Elephant seals are pinnipeds
Pinnipeds are carnivorous marine mammals with fin-like limbs. They breathe air like humans and other mammals, reptiles and birds, but they spend most of their time at sea.
Pinnipeds include "true seals", sea lions, fur seals and walruses. Elephant seals are the largest of the true seals.
Hear elephant seals
The differences between "true seals" and "sea lions"
True seals, such as elephant seals, differ from other pinnipeds in two easy-to-recognize ways. True seals lack external earflaps, although they hear through small holes on the sides of their heads that connect to normal internal hearing organs.
True seals can't "walk" on all fours like a sea lion because their back flippers don't bend forward like human feet. They move like an inchworm by pulling their heavy bodies forward with strong front flippers.
Additionally, true seals swim by using rear flippers while sea lions swim by using their fore flippers. When it comes to scratching, it's the opposite, true seals use their fore flippers while sea lions use their rear flippers. And in general, true seals have more blubber and are more streamlined.
Whether obvious or hidden, physical attributes allow northern elephant seals to excel in the deep marine environment and to coexist with other seals and sea lions in the eastern North Pacific.
Photographer - Mike Baird.
License - Creative Commons Attribution-Noncommercial 2.0 Generic
The two species of elephant seals
There are two living species of elephant seals, the northern elephant seal, Mirounga angustirostris and the southern elephant seal, Mirounga leonina. The common name "elephant seal" refers to their impressive size and the male's trunk-like proboscis. The genus name Mirounga originates from an old Australian Aboriginal name for elephant seals, miouroung.
In their classic book on elephant seals, experts Le Boeuf and Laws  mention the particularly poor choice of names given to both the northern and southern species - angustirostris that refers to the narrowness of the female's nose rather than the remarkable size of the male's proboscis; and leonina that refers to a "lion" or "sea lion" which elephant seals are not.
Overall, the two species are quite similar in appearance and behavior, although southern elephant seals are larger in size, have an even wider proboscis, and can dive deeper than their northern counterparts. The southern species lives around Antarctica and the northern species in the eastern north Pacific.
Males and females are so different in size and appearance that they may not be recognized as members of the same species. The differences in form between females and males of the same species is called sexual dimorphism. Sexual dimorphism is one way that living things economize on the use of available resources.
Both species spend most of their lives at sea and return to land to breed and molt. Breeding colonies in the north include the beaches of California and Baja California. Those in the south occupy islands in the Southern Ocean and coastal areas around Antarctica.
Both species are thought to have originated from monachinoid seals that made their way from the Atlantic into the Pacific about 15 to 20 million years ago when the barrier between the two oceans was open across central America. Which species developed first and how they became separated between the two hemispheres is not known.
Family Odobenidae - Walruses
Odobenus rosmarus, walrus
Family Otariidae - Eared seals or sea lions and fur seals
Callorhinus ursinus, northern fur seal
Otaria byronia, southern sea lion
Eumetopias jubatus, northern sea lion
Zalophus californianus, Californian sea lion
Arctocephalus pusillus, South African fur seal
Phocarctos hookeri, Hooker's sea lion
Neophoca cinerea, Australian sea lion
Arctocephalus townsendi, Guadalupe fur seal
Arctocephalus forsteri, New Zealand fur seal
Arctocephalus australis, South American fur seal
Family Phocidae - Earless seals or true seals
Monachinae - monk seals and southern true seals
Monachus monachus, Mediterranean monk seal
Monachus schauinslandi, Hawaiian monk seal
Mirounga leonina, southern elephant seal
Mirounga angustirostris, northern elephant seal
Ommatophoca rossi, Ross seal
Lobodon carcinophagus, crabeater seal
Leptonychotes weddelli, Weddell seal
Hydrurga leptonyx, leopard seal
Phocinae, northern true seals
Erignathus barbatus, bearded seal
Cystophora cristata, hooded seal
Histriophoca fasciata, ribbon seal
Pagophilus groenlandicus, harp seal
Phoca largha, largha seal
Phoca vitulina, harbor seal, Icelandic specimen
Phoca vitulina, harbor seal, Baltic Sea specimen
Phoca caspica, Caspian seal
Phoca sibirica, Baikal seal
Phoca sibirica, Baikal seal
Phoca hispida, ringed seal, Baltic Sea specimen
Halichoerus grypus (Phoca grypa), grey seal
Table adapted from Arnason et al., 2006. Pinniped phylogeny and a new hypothesis for their origin and dispersal. Molecular Phylogenetics and Evolution, 41, 345-354.