate to the open ocean to feed in order to avoid predators as much as possible. Northern elephant seals are important as predators to octopus, squid, small sharks, skates, and fish. In this way they impact the populations of these animals. They are also important as food for animals which prey on them, such as the Great white shark, Carcharodon carcharias, and killer whale, Orca orcinus. 

Food Habits

Northern elephant seals spend 90% of their lives in the water in order to obtain sufficient food. During their foraging migrations, they dive into the water repeatedly and continuously to hunt, not stopping to rest or sleep for months at a time.

Females and males feed separately from each other. Males travel north, remain closer to land, and tend to return to the same locations to feed year after year. Females migrate away from the land, west to the open ocean, and are less accurate in returning to the same places each year. Male foraging behavior is characterized by benthic dives to the sea floor. By contrast, females exhibit pelagic diving foraging, defined by a trip to the ocean floor and partial ascent, in repeated cycles.

There is some speculation as to the reason why male size is so large in relation to female size, and some suggestions indicate that food type may be a contributing factor. Males are more likely to eat food sources that are dense in mass such as sharks and skates, while females eat foods that are less dense such as squid. These differences in foods is a likely occurrence to the different locales in which they are foraging. This resource partitioning is likely to be the result of differences in body size. Males are less vulnerable to predators and are thus safer foraging in areas with more predators. Females are more vulnerable to predators and thus must forage in areas with fewer predators.

While elephant seals are on land they fast, sometimes going for extended periods of time without food, while they are reproducing and moulting. During this time, all nutrition and energy is broken down from fat that is stored on their bodies as blubber. It is believed that these animals never drink water. Their source of water comes from food sources and broken down fats. In addition they have developed physiological methods to retain water, such as producing a concentrated urine.

Another interesting phenomenon about these mammals is the behavior of eating stones before coming ashore. The true purpose of this behavior is not known. The stones are eliminated when they re-enter the water for migration, so it has been suggested that this phenomenon is in response to the long period of fasting. Foods consumed include: cephalopods, skates, small sharks and fish.

Conservation Status

Northern elephant seals are not presently endangered; however, in the early 20th century, the species was hunted to near extinction. (New York Zoological Society.1907) They were presumed extinct by the 1880's, after being exploited by hunters and whalers seeking to use the thick layer of blubber as an oil source. Remnant 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 likelihood that the population lacks the genetic diversity to overcome future disease or reproductive failure.

Economic Importance for Humans

Northern elephant seals are a major attraction for tourists along the Pacific Coast of parts of North America, notably the Año Nuevo State Reserve and along parts of the Pacific Coast in San Luis Obispo County in California. In these locations visitors can watch these magnificent animals from a safe distance, during breeding season. Northern elephant seals were once hunted for their blubber, which was refined to use as oil.

Elephant seals are the only known animals capable of filling collapsed lungs, which collapse during dives. The surfactant/lubricant responsible for this ability is being studied at the Scripps Institute in San Diego, California for the potential benefit to premature humans with immature lungs.

Northern elephant seals have also been used in research related to the effect of weightlessness on bone density because they spend 90% of their time in a neutrally buoyant environment. NASA has used this research in their efforts to counteract the effect of weightlessnes on bone density in astronauts.

Because northern elephant seals can dive to extreme depths it has been suggested that they can greatly aid human efforts to explore and map the deep ocean cryptic habitats, once instruments that can withstand extreme pressures are developed. Northern elephant seals may consume some fish and other prey that are important to the fishing industry, but this impact is not well documented and likely exaggerated.

References

Glossary

Citation

Life, E. (2013). Northern elephant seal. Retrieved from http://www.eoearth.org/view/article/154935

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