Elephant seals
Description
Southern elephant seals (Mirounga leonina) are named after the large proboscis (nose) of the adult males, which is used to make loud roaring sounds, especially during the mating season.
They are large ocean-going mammals with adult males weighing up to 3000 kg and adult females between 300 and 900 kg just prior to giving birth. Pups weigh about 40 kg at birth and are weaned after 24 days by which time they weigh on average 120 kg. A large weaner may weigh in excess of 220 kg.
Recently weaned elephant seal pups (weaners).
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Adult male Elephant seal at Macquarie Island station.
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Male elephant seal showing inflated proboscis
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They are big and cumbersome on land, but are superb swimmers and divers. Biologists have recorded them diving up to 2 km deep and holding their breath under water for up to 2 hours.
Diet
Elephant seals migrate south to Antarctica, after breeding or moulting, to feed on squid and fish at the edge of the sea-ice.
They travel long distances to their foraging areas. Males forage mainly on the Antarctic continental shelf while females forage in more pelagic areas, such as off the Antarctic shelf within the pack ice, or near the Antarctic Polar Frontal Zone. Foraging areas can be several thousand kilometres away from their breeding islands.
Breeding
Adult elephant seals return from Antarctica to subantarctic islands such as Heard and Macquarie Islands for the breeding season (September-November).
They breed in colonial groups called harems. A single dominant male usually can control a harem of up to 50 females. Any male intruder will be fought with to decide who dominates the group. Once harems get larger than this the dominant beachmaster will allow other males (assistant beachmasters) to reside within the harem. Harems can grow to over 1000 females with over 30 males.
Male elephant seals fighting at Macquarie Island station.
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Fighting on the beach.
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Abundance and distribution
Southern elephant seals have a circumpolar distribution and visit subantarctic islands to breed (September-November) and and to moult their hair and skin (January to April). There are four main stock groups: South Georgia, Peninsula Valdez, Iles Kerguelen (including Heard Island), and Macquarie Island.
New-born elephant seal pup
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Attaching a Time-Depth Recorder (TDR) to an adult female elephant seal (she's fast asleep).
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From 1950 to 1985 the elephant seal populations at Macquarie Island, Heard Island, and others of the Iles Kergulen stock declined by about 50%. Since then the population at Heard and Kerguelen has remained relatively stable but the Macquarie population has continued to decline at about 1.2% per year.
Australian scientists have tracked these seals using plastic ear tags. Elephant seals cruise the whole Southern Ocean and can swim enormous distances. Individuals spotted on Kerguelen Island, for example, have later been seen at Davis then Casey station in Antarctica.
Special adaptations
Elephant seals can navigate very accurately to feed. They can dive to over 1500 m and can stay submerged for up to 2 hours. Most dives are about 30 minutes duration and of depths between 300 and 800 m. The seals can dive constantly while at sea, spending about 2 minutes on the surface between dives.
Elephant seals are able to accomplish these amazing diving feats because they have evolved some special adaptations, by which they conserve energy very economically.
They have a torpedo shape, which accounts for their prowess in swimming and diving. And an enormous volume of blood in which to store oxygen, which they use very efficiently. Elephant seals even have extra spaces called sinuses in their abdomens to store extra blood.
Haemoglobin in red blood cells carries oxygen and elephant seals have a lot more red blood cells per unit of blood than other animals. Their red blood cells may, as an adaptation for diving, contain more haemoglobin than normal. These extra red blood cells make elephant seals' blood very thick. Elephant seal muscles are also used to store oxygen. Molecules of oxygen-carrying myoglobin are present in the muscles and colour them black. The analogy of fully charging a battery is sometimes used to describe their ability to take a breath and make the oxygen last for up to two hours.
Southern elephant seals have a thick layer of blubber that sustains them during the breeding season as they do not feed during this time. For many years seals were killed for their blubber which was boiled down to make oil.
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