Eared seals are marine mammals that belong to the family Otariidae, one of the three groups of pinnipeds, with the other two being the earless or true seals (family Phocidae) and the walrus (family Odobenidae). Unlike true seals, eared seals are easily recognized by prominent external ear flaps or pinnae.
These semi-aquatic mammals hunt in the water but return to land or ice for breeding and resting. They primarily feed on fish, squid, krill, and occasionally penguins.
They are mainly spread across the subpolar and temperate regions of the Pacific and Southern Oceans, the South Atlantic Ocean, and the southern Indian Ocean. The Galapagos fur seal is the only species found in the equatorial waters of the Pacific Ocean.
Currently, 15 extant species of eared seals are classified into two subfamilies: fur seals (Arctocephalinae) and sea lions (Otariinae).
They vary significantly in size, ranging from the smallest species, the Galápagos fur seal, which weighs around 150 lb (68 kg), to the largest, the Steller sea lion, which can reach up to 2,200 lb (998 kg).
Among the two subfamilies, the fur seals are smaller than sea lions and exhibit more pronounced sexual dimorphism. Moreover, adult male fur seals are often up to five times as heavy as their female counterparts.
These animals have slender, elongated bodies and small yet distinct ear flaps, a feature that gives them their name. While fur seals are covered with a thick underfur, sea lions possess coarse guard hairs on their bodies for insulation.
When compared to true seals, their foreflippers and pectoral muscles are larger. Moreover, they can easily twist their hind limbs forward and walk on all four limbs.
They have a robust, bear-like skull characterized by the presence of pre- and postorbital processes, as well as an alisphenoid canal. Their occipital condyles are located at the back of the cranium.
When compared to true seals, eared seals have large, well-developed canines, followed by simple, conical postcanine teeth. Moreover, the first two incisors are marked by distinct transverse grooves that resemble two prominent cusps.
The dental formula for this group is 3.1.4.1-3/2.1.4.1 = 34-38.
The name of the family Otariidae stems from the Greek word otarion, which translates to ‘little ear.’
All eared seal species are divided into 2 subfamilies and 7 genera.
They first appeared in the Miocene Epoch, between approximately 15 and 17.1 million years ago, in the North Pacific Ocean.
Their earliest fossils are represented by remains of the species Eotaria crypta unearthed from southern California.
In the Pacific Ocean, these seals are found along North and South America, the coasts of central and northern Asia, and New Zealand, including several islands, like the Galapagos. In the South Atlantic Ocean, their range is limited to the South American coastline. Meanwhile, in the Indian Ocean, they are confined to the southwestern coast of Australia, including its nearby islands.
Being carnivorous, these mammals primarily consume fish, squid, and krill. However, they may occasionally feed on penguins, too.
While sea lions typically feed on larger fish found near the shore, fur seals move offshore to consume large quantities of smaller fish.
Depending on the species, fur seals live between 13 and 25 years, whereas sea lions survive longer, up to 30 years in the wild.
All eared seals are polygynous (single males typically mating with multiple females simultaneously), and they usually gather on beaches or rocky substrates for annual breeding. However, the Australian sea lion has an unusual breeding cycle of more than a year (about 17.5 months).
Males arrive at the breeding ground first, establishing territories through vocal displays and, if necessary, physical confrontations with other males. Females arrive shortly (a day or so) before giving birth to pups of the previous breeding cycle, and within a short time, males form harems comprising up to 50 females. They mate within a few weeks after giving birth.
The fertilized egg undergoes delayed implantation of up to 6 months (depending on the species), followed by a year-long gestation period (in most species). This late implantation allows females to conceive immediately after giving birth to a pup from the previous breeding season.
Once born, the pups, particularly those of the New Zealand sea lion, are led by their mothers up to 2 km (1.2 mi) inland, into dense forests for protection from predators. The pups are nursed for anywhere between 4 and 30 months, depending on the species, and they are gradually weaned as they reach about 40% of their mother’s body weight.
These seals are killed by sharks, killer whales, and leopard seals. Moreover, humans, too, pose a significant threat to their survival.
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