TRUE SEALS

Southern Elephant Seal (Mirounga leonina)

Southern Elephant Seals (Mirounga leonina) are named for their huge size, and trunk-like proboscis developed by mature males. The seals were given their latin name by native people of Australia, with ‘leonina’ referring to their enormity, and moreover their lion-like roaring produced during sexual competition. This elephant seal of the Southern Oceans is found all around the Antarctic continent, and as far north as the subantarctic countries and the southern tip of South America. M. leonina are classified as a Least Concern species. Estimated population size = 650,000.
M. leonina are huge true seals, substantially bigger than their northern hemisphere cousins, and the largest pinniped in the world (males ♂ up to 6.5 m long/ 3,700 kg, females ♀ up to 4 m/ 800 kg). Like all other true seals, their hind flippers are backward-facing and broad, with clawed, elongated fore flippers which are still short relative to the size of the seal. They are thick and heavy bodied, with extreme sexual dimorphism. The males have bulky, heavily scarred necks with rows of skin folds behind the head, and the characteristic nose proboscis hanging in front of the mouth. Females are considerably smaller, have rounder heads with blunt, downward-facing noses, and lack the conspicuous proboscis. Males are usually dark brown with pale and cratered neck skin, while females range from paler brown to tan in colour. Blood and battle damage is a common sight in sexually mature males.
Feeding behaviour:
The diet of M. leonina varies according to location and season. Spending large amounts of time at sea and routinely diving to depths of 200-700 m, sometimes far greater, means that they can exploit a range of habitats from ice-edge to pelagic to ocean floor. Studies have shown that animals such as lanternfish, squid and other twilight zone species make up a large bulk of the diet. There is also diversity in male and female foraging preferences.
Breeding behaviour:
Sexual maturity – males ♂ 4-6 years old/ females ♀ 3-5 years old.
M. leonina breed in a polygynous system, with males fiercely defending their females. Males haul out earliest in the season and engage in aggressive displays and agonistic exchanges for control of large harems of females. The successful dominant males or ‘beachmasters’ then guard these territories for the full breeding season without eating, as a result gaining the rights to copulate with in excess of 50 (and occasionally 100) cows on land. Gestation lasts approximately 11 months (with a delayed implantation period of maybe 4 months) and pups are mostly born in the Antarctic spring during October. Females fast whilst nursing their black-coated youngster for ~ 3 weeks before they are weaned. She will then depart to feed, leaving the pup onshore for a further 4-8 weeks. During this time it will complete its moult into a waterproof silvery grey-brown juvenile pelt, and form groups with conspecifics, living on blubber reserves and slowly venturing into sheltered waters at night to learn to dive and forage. The annual catastrophic moult for adults takes place during January and February and lasts around 1 month with females slightly earlier than males. The lifespan of M. leonina is 15-23 years, though they are generally longer-lived than the Northern Elephant Seal (Mirounga angustirostris).
Though M. leonina exist in distinct geographical subpopulations, they are not distinguishable on a subspecies level.

Photo: Tom Mason

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