The southern elephant seal (Mirounga leonina)
The southern elephant seal (Mirounga leonina) is one of two species of elephant seals. It is the largest member of the clade Pinnipedia and the order Carnivora, as well as the largest extant marine mammal that is not a cetacean. It gets its name from its massive size and the large proboscis of the adult male, which is used to produce very loud roars, especially during the breeding season. A bull southern elephant seal is about 40% heavier than a male northern elephant seal (Mirounga angustirostris), which is nearly twice the weight of a male walrus (Odobenus rosmarus), or 6–7 times heavier than the largest living mostly terrestrial carnivorans, the Kodiak bear and the polar bear.
Taxonomy
The southern elephant seal was one of the many species originally described by Swedish zoologist Carl Linnaeus in the landmark 1758 10th edition of his Systema Naturae, where it was given the binomial name of Phoca leonina. John Edward Gray established the genus Mirounga in 1827.
In the nineteenth century, the species was often called "bottle-nosed seal".
Appearance
Mirounga leonina is a species of pinniped mammal in the family Phocidae.
Mirounga leonina is the largest seal species in the world. The trunk of Mirounga leonina is considerably shorter than that of its northern counterpart, measuring about 10 cm. This huge, enlarged nose is absent in females and young males. After continuous growth, the trunk reaches its full size by the eighth year of life and hangs over the mouth with the nostrils downwards. During the rutting season, this trunk swells even more due to the increased tide of blood. It happens that during fights the more aggressive males tear each other's trunks apart. Size differences between males and females are substantial. The male can reach a size of up to six and a half metres, while the female is only three and a half metres long. The male weighs up to three and a half tonnes, the female weighs a maximum of 900 kg.
Habitat and feeding area
There are large colonies of Mirounga leonina in the following archipelagos and sub-Antarctic islands: South Georgia, Kerguelen, Herd, Macquarie. Outside the breeding season, individuals can be found off the coasts of South Africa, Australia, New Zealand, Patagonia and Antarctica. These animals can travel sea distances of up to 4 800 km.
The prey of Mirounga leonina are fish and cephalopods. Mirounga leonina is able to dive for prey up to a depth of 1400 m. This is possible thanks to its large mass of fish and cephalopods. This is made possible by its large mass and large blood volume, which is able to store a lot of oxygen. As with whales, the internal organs of Mirounga leonina slow down when diving deeper, so oxygen consumption is reduced. The natural enemies of Mirounga leonina are white sharks and killer whales that hunt in the upper layers of the water.
Social behavior
Elephant seals are among the seals that can stay on land for the longest periods of time, as they can stay dry for several consecutive weeks each year. Males arrive in the colonies earlier than the females and fight for control of harems when they arrive. Large body size confers advantages in fighting, and the agonistic relationships of the bulls gives rise to a dominance hierarchy, with access to harems and activity within harems being determined by rank. The dominant bulls (“harem masters”) establish harems of several dozen females. The least successful males have no harems, but they may try to copulate with a harem male's females when the male is not looking. The majority of primiparous females and a significant proportion of multiparous females mate at sea with roaming males away from harems.
An elephant seal must stay in his territory to defend it, which could mean months without eating, having to live on his blubber storage. Two fighting males use their weight and canine teeth against each other. The outcome is rarely fatal, and the defeated bull will flee; however, bulls can suffer severe tears and cuts. Some males can stay ashore for more than three months without food. Males commonly vocalize with a coughing roar that serves in both individual recognition and size assessment. Conflicts between high-ranking males are more often resolved with posturing and vocalizing than with physical contact.
Feeding and diving
Satellite tracking revealed the seals spend very little time on the surface, usually a few minutes for breathing. They dive repeatedly, each time for more than 20 minutes, to hunt their prey—squid and fish—at depths of 400 to 1,000 m (1,300 to 3,300 ft). They are the deepest diving air-breathing non-cetaceans and have been recorded at a maximum of 2,388 m (7,835 ft) in depth.
As for the duration, depth and sequence of dives, the southern elephant seal is the best performing seal. In many regards, they exceed even most cetaceans. These capabilities result from nonstandard physiological adaptations, common to marine mammals, but particularly developed in elephant seals. The coping strategy is based on increased oxygen storage and reduced oxygen consumption.
In the ocean, the seals apparently live alone. Most females dive in pelagic zones for foraging, while males dive in both pelagic and benthic zones. Individuals will return annually to the same hunting areas. Due to the inaccessibility of their deep-water foraging areas, no comprehensive information has been obtained about their dietary preferences, although some observation of hunting behavior and prey selection has occurred.
While hunting in the dark depths, elephant seals seem to locate their prey, at least in part, using vision; the bioluminescence of some prey animals can facilitate their capture. Elephant seals do not have a developed system of echolocation in the manner of cetaceans, but their vibrissae (facial whiskers), which are sensitive to vibrations, are assumed to play a role in search of food. When at the subantarctic or Antarctic coasts, the seals forage largely on deep-sea cephalopod species such as Psychroteuthis glacialis, Alluroteuthis antarcticus, Histioteuthis eltaninae, Onykia ingens, Gonatus antarcticus. Martialia hyadesi and other molluscs, various fish species, including lanternfish (i.e. Electrona spp. and Gymnoscopelus spp.), nothothens (i.e. Genera Lepidonotothen, Pleuragramma, Trematomus, Pagothenia,), Channichthyidsae spp., Bathylagidae spp., krill (mostly Euphausia spp.) and other crustaceans, and even algae.
Reproduction
Mirounga leonina calves are born in October. The mother feeds them with milk for 20 days and then leaves them. In the 19th century, Mirounga leonina were heavily hunted, but their situation was even less critical than that of the northern Mirounga leonina. There were formerly established colonies of Mirounga leonina on Tasmania, King's Island, Juan Fernandez Islands and St. Helena, but these animals were extirpated there. Today, the total population of Mirounga leonina is about 750,000, more than half of which are found on South Georgia.
During the mating season, Mirounga leonina, which is usually solitary, gathers in large colonies. The ratio of males to females is approximately one to ten, sometimes even one to twenty. There are fierce fights between males for control of the harem. Weaker or younger males are pushed to the edge of the colony, where they find less favourable conditions for mating.
However, they constantly try to penetrate the centre, resulting in new fights from time to time. Under the protection of the dominant male, the fertilised females give birth to their offspring conceived a year earlier. They suckle their young for a few weeks before mating with the male again. If the male wants to mate with a female, he places one of his front flippers on her and bites her on the back of the neck. Copulation then begins. If the female resists, the male crawls over her and prevents her from moving under the pressure of his weight.
Mirounga leonina reaches sexual maturity at three to four years of age. However, males are not strong enough to defend their own harem until they are eight years old. Mating at an earlier age is unlikely for them. The average life expectancy of males is lower than that of females due to multiple fights and is only 14 years. Females live on average four years longer.
Predation
Weaned pups and juveniles may fall prey to orcas. Cases where weaned pups have been attacked and killed by leopard seals (Hydrurga leptonyx) and New Zealand sea lions (Phocarctos hookeri), exclusively small pups in the latter case, have been recorded. Great white sharks (Carcharodon carcharias) have hunted elephant seals near Campbell Island, while bite marks from a southern sleeper shark (Somniosus antarcticus) have been found on surviving elephant seals in the Macquarie Islands.
Conservation
After their near extinction due to hunting in the 19th century, the total population was estimated at between 664,000 and 740,000 in 2005, but as of 2002, two of the three major populations were declining. The reasons for this are unclear, but are thought to be related to the distribution and declining levels of the seals' primary food sources. Most of their important breeding sites are now protected by international treaty, as UNESCO World Heritage Sites, or by national legislation.
