An Australian Sea Lion
Eared seal is the common name for any of the marine mammals comprising the pinniped family Otariidae, characterized by presence of a pinna (external part of ear), the ability to invert their hind-flippers under the body, aiding land movement, and a swimming motion using their long front flippers to propel them through the water. These characteristics help distinguish otariids from the earless seals of the family Phocidae. Extant eared seals comprise 16 species in seven genera commonly known either as sea lions or fur seals.
Otariids are adapted to a semi-aquatic lifestyle, feeding and migrating in the water but breeding and resting on land or ice. They reside in subpolar, temperate, and equatorial waters throughout the Pacific and Southern oceans and the southern Indian and Atlantic oceans. These marine mammals are conspicuously absent in the north Atlantic.
Eared seals play key roles in food chains, consuming fish, mollusks, and sometimes penguins, and being consumed by killer whales, sharks, and bears. They also provide direct values for human beings. Seals have traditionally been hunted for their furs, while sea lions have been trained for such underwater tasks as finding objects or detecting and attaching a clamp to any person underwater who may be approaching military ships or piers (Leinwand 2003).
Eared seals are one of the three main groups of mammals within the taxonomic group Pinnipedia. Pinnipeds are aquatic (mostly marine) mammals that are characterized by having both front and hind limbs in the form of flippers. In addition to eared seals, other pinnipeds are walruses and earless seals.
Eared seals are one of two groups of seals (any pinniped other than walruses): Earless seals, comprising the taxonomic family Phocidae (phocids), and eared seals comprising the family Otariidae (otariids). Walruses generally are considered a separate family of pinnipeds, the Obobenidae, although sometimes they are included with the phocids (McKenna and Bell 1997).
One way of differentiating between the two main groups of seals is by the presence of the pinna, a small furry earflap (external ears), found on the otarids and missing from phocids. Phocids are referred to as "earless seals" because their ears are not easily seen, while otarids are referred to as "eared seals." The name otariid comes from the Greek otarion meaning "little ear," referring to the small but visible external ear flaps (pinnae).
In addition to the presence of the pinna, there are other obvious differences between otarids and phocids. Otarids have hind-flippers that can be inverted under the body, aiding their movement on land, while the hind-flippers of phocids cannot be turned forward under the body, causing their movement on land to be slow and awkward (Riedman 1990). Otarids also swim using their long front flippers to move themselves through the water, while phocids swim by using their rear flippers and lower body in a side-to-side motion (Riedman 1990). There are also behavioral differences, including the breeding systems.
The eared seals include both fur seals and sea lions. Traditionally, the fur seals were placed in the otariid subfamily Arctocephalinae and the sea lions in the subfamily Otariinae. However, recent studies have suggested that the differences between the fur seals and the sea lions are not great enough to separate them into these two subfamilies (Riedman 1990; ITIS 2006; Brunner 2003).
There are no otariids living in the extreme polar regions; among seals, only earless seals live and breed in the Antarctic and Arctic (Riedman 1990). On the other hand, a number of fur seals and sea lions live in tropical and subtropical areas, while only two species of phocids (the endangered Hawaiian and Mediterranean monk seals) are found in the tropics and these are small populations; fur seals also range widely into colder climates as well (Riedman 1990). The Antarctic fur seal (Arctocephalus gazella), which breeds on islands between 45° S and 60° S (95 percent of the population at South Georgia), likely has a winter range that includes spending time close to the Antarctic ice.
Otariids have proportionately much larger fore-flippers and pectoral muscles than phocids, and have the ability to turn their hind limbs forward and walk on all fours, making them far more maneuverable on land. They are generally considered to be less adapted to an aquatic lifestyle, since they breed primarily on land and haul out more frequently than true seals. However, they can attain higher bursts of speed and greater maneuverability in the water. Their swimming power derives from the use of flippers more so than the sinuous whole body movements typical of phocids and walruses.
Otariids are further distinguished by a more dog-like head, sharp, well-developed canines, and the aforementioned visible external pinnae. Their postcanine teeth are generally simple and conical in shape. The dental formula for eared seals is:
In general, fur seals have a more pointed snout and longer fore-flippers than sea lions, and they have a thick, luxuriant coat of fur (pelage) (Riedman 1990). The underfur, which is waterproof, is covered with long, "guard" hairs that give them a "somewhat grizzled appearance" (Riedman 1990). The thick underfur of fur seals have historically made them the objects of commercial exploitation. Sea lions, which generally are larger than fur seals, have a more rounded muzzle and shorter fore-flippers than fur seals, and their pelage is more short and coarse (Riedman 1990).
Male otariids range in size from the 70 kilograms (150 pounds) Galapagos fur seal, smallest of all pinnipeds, to the over 1000 kilogram (2200 pounds) Steller sea lions. Mature male otariids weigh two to six times more than females with proportionately larger heads, necks, and chests, making them the most sexually dimorphic of all mammals (Weckerly 1998).
All otariids breed on land during well-defined breeding seasons. Except for the Australian sea lion, which has an atypical 17.5 month breeding cycle, they form strictly annual aggregations on beaches or rocky substrates, often on islands. All species are polygynous; that is, successful males breed with several females.
In most species, males arrive at breeding sites first and establish and maintain territories through vocal and visual displays and occasional fighting. Females typically arrive on shore shortly before giving birth to pups from the mating of the previous year. Females go into estrous sometime after giving birth, perhaps a week or two weeks later, and they breed again, but implanting of the embryo is delayed, allowing an annual cycle in most species.
While considered social animals, there are no permanent hierarchies or statuses established on the colonies. The extent to which males control females or territories varies between species. Northern fur seals and South American sea lions tend to herd specific harem-associated females, occasionally injuring them, while Steller sea lions and New Zealand sea lions control spatial territories but do not generally interfere with the movement of the females.
Otariids are carnivorous, feeding on fish, squid, and krill. Sea lions tend to feed closer to shore in upwelling zones feeding on larger fish while the smaller fur seals tend to take longer, offshore foraging trips and can subsist on large numbers of smaller prey items. They are visual feeders and some females are capable of dives up to 400 meters (1300 feet).
Along with the Phocidae and Odobenidae, the two other members of Pinnipedia, Otаriidae are considered to be descended from a common ancestor most closely related to modern bears (Lento et al. 1995). There remains debate as to whether the phocids diverged from the otariids before or after the walruses.
Otariids arose in the late Miocene (10 to 12 million years ago) in the North Pacific, diversifying rapidly into the Southern Hemisphere, where most species now live. The Callorhinus (northern fur seal) genus is considered to have the oldest lineage.
Traditionally, otariids had been subdivided into the fur seal (Arctocephalinae) and sea lion (Otariinae) subfamilies, with the major distinction between them being the presence of a thick underfur layer in the former. Under this categorization, the fur seals comprised two genera: Callorhinus in the North Pacific with a single representative, the northern fur seal (C. ursinus) and eight species in the southern hemisphere under the genus Arctocephalus, while the sea lions comprise five species under five genera (King 1983).
Recent analyses of the genetic evidence suggests that the Callorhinus ursinus is in fact more closely related to several sea lion species (Wynen et al. 2001). Furthermore, many of the Otariinae appear to be more phylogenetically distinct than previously assumed; for example, the Zalophus japonicus is now considered a separate species, rather than a subspecies of Zalophus californius. In light of this evidence, the subfamily separation generally has been removed entirely and the Otariidae family has been organized into seven genera with 16 species and two subspecies (Brunner 2003; ITIS 2006).
Nonetheless, because of morphological and behavioral similarity among the "fur seals" and among "sea lions," these remain useful categories when discussing differences between groups of species.
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