Southern Elephant Seal
Contents
Southern elephant seal
Southern elepant seals, Tierra del Fuego, Chile. @ C. Michael Hogan
The Southern elephant seal, Mirounga leonina, 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.
Instantly recognizable by the large, inflatable proboscis, the southern elephant seal is the biggest seal in its family. Males are up to five times larger than females, and have much larger canine teeth.
Physical Description
Both sexes have robust bodies, thick necks and broad heads. The fore flippers have a large black nail. Adults have a grey-brown coat, and males develop a chest shield of thickened, creased and heavily scarred skin as they age; they also become paler across the face, proboscis and head with age. Pups are black when born, before growing the waterproof coat needed for entering the sea. During the breeding season, southern elephant seals become stained rusty orange and brown from lying in their own excrement.
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Conservation Status  |
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Scientific Classification Kingdom: Anamalia (Animals) |
Male Southern elephant seals are the largest pinnipeds, larger even than Northern elephant seals, Mirounga angustirostris, their closest relatives. Mirounga leonina males have been documented reaching over six meters long and weighing over 4,000 kg. This is in sharp contrast to females, which are rarely over 800 kg or four meters long. In fact, both species of the genus Mirounga are more sexually dimorphic than any other mammal. This dimorphism stretches beyond just size. Males also have a large, inflatable proboscis, which enhances vocalizations used to challenge other males for mating rights. The southern elephant seal proboscis is somewhat smaller than the proboscis of Northern elephant seals, overhanging the mouth by only about 10 centimeters compared to 30 centimeters in their northern relatives.
Southern elephant seal. Source: Yan Ropert-Coudert/WoRMS/Encyclopedia of Life Breeding populations vary in size. In the South Georgian population, males average 4.5 meters in length and weigh 4000 kg. Females average 2.8 meters and weigh 900 kg. The seals from the Macquarie Island population are somewhat smaller, with males averaging 4.2 meters in length and 3000 kg and females averaging 2.6 meters and 400 kg.
Despite the large difference in size, male and female Southern elephant seals share a number of physical traits. The sexes have similar body typex, including short front flippers used primarily for steering in the water, and very strong, fully webbed, rear flippers that propel them through the water with remarkable speed and agility. They also have a layer of short, stiff hair covering their bodies. At birth this fur is very dark in color, but lightens after the first molt. New fur after a molt is typically a dark gray/brown with lighter underside and lightens over the course of the year. It is also common for the bodies of both sexes to have scars, usually around the neck, from fighting and mating
Reproduction
Once a year, from August to November, Southern elephant seals return to land to breed. Amazingly, most return to the very same breeding grounds on which they were born. Male Southern elephant seals arrive at breeding grounds several weeks before females and, through vocalizations, body positions, and occasional fighting, claim territories on the beach. The best and largest territories go to the largest and strongest males. One of these alpha-males become the head of a harem when the females arrive, sequentially mating with up to 60 females. If harems exceed this size, additional beta-males may be present, each claiming as many females as they can. Females become a part of a harem simply through their position on the beach and may move from one harem to another with reasonable fluidity.
Southern elephant seal. Source: Yan Ropert-Coudert/WoRMS/Encyclopedia of Life In addition to their mating duties, alpha males are responsible for keeping unwanted males away from the harems. This is done through the same vocalizations and aggressive body postures that were used originally to claim their harem. Males must remain on their territory to defend it and, therefore, go for periods of months without eating. This, and the stress of aggressive encounters with other males and the energy expense of mating with multiple females, can take a significant toll on male physical condition. Only males in the best physical condition at the beginning of breeding season will successfully defend their territory and breed with multiple females. Subordinate males attempt to copulate with females on the edges of territories or in the surf as they leave the beach.
Females that were pregnant from the previous year’s mating give birth to one pup five to seven days after arriving on land. Occasionally twin pups are born but one typically dies soon afterwards.
Following birth, mothers bond vocally and through smell with their pup. For the next 20 to 25 days (sometimes as long as 35 days) mothers are responsible for providing milk and protecting pups. Mothers are typically less than one-meter from their pups during the stage of suckling, regardless of tide, the position in the harem, or the time in the breeding season. A pup might get separated from its mother due to male harassment and herding of females. This can result in an abandoned pup. Once a pup is separated from its mother the results are fatal. Alien suckling (nursing between unrelated cows and pups) isn't tolerated in this species. If an orphan pup attempts to steal milk from a sleeping or resting cow, it usually is bitten and will succumb to starvation or the effects of the bites. The most dire threat to young pups is adult males who crush pups as they travel and fight on beach territories. During lactation, mothers do not return to the water to feed and instead live on fat reserves built up during the previous foraging season. At weaning pups weigh from 120 to 130 kg, a weight gain of as much as 105 kg in a few weeks!
Towards the end of this period, females enter estrus and mate with the alpha male or a successful beta male. Shortly following mating, males return to sea. Females return to the sea immediately after the pups are weaned. Pups then forage on their own for several weeks before venturing out to sea in small groups.
Female southern elephant seals typically reach sexual maturity by the age of three and participate in the annual breeding cycle by age six. Males reach sexual maturity by age five or six, but rarely are developed enough to compete for mates until they reach 10 to 12 years of age.
The gestation period of female M. leonina is about eight months. There is a period of several weeks during late October when all mature females mate. In order to maintain the yearly birthing cycle with an eight-month gestation period, there is delayed implantation of the fertilized egg for about three months. After the three-month delay, the egg implants and begins to develop to become mature enough for birth during the next breeding season.
Southern elephant seal. Source: Yan Ropert-Coudert/WoRMS/Encyclopedia of Life Immediately following weaning, female southern elephant seals return to sea, leaving their pups alone on the beach. Eventually the pups begin to get hungry and find their way to the ocean, learning to feed and swim on their own. After weaning, there is no interaction between parents and pups. Approximately 30% of these pups will not live through their first year.
Lifespan/Longevity
There is little known about lifespan of Southern elephant seals, due to the lack of substantial information concerning the periods of the year when they are at sea. Average life expectancy in the wild, as seen during the breeding season, is about 23 years. However, about 30% of pups die in their first year. Captive M. leonina have lived to 15 years of age. Not much is known concerning the deaths of these mammals but, in addition to predation, weather and disease may play a large role in limiting their lifespan.
Behavior
Southern elephant seals have both solitary and social seasons. During breeding and molting, large colonies gather along the same beaches where they were born. Alpha males defend breeding territories in these colonies through a series of specific behaviors. First the outsider vocally challenges the alpha-male. The alpha male rises up on his rear fins and responds in kind. If this does not scare off the challenger, vicious fights can break out with the winner claiming the territory as his own. Colonies reform in the spring when M. leonina go through a molting process that involves the shedding of all fur and the outer layer of skin. In the next three to five weeks they grow new fur and return to the ocean.
Other than the time spent on land for breeding and molting, southern elephant seals live a solitary life in the waters of the southern oceans. Their time at sea is spent diving for food to replenish their weight lost during fasts when on land. Dives occur all day long, with perhaps some times spent resting.
Southern elephant seals are amazing divers and will usually spend at least thirty minutes underwater, come up for a brief two-minute period, and then return underwater for another thirty minutes. While most dives are only between 300 and 800 m, dives of over 1500 m have been recorded, nearing depths only surpassed in mammals by sperm whales. Dives can last well over an hour. Some researchers think they may enter a sleep-like state when diving. Not very much is known about the habits of foraging southern elephant seals due to the solitary nature and the extreme depths to which they dive. During most trips at sea, they are underwater for 90% of the time, day and night.
When at sea, Southern elephant seals rarely encounter each other and thus have little need for communication. The only time communication is used is during breeding. Males use their large proboscis as a sound chamber for amplifying their bellows. These sounds are made to establish territories and challenge males for established harems. Upright posturing often accompanies these vocalizations and males are known to visually assess their competitor before fighting. Lesser males will also exhibit a flattened posture without inflating their proboscis when near another male’s harem to demonstrate that they are not threats.
A threat vocalization is a low-pitched harsh vocalization. While the seal is doing this it will raise its head and forequarters off of the ground, supporting itself without fore flippers. A lunge from an animal is a rapid movement of the head towards an opponent or invader. This is done with an open mouth. A high rear is the raising of the front half of the body then delivering blows to another animal with the neck or chin. A bite may also be used, mainly from a low rear or a high rear position.
Females are known to communicate with newborn pups through vocalizations. Females and pups recognize each other through these vocal cues and through their individual smells.
Distribution
The Southern elephant seal is found widely distributed in the Southern Ocean during the non-breeding season, but in the breeding season, it splits into three discrete populations on islands north of the Antarctic's pack ice. The main islands to support the breeding seals are South Georgia, near the southern-most tip of South America, Macquarie Island, 1,500 km south-south-east of Tasmania, and Kerguelen Island, midway between Africa, Antarctica and Australia. Southern elephant seals are also found on Heard Island, Kerguelen Island, Tierra del Fuego and the Peninsula Valdez in Argentina. When at sea, Southern elephant seals often journey thousands of miles from their breeding grounds.
Habitat
When southern elephant seals are on land, they are typically found along the coast of sub-Antarctic islands including the Islands of Tiera del Fuego on smooth beaches of sand or small rocks. Although they previously bred well north into temperate regions, M. leonina are now only found farther south. They are found on land during the breeding season, from August to November, and the molting season, which lasts three to five weeks in the spring.
The rest of the year is spent entirely at sea. During this time they can be found from sub-Antarctic waters to almost as far north as the equator, often venturing thousands of kilometers from their breeding grounds. While males typically forage on the Antarctic continental shelf, females travel farther into open waters.
Predators
Knowledge of predation of southern elephant seals is limited due to their deep ocean habitat. Known predators include large sharks, specifically Carcharodon carcharias. The Killer Whale, Orcinus orca, and Leopard Seal, Hydrurga leptonyx, are also known to prey on pups. In order to avoid predation, Southern elephant seals have dark dorsal surfaces with lighter undersides. This allows camouflage by blending in with the lighter water when viewed from below and the darker water when seen from above.
Another important role they play in the ecosystem is as a host for many kinds of parasites. Some of the known parasites include cestoda, acanthocephalans, and the louse Lepidophthirus macrorhini.
Feeding Habit
Southern elephant seals feed exclusively when they are at sea. For this reason not much is known about what they eat. The main known sources of food are squid, crabs, shrimp, fish, and sharks. This prey is obtained both near the surface and also during very deep dives. They have been known to eat bottom dwelling fish.
Conservation Status
In the 18th and 19th centuries southern elephant seals were hunted for fur for clothing, and oil for mechanical lubrication, but this no longer occurs, and the population recovered under later protection. A large-scale sealing industry was allowed in the 20th century by the government of South Georgia from 1909 to 1964. It was restricted to adult males but caused population decline, which again was reversed under a research-based management plan.
The southern elephant seal population is currently estimated at 650,000 individuals, but has decreased significantly over the last 40 years. Reasons for this decline are unknown, as other mammals in the southern oceans do not appear to have suffered similar declines. It has been suggested that populations increased rapidly following the drop in commercial sealing, overshooting the maximum population that could be supported by the ecosystem, and are now dropping back to equilibrium.
An increase in commercial fishing in this area could cause future problems. Seals are seen entangled in net as well as covered in oil. A research group studying southern elephant seals had been using hot iron branding to mark individuals, but the Tasmanian government banned this activity after it was found that branded seals were more likely to be in poor condition than unbranded seals.
Southern elephant seals are protected by the U.S. Fish and Wildlife Service's Marine Mammal Act 1972, but research into the reasons behind the decline of this species must be conducted before management plans can be drawn up. Priorities for research need to focus on the continuation of census programmes, demographic studies and investigations into several aspects of the biology of first-year seals, particularly diet and foraging ranges. Some populations of southern elephant seals are found in a 16 million hectare protected Marine Park on the eastern side of Macquarie Island.
Although once hunted by humans, southern elephant seals were never near extinction like elephant seals]. This is largely because most of the breeding grounds of Southern elephant seals were out of reach of hunting boats. Hunting did have some impact, but numbers have recovered since hunting has ceased. Some populations are are experiencing declines. This may be normal population fluctuations, however.
Economic Importance for Humans
In the past, southern elephant seals were hunted for their blubber that was boiled down into oil. A typical male could produce about 350 liters of oil. Some aboriginal people also hunted them for food and skins. This activity has ceased and killing is now controlled by the Convention for the Conservation of Antarctic Seals. The only use of southern elephant seals to man today is for purely scientific purposes.Southern elephant seals may occasionally compete with some fisheries, but this is unlikely. Southern elephant seals live in remote regions where they have few interactions with humans.
Further Reading
- Mirounga leonina (Linnaeus, 1758) Encyclopedia of Life (accessed Feb. 22, 2010)
- Mirounga leonina, Block, D., P. Meyer and P. Myers, 2004, Animal Diversity Web (accessed April 15, 2009)
- Southern elephant seal, Seal Conservation Society (accessed Feb. 22, 2010)
- Mirounga leonina IUCN Red List of Threatened Species (accessed Feb. 22, 2010)
- The Pinnipeds: Seals, Sea Lions, and Walruses, Marianne Riedman, University of California Press, 1991 ISBN: 0520064984
- Encyclopedia of Marine Mammals, Bernd Wursig, Academic Press, 2002 ISBN: 0125513402
- Marine Mammal Research: Conservation beyond Crisis, edited by John E. Reynolds III, William F. Perrin, Randall R. Reeves, Suzanne Montgomery and Timothy J. Ragen, Johns Hopkins University Press, 2005 ISBN: 0801882559
- Walker's Mammals of the World, Ronald M. Nowak, Johns Hopkins University Press, 1999 ISBN: 0801857899
- Southern elephant seal, MarineBio.org (accessed Feb. 22, 2010)
- CITES
- Laws, R.M. and Le Boeuf, B.J. (1994) Elephant seals: population ecology, behaviour and physiology. University of California Press, Berkeley, California.
