provided by EG-BAMM, Iain Field
Southern elephant seals are phocids, or true seals, and are the largest of all seal species. They have a circumpolar distribution, breeding mainly on subantarctic islands. At sea they have been found to inhabit almost all of the Southern Ocean and travel long distances during their foraging migrations. They are highly sexually dimorphic, with males (over 4000 kg) being up to ten times larger than females (~450 kg). Males will attempt to join the breeding system at around seven years of age and may live to 14 years old, whereas the females are recruited into the breeding population from age four and may live to 24 years of age.
Their scientific name, Mirounga leonina, is thought to be a combination of the Australian aboriginal name for them ‘miouroung’ for the genus and the latin word for lion as the specific part, due to their roar and threat vocalisations.
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The distribution of southern elephant seals is circumpolar and ranges mainly in subantarctic waters from 16° S at Saint Helena to 78° S. The seals’ haul-out locations are typically subantarctic islands lying between 40 and 62° S of the Atlantic and Indian Ocean sectors of the Southern Ocean. While at sea the seals forage widely in the Southern Ocean from the high latitudes around the Antarctic continent to temperate waters around Argentina, Chile, southern Africa, Australia and New Zealand. During these long foraging trips the seals may spend more than 9 months of the year at sea and travel over 5000 km in a round trip. On the basis of their chief haul-out locations, four main breeding populations have been identified: South Georgia (population size ~ 400, 000) in the south Atlantic, Iles Kerguélen and Heard Island (~ 220,000) in the Indian Ocean, Macquarie Island (~ 76,000) in the south Pacific Ocean, and on Peninsula Valdez (~ 42,000) in Argentina. There is estimated to be little gene flow between these populations. The global population in recent years has increased from 664,000 in 1994 to 740,000 in 2001. The increasing population at Peninsula Valdéz has mainly driven this overall increase. The South Georgia population has remained stable over the past few decades. The population in the Indian Ocean at Iles Kerguélen and Heard Island has remained stable since 1990 after declining since the 1950s, though the Macquarie Island population has continued to decrease for reasons that are remain unclear. Though only a small population, the Marion Island population, in the south Indian Ocean, has also continued to decrease until recently. The primary reason for these declines between the 1950s and 1990 has been suggested to result from food limitation with inter-island differences attributed to factors such as competition with other species and predation.
Extreme dives have been recorded to depths greater than 2000 m. Elephant seals commonly dive between 300 to 1500 m.
Southern elephant seals are major consumers of biomass, primarily squid and fish in the Southern Ocean. The life cycle of southern elephant seals is a combination of terrestrial haul-outs required for breeding (September to November) and moulting (December to March) interspersed with long periods at sea foraging. In the case of juveniles, the adult breeding haul-out is replaced with a mid-year haul-out (April to August).
While ashore the seals are found on sandy or pebble beaches during the breeding season on subantarctic Islands, during the moult and midyear haulouts their habitat extends into the vegetated fringes of these islands. At sea the seals will forage pelagically or on the bottom and range across all the physical divisions of the Southern Ocean from temperate waters to the high Antarctic coastline. In the Antarctic the seals commonly live within or at the edge of the sea ice, in the open ocean the seals are thought to concentrate their foraging activity around fronts and ocean features.
Southern elephant seals are an important predator within the southern ocean ecosystem, with a broad foraging range and are generalist foragers. The diet of the seals’ consists mainly of squid and fish, although as the seals get older their diet changes from being dominated by more fish to more squid. There are also differences in the diet of the seals depending on their foraging location which is related to the variability in productivity and ecosystem dynamics of the Southern Ocean. Tracking studies have shown that adults seals can be to deeper than 2000 m and hold their breath for over two hours. Commonly female seals are foraging between 300 and 800 m and males between 300 m and 1500 m.
The breeding season starts early in the austral summer when the breeding males haul out on the beaches in August to establish territories, followed by the pregnant females coming ashore in September. Elephant seals have been observed to display a high degree of philopatry, retuning to breed close to or at their natal beaches. The breeding strategy used is a polygamous system where one male will serve many females. The female seals form large groups, known as harems, often dominated by a single male (a 'beachmaster'). The position of a beachmaster is often challenged and may be relinquished several times in a season. These challenges are often settled by posturing and vocalisation; however, there can be fierce battles between equally sized males. Once a harem size grows beyond the control of one male, he will tolerate assistant beachmasters to protect the harems from the other ‘challengers’ and ‘sneakers’ at the edge of the harem. Over the entire breeding season the males are solely interested in access to females and play no role in provisioning or protection of the pups. Each female gives birth to a single pup weighing on average 40 kg. The females lactate for ~ 23 days converting ~ 30% of their energy stores (blubber) to milk for transfer to their pups. The energy transfer is highly efficient and it is expected that the pups will weigh on average 120 kg at weaning. Although rare, some females have been known to produce two pups though one may be favoured or the weaker abandoned. There is also a degree of natural abandonment by young and inexperienced mothers. The females become sexually receptive at ~18 days after birth and may be mated several times before weaning. The pups are weaned when the females abandon them and return to the sea to replenish their energy reserves before their annual moult. Once weaned, the pups, or ‘weaners’, spend between 3 and 10 weeks fasting on land. The weaners may initially remain around the harems though are often bitten by other females and so move away from the harems to the back beach areas until the females have gone to sea. By early November most of the females have left to forage and the weaners move freely around the beaches with most having departed for their first foraging trip by the end of December. During this time, they lose ~30% of their weaning mass, develop their muscle condition and learn to swim. They spend successively more time in the water towards the end of the fast. The amount of time spent on the beach is most likely influenced by the resources available to them at weaning, and large weaners stay longer than small weaners. This may give them more time to develop the breath holding and heart rate control mechanisms crucial for effective diving and foraging. The breeding season ends in late November when the last of the males and females leave the beaches to forage and fatten up after the breeding fast.
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