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Pinniped

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Pinnipeds
Temporal range: Oligocene – Holocene, 23–0 Ma
Southern elephant seal (Mirounga leonina) (upper right), New Zealand fur seal (Arctocephalus forsteri) (upper left), Grey seal (Halichoerus grypus) (middle left), Steller sea lion (Eumetopias jubatus) (lower right) and Walrus (Odobenus rosmarus) (lower left)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Clade: Pinnipediformes
Clade: Pinnipedia
Illiger, 1811[1]
Subgroups

Pinnipeds, often generalized as seals[a], are a widely distributed and diverse group of fin-footed, semiaquatic, marine mammals of the order Carnivora. They comprise the extant families Odobenidae (whose only living member is the walrus), Otariidae (the eared seals; includes sea lions, and fur seals), and Phocidae (the earless, or "true" seals). There are 33 extant species of pinnipeds, and more than 50 extinct species have been described from fossils. While they were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic lineage. Their closest living relatives are bears and musteloids.

Seals range in size from the 1 m (3 ft 3 in) and 45 kg (99 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal (which is also the largest carnivoran). Several species exhibit sexual dimorphism. They have streamlined bodies and four limbs that are modified into flippers. Though not as fast in the water as dolphins, seals are more flexible and agile. Otariids use their front limbs primarily to propel themselves through the water while phocids and walruses use their hind limbs. Otariids and walruses have hind limbs that can be rotated at right angles to the body axis and used as legs on land. By comparison, terrestrial locomotion by phocids is more cumbersome. Otarids have visible external ears, while phocids and walruses lack these. Pinnipeds have well-developed senses—their eyesight and hearing are adapted for both air and water and they an advanced tactile system in their vibrissae. Some species are also well adapted for diving at great depths. They have layers of fat, or blubber, under the skin to keep warm in the cold water, and, other than the walrus, all species have fur coats.

Although pinnipeds are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres. They spend of their lives in the water, but come ashore to mate, give birth, molt or escape from predators like sharks and killer whales. They feed largely on fish and marine invertebrates, but a few, like the leopard seal, feed on large vertebrates such as penguins and other seals. Walruses are specialized for feeding on bottom-dwelling mollusks. Pinnipeds are typically polygynous although the degree of polygyny varies with the species. Land-breeding species tend towards greater polygyny while ice-breeding species are less polygynous. Some species are even serially monogamous. Male pinniped strategies for reproductive success vary between female defense, territorial defense and lekking. Pups are typically born in the spring and summer months. Females bear almost all the responsibility for raising the young. Mothers of some species fast and nurse their young for a relatively short period of time while others take foraging trips at sea between nursing bouts. Walruses are known to nurse their young while at sea. Seals produce a number of vocalizations, notably the barks of California sea lions, the gong-like calls of walruses and the complex songs of Weddell seals.

The meat, blubber and fur coats of pinnipeds have traditionally been used by indigenous peoples of the Arctic, and seals have been depicted in various cultures worldwide. They are commonly kept in captivity and are even sometimes trained to perform tricks and tasks. Seals were at one time relentlessly hunted by commercial industries for their products, but now they are protected by international law. The Japanese sea lion and the Caribbean monk seal have vanished in the past century, while the Mediterranean monk seal and Hawaiian monk seal are ranked Critically Endangered by the International Union for Conservation of Nature. Besides exploitation, pinnipeds are also threatened by conflicts with local people, bycatching and marine pollution.

Taxonomy

Cladogram showing relationships among the pinnipeds, combining several phylogenetic analyses.[2]

German naturalist Johann Karl Wilhelm Illiger was the first to recognize the pinnipeds as a distinct taxonomic unit, and, in 1811, gave the name Pinnipedia to both a family and an order.[3] American zoologist Joel Asaph Allen reviewed the world's pinnipeds in an 1880 monograph. In this publication, he traced the history of names, gave keys to families and genera, described North American species and provided synopses of species in other parts of the world.[4] In 1989, Annalisa Berta and colleagues proposed the Pinnipedimorpha, an unranked clade to contain the fossil genus Enaliarctos and modern seals as a sister group.[5] Pinnipeds belong to the order Carnivora and the suborder Caniformia (known as dog-like carnivorans).[6] of the three extant families, the Otariidae and Odobenidae are grouped in the superfamily Otarioidea[7] while the Phocidae belongs to the superfamily Phocoidea.[8] The name pinniped derives from the Latin pinna, feather or wing, and ped-, foot.[9]

Otariids are also known as eared seals due to the presence of pinnae. These animals rely on their well-developed fore-flippers to propel themselves through the water. They can also turn their hind-flippers forward and "walk" on land.[10] The anterior end of an otariid's frontal bones extends between the nasal bones and the supraorbital foramen is large and flat horizontally. The supraspinatous fossas are divided by a "secondary spine" and the bronchus are divided anteriorly.[11] Otariids consist of two types, sea lions and fur seals. Sea lions are distinguished by their rounder snouts and shorter, rougher pelage, while fur seals have more pointed snouts, longer fore-flippers and thicker fur coats that include an undercoat and guard hairs. The former also tend to be larger than the latter.[12] Five genera and species of sea lion are known to exist while two genera and nine species of fur seal exist. While sea lions and fur seal have historically been considered separate subfamilies (Otariinae and Arctocephalinae respectively), a 2001 genetic study found that the northern fur seal is more closely related to several sea lion species.[13] This is supported by a 2006 molecular study that also found that the Australian sea lion and New Zealand sea lion are more closely related to Arctocephalus than to other sea lions.[14]

Odobenidae consists of only one living member, the modern walrus. This animal is easily distinguished from other extant pinnipeds by its larger size (exceeded only by the elephant seals), nearly hairless skin and long upper canines, known as tusks. Like otariids, walruses are capable of turning their hind-flippers forwards and can walk on land. When moving in water, however, the walrus relies on its hind-flippers for locomotion while its fore-flippers are used for steering. In addition, the walrus lacks external ear flaps.[15] Walruses have pterygoid bones that are broad and thick, frontal bones that are V-shaped at the anterior end and calcaneuses with pronounced tuberosity in the middle.[11]

Phocids are known as true or "earless" seals. These animals lack external ear flaps and are incapable of turning their hind-flippers forward, which makes them more cumbersome on land. In water, true seals swim by moving their hind-flippers and lower body from side to side.[10] Phocids have thickened mastoids, enlarged entotympanic bones, everted pelvic bone and a massive ankle bone. They also lack supraorbital processes on the frontal and have underdeveloped calcaneal tubers.[11] A 2006 molecular study supports the division of phocids into two monophyletic subfamilies: Monachinae, which consists of the elephant seals, monk seals and lobodontine seals, and Phocinae, which includes the genera Pusa, Phoca, Histriophoca, Pagophilus, Erignathus and Cystophora.[14]

In a 2012 review of pinniped taxonomy, Bertha and Morgan Churchill suggest that, based on morphological and genetic criteria, there are 33 extant species and 29 subspecies of pinnipeds, although five of the latter lack sufficient support to conclusively consider them as subspecies. They recommend that the genus Arctocephalus should be limited to Arctocephalus pusillus, and resurrected the name Arctophoca for several species and subspecies formerly placed in Arctocephalus.[2] More than 50 fossil species have been described.[16]

Evolutionary history

Restoration of Puijila darwini

Traditionally, pinnipeds were thought to be diphyletic (descended from two ancestral lines), with walruses and otariids sharing a recent common ancestor with bears and phocids sharing one with Musteloidea. However, morphological and molecular evidence support a monophyletic origin.[11] Nevertheless there is some dispute as to whether pinnipeds are more closely related to bears or musteloids, as some studies support the former theory[17][18][19] and others the latter.[14][20][21] Pinnipeds split from other caniforms 50 million years ago (mya) during the Eocene.[22] Their evolutionary link to terrestrial mammals was unknown until the 2007 discovery of the otter-like Puijila darwini in early Miocene deposits in Nunavut, Canada. The researchers who found Puijila placed it in a clade with Potamotherium (traditionally considered a mustelid) and Enaliarctos. Of the three, Puijila was the least specialized for aquatic life. The discovery of Puijila in a lake deposit suggests that pinniped evolution went through a freshwater transitional phase.[23] Enaliarctos of late Oligocene/early Miocene (27–18 million years ago) California closely resembled modern pinnipeds, as it was fully aquatic with a flexible spine and its limbs were modified into flippers. Its teeth were adapted for shearing (like terrestrial carnivorans) and may have stayed near shore more often. Enaliarctos was capable of swimming with both the fore-flippers and hind-flippers but it may have been more specialized for the former. A closer relative of extant pinnipeds was Pteroarctos, which lived in Oregon 19–15 million years ago (mya). As in modern seals, Pteroarctos had an orbital wall that was not limited by certain facial bones (like the jugal or lacrimal bone) but was mostly shaped by the maxilla.[11]

The lineages of Otariidae and Odobenidae split almost 28 mya.[14] Otariids originated in the North Pacific. The earliest fossil Pithanotaria, found in California, is dated to 11 mya. However, the Callorhinus lineage split earlier at 16 mya. Zalophus, Eumetopias and Otaria diverged next with the latter colonizing the coast of South America. Most of the other otariids diversified in the Southern Hemisphere. The earliest fossils of Odobenidae, Prototaria of Japan and Proneotherium of Oregon, date 18–16 mya. These primitive walruses had much shorter canines and lived on a fish diet rather than a specialized mollusk diet like the modern walrus. Walruses further diversified in the middle and late Miocene. Several species had enlarged upper and lower canines. The genera Valenictus and Odobenus developed elongated tusks. The lineage of the modern walrus may have spread from the North Pacific to the Caribbean (via the Central American Seaway) 8–5 mya and subsequently made it to the North Atlantic and returned to the North Pacific via the Arctic 1 mya. Alternatively, this lineage may have spread from the North Pacific to the Arctic and subsequently the North Atlantic during the Pleistocene.[11]

Fossil skull cast of Desmatophoca oregonensis from the extinct Desmatophocidae

The ancestors of the Otarioidea and Phocoidea diverged 33 mya.[22] The Phocidae are likely to have descended from the extinct family Desmatophocidae in the North Atlantic. Desmatophocids lived 23–10 mya and had elongated skulls, fairly large eyes, cheekbones connected by a mortised structure and rounded cheek teeth. They also were sexually dimorphic and may have been capable of propelling themselves with both the foreflippers and hindflippers. Phocids are known to have existed since at least 15 mya[11] and molecular evidence supports a divergence of the Monachinae and Phocinae lineages 22 mya.[14] The fossil monachine Monotherium and phocine Leptophoca were found in southeastern North America. The deep split between the lineages of Erignathus and Cystophora 17 mya suggests that the phocines migrated eastward and northward from the North Atlantic. The genera Phoca and Pusa could have arisen when a phocine lineage traveled from the Paratethys Sea to the Arctic Basin and subsequently went eastward. The Baikal seal migrated into Lake Baikal from the Arctic (via the Siberian ice sheet) and became isolated there. The Caspian seal became isolated as the Paratethys shrunk leaving the animal in a small remnant sea, the Caspian sea.[11] The monochines diversified southward. Monachus emerged in the Mediterranean and migrated to the Caribbean and then the central North Pacific.[24] The two extant elephant seal species diverged close to 4 mya after the Panamanian isthmus was formed.[14] The lobodontine lineage emerged around 9 mya and colonized the southern ocean in response to glaciation.[24]

Anatomy and physiology

Skeleton of adult female Steller sea lion

Pinnipeds have streamlined, spindle-shaped bodies with reduced or non-existent external ear flaps, small tails, rounded heads, flexible necks and limbs modified into flippers.[25][26] Their mammary glands and genitals retract into the skin. They range in size from the 1 m (3 ft 3 in) and 45 kg (99 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal. Overall, they tend to be larger than other carnivorans (the southern elephant seal is the largest carnivoran).[25] Several species have male-biased sexual dimorphism that correlates with the degree of polygyny in a species: highly polygynous species like elephant seals are extremely sexually dimorphic, while less polygynous species have males and females that are closer in size. In lobodontine seals, females are slightly larger than males. Males of sexually dimorphic species also tend to have secondary sex characteristics such the prominent proboscis of elephant seals, the inflatable red nasal hood of the hooded seals and the thick necks of manes of otariids.[27][28] Pinniped skulls have large eye orbits, short snouts, and a constricted interorbital region.[29] They are unique among carnivorans in that their orbital walls are significantly shaped by the maxilla and are not limited by certain facial bones.[11] Compared to other carnivorans, their teeth are pointed and cone-shaped, they lack carnassials, and they tend to be fewer in number (especially incisors and back molars).[30] The walrus has unique upper canines that are elongated into tusks.[31]

Male and female South American sea lions, showing sexual dimorphism

Almost all pinnipeds have fur coats, the exception being the walrus, which is only sparsely covered. However, even some fully furred species(particularly sea lions) are more sparsely haired than most land mammals.[32] In species that live on ice, young pups have thicker coats than adults, known as lanugo. The individual hairs on the coat can trap heat from sunlight and keep the pup warm.[33] Pinnipeds typically have countershaded pelage colors, and are darker colored dorsally and lighter colored ventrally, which serves to eliminate shadows caused by light shining over the ocean water. The pure white fur of harp seal pups conceals them in their Arctic environment.[34] Some species like ribbon seals, ringed seals and leopard seals have patterns of contrasting light and dark coloration. All fully furred species molt; phocids molt once a year, while otariids gradually molt all year.[35] Seals have a layer of subcutaneous fat known as blubber that is particularly thick in phocids and walruses.[25] Blubber serves both to keep the animals warm and to provide energy and nourishment when they are fasting. Blubber can make up as much as 50% of a pinniped's body weight and the blubber layers of walruses and elephant seals can be as thick as several inches. Pups are born with a thin layer of blubber and some species make up for this with their thick lanugos.[33]

Pinnipeds have a simple stomach that is similar in structure to terrestrial carnivores. Most species have neither a cecum nor a clear demarcation between the small and large intestines; the large intestine is comparatively short and only slightly wider than the small intestine. Small intestine lengths range from 8 (California sea lion) to 25 times (elephant seal) the body length. The length of the intestine may be an adaptation to frequent deep diving, as the increased volume of the digestive tract serves as an extended storage compartment during submersion. Pinnipeds do not have an appendix.[36] As in most marine mammals, the kidneys are divided into small lobes and can effectively absorb water and filter out salt.[37]

Locomotion

Harbor seal (above) and California sea lion swimming. The former swims with its hind-flippers, the latter with its fore-flippers.

Unlike other marine mammals, pinnipeds have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body.[38] They tend to be slower swimmers than cetaceans, typically cruising at 5–15 kn (9.3–27.8 km/h; 5.8–17.3 mph) compared to around 20 kn (37 km/h; 23 mph) for several species of dolphin. However, seals are more agile and flexible.[32] Otariids, like the California sea lion, are capable of bending their necks backwards far enough to reach their hind-flippers and can make dorsal turns.[39] Pinnipeds have several adaptions for reducing drag. In addition to their streamlined bodies, they have smooth networks of muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water. They also lack arrector pili so their fur can lay flat as they swim.[32]

When swimming, otariids rely on their fore-flippers for locomotion in a manner similar to penguins and sea turtles.[40] Fore-flipper movement is not continuous and the animal glides between each stroke.[39] Compared to territorial carnivorans, the fore-limbs of otariids are reduced in length. This gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage.[38] The hind-flippers of otariids serve as stabilizers.[32] Phocids and walruses swim by moving their hind-flippers and lower body from side to side.[40] Their fore-flippers are mainly used for steering.[38] Some species leap out of the water. This may allow then to travel faster. In addition, sea lions are known to "ride" waves, which probably helps them decrease their energy usage.[32]

Pinnipeds can move around on land, though not as well as terrestrial animals. Otariids and walruses are capable of turning their hind-flippers forward and under the body so they can "walk" on all fours.[41] The fore-flippers move in a transverse, rather than a sagittal fashion. Otariids rely on the movements of their heads and necks more than its hind-flippers during terrestrial locomotion.[42] By swinging their heads and necks, otariids create momentum while they are moving. Sea lions have been recording climbing up flights of stairs. Phocids are less agile on land. They can not pull their hind-flippers forward and move on land by lunging, bouncing and wiggling while their fore-flippers keep them balanced. Some species use their fore-flippers to pull themselves forward. Terrestrial locomotion is easier for phocids on ice as they can sled along.[41]

Senses

Frontal view of brown fur seal head

The eyes of pinnipeds are relatively large for their size and are positioned near the front of the head. One exception is the walrus, whose smaller eyes are located on the sides of its head.[43][44] This is because it feeds on immobile bottom dwelling mollusks and hence does not need acute vision.[43] A seal's eye is adapted for seeing both underwater and in air. The lens is mostly spherical and much of the retina is at equal distance from the lens center. The cornea has a flattened center where refraction is nearly equal in both water and air. Pinnipeds also have very muscular and vascularized irises. The well-developed dilator muscle gives the animals a great range in pupil dilation. When contracted, the pupil is typically pear-shaped although the bearded seal's is more diagonal. In species like harbor seals and California sea lions, which live in shallow water, dilation varies little while the deep- diving elephant seals have much greater variation.[45] On land, pinnipeds are near-sighted in dim light. This is reduced in bright light as the retracted pupil reduces the lens and cornea's ability to bend light. They also have a well-developed tapetum lucidum, a reflecting layer that increases sensitivity by reflecting light back through the rods. This helps them see in darkness.[43] Ice-living seals like the harp seal have corneas that can tolerate high levels of ultraviolet radiation typical of bright, snowy environments. As such, they do not suffer snow blindness.[46] Pinnipeds appear to have limited color vision as they lack S-cones.[47] Flexible eye movement has been documented in seals.[48] The extraocular muscles of the walrus are well developed. This and its lack of orbital roof allow it to protrude its eyes and see in both frontal and dorsal directions.[49] Seals release large amounts of mucus to protect their eyes.[32] The corneal epithelium is keratinized and the sclera is thick enough to withstand the pressures of diving. As in many mammals and birds, pinnipeds possess nictitating membranes.[50]

Photo of walrus head in profile showing one eye, nose, tusks, and "mustache"
Vibrissae of walrus

The pinniped ear is adapted for hearing underwater, where they can hear sound frequencies at up to 70,000 hz. In air, hearing is somewhat reduced in pinnipeds than in many terrestrial mammals. While they are capable of hearing as many sound frequencies as humans their hearing sensitivity is overall weaker.[51] One study of three species, the harbor seal, California sea lion and northern elephant seal, found that the sea lion was best adapted for airborne hearing, the harbor seal was equally capable of hearing in air and water, and the elephant seal was better adapted for underwater hearing.[52] Although pinnipeds have a fairly good sense of smell on land,[53] it is useless underwater as their nostrils are closed.[54]

Pinnipeds have well-developed tacile senses. Their vibrissae have ten times the innervation of terrestrial mammals, allowing them to effectively detect vibrations in the water.[55] This is useful when the animals are foraging and may add to or replace vision, particularly in darkness.[56] The vibrissae of some otariids grow quite long—those of the Antarctic fur seal can reach 41 cm (16 in).[57] Walruses have the most vibrissae, at 600–700 individual hairs. These are important for detecting their prey on the muddy sea floor.[58] Unlike terrestrial mammals such as rodents, pinnipeds do not move their vibrissae when examining an object but extend their moveable whiskers and keep them in the same position.[56] In addition to foraging, vibrissae may also play a role in navigation. Spotted seals appear to use them to detect ice holes.[58]

Diving adaptations

File:Diving weddell seals.jpg
Diving Weddell seals

Before diving, pinnipeds typically exhale to empty their lungs of half the air[59] and then close their nostrils and throat cartilages to protect the trachea.[60] Their unique lungs have airways that are highly reinforced with cartilaginous rings and smooth muscle, and alveoli that completely collapse during deeper dives.[61] While overemptying of the lungs in humans classifies as a lung disease and terrestrial mammals are in general unable to empty their lungs,[62] pinnipeds can reinflate their lungs even after complete respiratory collapse.[63] The middle ear contains sinuses that probably fill with blood during dives, preventing middle ear squeeze.[64] The heart of a seal is moderately flattened to allow the lungs to collapse. The trachea is flexible enough to collapse under pressure.[59] During deep dives, they store the air expelled from their collapsed lung into their bronchioles and trachea, which prevents them from experiencing decompression sickness, oxygen toxicity and nitrogen narcosis. In addition, they can tolerate large amounts of lactic acid.[64]

The main adaptations of the pinniped circulatory system for diving are the enlargement and increased complexity of veins to increase their capacity. The retia mirabilia form blocks of tissue on the inner wall of the thoracic cavity and the body periphery. These tissue masses, which contain extensive contorted spirals of arteries and thin-walled veins, act as blood reservoirs that increase oxygen stores for use during diving.[63] As with other diving mammals, pinnipeds have high amounts of hemoglobin and myoglobin stored in their muscles and blood. This allows them to stay submerged for long periods of time while still having enough oxygen. Deep-diving species such as elephant seals have blood volumes that make up to 20% of their body weight. When diving, they reduce their heart rate and maintain blood flow only to the heart, brain and lungs. To keep their blood pressure stable, phocids have an elastic aorta that stores some energy of each heartbeat and releases it.[64]

Thermoregulation

Northern elephant seal resting in water

Pinnipeds conserve heat with their large and compact body size, insulating blubber and fur, and high metabolism.[65] In addition, the blood vessels in their flippers are adapted for countercurrent exchange. Veins containing cool blood from the body extremities surround an artery with warm blood received from the core of the body. The blood from the artery transfers heat to the vessels, which then recirculates blood back to the core.[66] The same adaptations that conserve heat while in water tend to inhibit heat loss when out of water. To counteract overheating, many species, such as South American and New Zealand sea lions, and northern elephant seals, cool off by flipping sand onto their backs, which adds a layer of cool, damp sand that enhances heat loss. The northern fur seal pants to help stay cool, while monk seals often dig holes in the sand to expose cooler layers to rest in.[67]

Sleep

Pinnipeds spend many months at a time at sea, so they must sleep in the water. Scientists have recorded them sleeping for minutes at a time while slowing drifting downward in a belly-up orientation. Like other marine mammals, seals sleep in water with half of their brain awake so that they can detect and escape from predators.[68] When they are asleep on land, both sides of their brain go into sleep mode.[69]

Distribution and habitat

Walrus on ice off Alaska. This species has a discontinuous distribution around the North Pole.

Living pinnipeds mainly inhabit polar and subpolar regions, particularly the North Atlantic, the North Pacific and the Southern Ocean. They are entirely absent from Indo-Malayan waters.[70] Monk seals and some otariids live in tropical and subtropical waters. Seals usually require cool, nutrient-rich waters with temperatures lower than 20 °C (68 °F). Even those that live in warm or tropical climates live in areas that become cold and nutrient rich due to current patterns.[70][71] Only monk seals live in waters that are not typically cool or rich in nutrients.[70] Two species, the Caspian seal and Baikal seal, are found in large landlocked bodies of water (the Caspian Sea and Lake Baikal respectively).

As a whole, pinnipeds can be found in a variety of aquatic habitats including coastal water, open ocean, brackish water and even freshwater lakes and rivers. Most species inhabit coastal areas though some travel offshore and feed in deep waters off oceanic islands.[72] The Baikal seal is the only freshwater species. However, two subspecies of ringed seals live in landlocked lakes in Russia close to the Baltic sea. In addition, harbor seals may visit estuaries, lakes and rivers and sometimes stay as long as a year. Other species known to enter freshwater include California sea lions and South American sea lions.[72] Pinnipeds also use a number of terrestrial habitats and substrates, both continental and island. In temperate and tropical areas, they haul-out on sandy and pebble beaches, rocky shores, shoals, mud flats, sea caves and tide pools. Some species also exploit man-made structures like piers, jetties, buoys and oil platforms. Pinnipeds may move further inland and rest in sand dunes or vegetation and even climb steep cliffs.[73] Polar-living species utilize either fast ice or pack ice.[74]

Behavior and life history

Habor seal hauled out on rock

Pinnipeds have an amphibious lifestyle; they spend most of the time at sea but haul-out to mate, raise young, molt, rest, thermoregulate or escape from aquatic predators. Several species are known to migrate vast distances, particularly in response to extreme environmental changes like El Niño and changes in ice cover. Elephant seals stay at sea 8–10 months a year and migrate between breeding and molting sites. The northern elephant seal has the longest recorded migration distance of any mammal, at 18,000–21,000 km (11,000–13,000 mi). Phocids tend to migrate more than otariids.[75] Travelling seals may use various features of their environment to reach their destination including geomagnetic fields, water and wind currents, the position of the sun and moon and the taste and temperature of the water.[76] Pinnipeds may dive during foraging and to avoid predators. When foraging, Weddell seals typically dive for less than 15 minutes at depths of around 400 m (1,300 ft) but can dive for as long as 73 minutes and at depths of up to 600 m (2,000 ft). Northern elephant seals commonly dive 350–650 m (1,150–2,130 ft) for as long as 20 minutes. They can also dive 1,259–4,100 m (4,131–13,451 ft) and for as long as 62 minutes. The dives of otariids tend to be shorter and less deep. They typically last 5–7 minutes with average depths at 30–45 m (98–148 ft). However, the New Zealand sea lion has been recorded diving at a maximum of 460 m (1,510 ft) and a duration of 12 minutes.[77] Walruses do not often dive very deep as they feed in shallow water.[78]

Pinnipeds have lifespans averaging 25–30 years. Females usually live longer, as males tend to fight and often die before reaching maturity.[79] The longest recorded lifespans include 43 years for a wild female ringed seal and 46 years for a wild female grey seal.[80] The age in which a pinniped sexually matures can vary from 2–12 years depending on the species. Female typically mature earlier than males.[81]

Foraging and predation

File:Sea lion with salmon.gif
California sea lion with chinook salmon

All pinnipeds are carnivorous and predatory. As a whole, they mostly feed on fish and squid followed by crustaceans and bivalves, and then zooplankton and warm-blooded prey like birds.[82] While most species are generalist and opportunistic feeders, a few are specialists. Examples include the crabeater seal, which primarily eats krill, the ringed seal, which eats mainly crustaceans, the Ross seal and southern elephant seal, which specialize on squid and the bearded seal and walrus, which feed on clams and other bottom-dwelling invertebrates.[70] Pinnipeds may hunt solitarily or cooperatively. The former behavior is typical with non-schooling fish, slow-moving or immobile invertebrates or warm-blooded prey. Solitary foraging species usually exploit coastal waters, bays and river. An exception to this is the northern elephant seal, which feeds on fish at great depths in the open ocean. In addition, walruses feed solitarily but are often near other walruses in small or large groups that may surface and dive in unison. When large schools of fish or squid are available, pinnipeds such as certain otariids hunt cooperatively in large groups. Some species, such as California and South American sea lions, may forage with cetaceans and sea birds.[83] Seals typically consume their prey underwater where it is swallowed whole. Prey that is too large or awkward is taken to the surface to be torn apart.[84] The leopard seal, a prolific predator of penguins, is known to violently swing its prey back and forth until it is decapitated.[85] The elaborately cusped teeth of filter-feeding species, such as crabeater seals, allow them to remove water before they swallow their planktonic food.[70] The walrus is unique in that it consumes its prey by suction feeding, using its tongue to suck the meat of a bivalve out of the shell.[44] While species mostly hunt in the water, South American sea lions are known to chase down penguins on land.[86] Some species may swallow stones or pebbles for reasons not understood.[87] Though they can drink seawater, pinnipeds get most of their fluid intake from the food they eat.[37]

Leopard seal capturing an emperor penguin

Pinnipeds themselves are also subject to predation. Most species are preyed on by the killer whale or orca. To subdue and kill seals, orcas continuously ram them with their heads, slap them with their tails and fling them in the air. They are typically hunted by whales in groups of 10 or less but are occasionally hunted by lone individuals or groups of 2–30. Pups are more commonly taken by orcas but adults can be targeted as well. Large sharks are another major predator of pinnipeds, usually the great white shark but also the tiger shark and mako shark. Sharks usually attack by ambushing them from below. However, the prey often escapes and seals are often seen with shark-inflicted wounds. Otariids typically have injuries in the hindquarters while phocids usually have injuries on the forequarters.[88] Pinnipeds are also targeted by terrestrial predators. The polar bear is well adapted for hunting Arctic seals and walruses, particularly pups. Bears are known to use sit-and-wait tactics was well as active stalking and pursuit of prey on ice or water. Other terrestrial predators include cougars, brown hyenas and various species of canids, which mostly target the young.[88] Pinnipeds lessen the chance of predation by gathering in groups.[89] Some species are capable of inflicting damaging wounds on their attackers with their sharp canines—an adult walrus is capable of killing polar bears.[88] When out at sea, northern elephant seals dive out of the reach of surface-hunting orcas and white sharks.[68] In the Antarctic, which lacks terrestrial predators, pinniped species spend more time on the ice than their Arctic counterparts.[90] Ringed seals build dens underneath fast ice to hide from predators like polar bears.[91]

Interspecific predation among pinnipeds does occur. The leopard seal is known to prey on numerous other species, especially the crabeater seal. Leopard seals typically target crabeater pups, which form an important of their diet from November to January. Older crabeater seals commonly bear scars from failed leopard seal attacks; a 1977 study found that 75% of a sample of 85 individual crabeaters had these scars.[92] Walruses, despite being specialized for feeding on bottom-dwelling invertebrates, occasionally prey on Arctic seals. They kill their prey with their long tusks and eat their blubber and skin. Steller sea lions have been recorded eating the pups of harbor seals, northern fur seals and California sea lions. New Zealand sea lions feed on pups of some fur seal species and the South American sea lion may prey on South American fur seals.[88]

Reproductive behavior

Northern fur seal breeding colony

The mating system of pinnipeds varies from extreme polygyny to serial monogamy.[93] The type of mating system depends on the substrate where mating takes place. Of the 33 species, 20 breed on land, and the remaining 13 breed on ice.[94] Species that breed on land are usually polygynous as females gather in large aggregations and males are able to mate with them as well as defend them from rivals. Polygynous species include elephant seals, grey seals and most otariids.[27] Land-breeding pinnipeds tend to mate on islands where there are fewer terrestrial predators. However, very few islands are favorable for breeding and thus are particularly crowded. Since the land they breed on is fixed, females of these species return to the same sites year after year. The males arrive earlier in the season and wait for them. The males stay on land and try to mate with as many females as they can; some of them will even fast. If a male leaves the beach to feed, he will likely lose mating opportunities and his dominance.[95] Polygynous species also tend to be extremely sexually dimorphic in favor of males. This dimorphism manifests itself as larger chests and necks, and longer canines and denser fur—all traits that help males in fights for females. Increased body weight in males increases the length of time they can fast due to the ample energy reserves stored in the blubber.[27] Other seals, like the walrus and most phocids, mate on ice or in the water.[27][96] Females of these species tend to aggregate less. In addition, since ice is less stable then solid land, breeding sites change location each year and males are unable to predict where females will stay during the breeding season. Hence polygyny tends to be weaker in ice-breeding species. An exception to this is the walrus, where females form dense aggregations perhaps due to their patchy food sources. Pinnipeds that breed on fast ice tend to cluster together more than those that breed on pack ice.[96] Some of these species are serially monogamous, including the harp seal, crabeater seal and hooded seal.[97] Seals that breed on ice or water tend to have little or no sexual dimorphism. In lobodontine seals, females are slightly longer than males. Walruses and hooded seals are unique among ice-breeding species in that they have pronounced sexual dimorphism in favor of males.[27][96]

Male northern elephant seals fighting for dominance and females

Adult male pinnipeds have several strategies to ensure reproductive success. Otariids establish territories containing resources that attract females, such as shade, tide pools or access to water. Territorial boundaries are usually marked by natural breaks in the substrate[98] and males defend their territorial boundaries with threatening vocalizations and postures, but physical fights are usually avoided.[99] Individuals also return to the same territorial site each breeding season. In certain species, like the Steller sea lion and northern fur seal, a dominant male can maintain a territory for as long as 2–3 months. The females can usually move freely between territories and males are unable to coerce them. However, some species like the northern fur seal, South American sea lion and Australian sea lion can successfully contain females in their territories and prevent them from leaving. In some phocid species like the harbor seal, Weddell seal, harp seal and bearded seal, the males have underwater territories called "maritories" near female haul-out areas.[98] The maritories of Weddell seal males can overlap with female breathing holes.[96] Lek systems are known to exist among some populations of walruses.[98] These walruses cluster around females and try to attract them with elaborate courtship displays and vocalizations.[98][100] Lekking may also exist among California sea lions, South American fur seals and New Zealand sea lions. In some species, including elephant seals and grey seals, males will try to lay claim to the desired females and defend them from rivals.[98] Elephant seal males establish dominance hierarchies with the highest ranking males—the alpha males—maintaining harems of as many as 50 females.[101] Harp seals, crabeater seals and hooded seals follow and defend lactating females in their vicinity and wait for them to reach estrus.[98][97]

Younger or subdominant male pinnipeds may attempt to achieve reproductive success in other ways. Subadult elephants seals will sneak into female clusters and try to blend in by pulling in their noses. They also harass and attempt to mate with females that head out to the water. In otariid species like the South American and Australian sea lions, non-territorial subadults form "gangs" and cause chaos within the breeding rookeries to increase their chances of mating with females.[102] Female pinnipeds do appear to have some choice in mates, particularly in lek-breeding species like the walrus, but also in elephant seals where the males try to dominate all the females that they want to mate with.[100] When a female elephant seal is mounted by an unwanted male she tries to squirm and get away while croaking and slapping him with her tail. This commotion attracts other males to the scene and the most dominant will end the copulation and attempt to mate with the female himself.[103] This behavior is also known to occur among grey seals.[104] Dominant female elephant seals stay in the center of the colony where they are more likely to mate with a dominant male, while peripheral females are more likely to mate with subordinates.[105] Female Steller sea lions are known to solicit mating with their territorial males.[100]

Birth and parenting

With the exception of the walrus, female pinnipeds enter estrous shortly after they give birth.[106] All species go through delayed implantation, which puts the embryo in suspended development for as long as a few weeks or several months before it is implanted in the uterus. Delayed implantation postpones the birth of young until the female hauls-out on land or until conditions for birthing are favorable.[106][107] Gestation in seals (including delayed implantation) typically lasts a year. However, walruses have five- to six-year-long inter-birth intervals.[106] For most species, birthing takes place in the spring and summer months.[108] Typically, single pups are born;[106] twins are uncommon and have high mortality rates.[107] Pups of most species are born precocial.[106]

Harp seal mother nursing pup

Mother pinnipeds have different strategies for maternal care and lactation. Elephant seals, grey seals and hooded seals remain on land or ice and fast during their relatively short lactation period–four days for the hooded seal and five weeks for elephant seals. The milk of these species consist of up to 60% fat, allowing the young to grow fairly quickly. In particular, northern elephant seal pups gain 4 kg (8.8 lb) each day before they are weaned. Some pups may try to steal extra milk from other nursing mothers and gain weight more quickly than others. Alloparenting occurs in these fasting species;[106] while most northern elephant seal mothers nurse their own pups and reject nursings from alien pups, some do accept alien pups with their own.[109] For otariids and some phocids like the harbor seal, mothers fast and nurse their pups for a few days at a time. In between nursing bouts, the females leave their young onshore to forage at sea. These foraging trips may last one day to two weeks depending on the abundance of food and the distance of foraging sites. When their mothers are away, the pups will fast.[106] Lactation in otariids may last 6–11 months; in the Galápagos fur seal it can last as long as 3 years. Pups of these species are weaned at lower weights than their phocid counterparts.[110] Walruses are unique in that mothers nurse their young at sea. Young accompany their mothers after they learn how to dive, at five months old. Lactation in this species lasts two years, which suggests that the young need time to learn aquatic nursing.[106] The female rests at the surface with its head held up and the young nurses upside down.[111]

Male pinnipeds generally play little role in raising the young.[112] However, male walruses may help inexperienced young as they learn to swim and have even been recorded caring for orphans.[113] Male California sea lions have been observed to help shield swimming pups from predators.[114] Males can also pose threats to the safety of pups. In terrestrially breeding species, pups may get crushed by fighting males.[112] Subadult male South America sea lions sometimes abduct pups from their mothers and treat them like adult males treat females. This helps them gain experience in controlling females. Pups can get severely injured or killed during abductions.[115]

Communication

Walrus males are known to use vocalizations to attract mates.

Pinnipeds can produce a number of vocalizations such as barks, grunts, rasps, rattles, growls, creaks, warbles, trills, chirps, chugs, clicks and whistles. Vocals are produced both in air and underwater. Otariids are more vocal on land while phocids are more vocal in water. Antarctic seals are also more vocal on land than Arctic seals due to a lack of terrestrial predators like the polar bear.[116] Male vocals are deeper than female vocals.[117] Vocalizations are particularly important during the breeding seasons. Dominant male elephant seals advertise their status and threaten rivals with "clap-threats" and loud drum-like calls. Male otarids do the same with strong barks, growls, roars and "whickers".[118] Male walruses are known to produce distinctive gong-like vocals when attempting to attract females. They can also create somewhat musical sounds with their inflated throats.[119]

The Weddell seal has perhaps the most elaborate vocal repertoire with separate sounds for airborne and underwater contexts.[116] Underwater vocals include trills, chugs, chirps, chugs and knocks. The calls appear to contain prefixes and suffixes that serve to emphasize a message.[120] The underwater vocals of Weddell seals can last 70 seconds, which is long for a marine mammal call. Some calls have around seven rhythm patterns and are comparable to songs (like those of birds and whales)[121] In some species there appear to be geographic differences in vocalizations, known as dialects.[122] Certain species may have individual variations in vocals.[118] These differences are likely important for mothers and pups who need to remain in contact in crowded beaches. Otariid females and their young use mother-pup attraction calls to help them reunite when the mother returns from foraging at sea. Female pinnipeds are also known to bellow when protecting their young.[117]

While most vocals are audible to the human ear, a captive leopard seal was recorded making ultrasonic calls underwater. In addition, the vocals of northern elephant seals may produce infrasonic vibrations. Non-vocal communication is not as common in pinnipeds as in cetaceans. Nevertheless, when disturbed by intruders harbor seals and Baikal seals may slap their fore-flippers against their bodies as warnings. Teeth chattering, hisses and exhalations are also made as aggressive warnings. Visual displays also occur: Weddell seals will make an S-shaped posture when patrolling under the ice and Ross seals will display the stripes on their chests and teeth when approached.[116]

Intelligence

Sea lions can recognize relationships between stimuli based on functions or connections made with their peers, rather than only the stimuli's common features.[123] They demonstrate the ability to understand simple syntax and commands when taught an artificial sign language. However, the animals only rarely used the signs semantically or logically.[124] In 2011, a captive California sea lion named Ronan was recorded bobbing its head in synchronization to musical rhythms. This "rhythmic entrainment" was previously seen only in humans, parrots and other birds possessing vocal mimicry.[125] In 1971, a captive harbor seal named Hoover was trained to imitate human words, phases and laughter.[126] For sea lions used in entertainment, trainers toss a ball at the animal so it may accidentally balance it or hold the ball on its nose, thereby gaining an understanding of what to do. A sea lion may go through a year of training before performing a trick for the public. However, its memory allows it to perform a trick even after three months of resting.[114]

Human relations

Cultural depictions

Moche stirrup spout vessel in shape of seal
See also: Pinnipeds in popular culture

Various human cultures depicted pinnipeds for millennia. The Celts of Orkney and Hebrides believed in the selkie—seals that could change into humans and walk on land. There are even stories of people mating with selkies in their human form. seals are also of great importance in the culture of the Inuits, who depend on them for survival.[127] In Inuit mythology, the goddess Sedna rules over the sea and marine animals. She is depicted as a mermaid, occasionally with a seal's lower body. In one legend, Sedna is about to be married to an evil bird spirit but is recused by her father. As they paddled home in their kayak, the bird causes a storm that forces the father to throw his daughter over. As she clings to the sides of the kayak he chops off her fingers, which become the seals, whales and other marine mammals hunted by the Inuit.[128] One of the earliest Ancient Greek coins depicted the head of a seal and the animals were mentioned by Homer and Aristotle. The Greeks believed that seals loved both the sea and sun and were considered to be under the protection of the gods Poseidon and Apollo.[129] The Moche people of ancient Peru worshipped the sea and its animals, and often depicted sea lions in their art.[130] In modern popular culture, pinnipeds are usually depicted as comical figures based on their performances in zoos, circuses and marine mammal parks.

In captivity

Performing sea lions being fed at the Memphis Zoo

Pinnipeds have been kept in captivity since at least the 17th century. Their large size and playfulness make them popular exhibits. In Europe, exhibits have rocky backgrounds with artificial haul-out sites and a pool in the center. In the United States, sea lions are kept in pens with small rocky, elevated shelters where they can jump into their oval pools. More elaborate exhibits have deep pools that can be viewed underwater with rock-mimicking cement as haul-out areas. The most common pinniped species kept in captivity is the California sea lion as it is fairly abundant and easy to train.[131] These animals are used to perform tricks and entertain visitors.[114] Other species popularly kept in captivity include the grey seal and harbor seal.[131] Some organizations, such as the Humane Society of the United States and the World Society for the Protection of Animals, object to keeping pinnipeds and other marine mammals in captivity. They state that the exhibits could not be large enough to house animals that have evolved to be migratory and a pool could never replace the size and biodiversity of the ocean. They also oppose using sea lions for entertainment, claiming the tricks performed are "exaggerated variations of their natural behaviors" and distract the audience from the animal's unnatural environment.[132]

California sea lions are used in military applications by the U.S. Navy Marine Mammal Program, including detecting naval mines and enemy divers. In the Persian Gulf, the animals can swim behind divers approaching a US naval ship and attach a clamp with a rope to the diver's leg. Navy officials say the sea lions can do this in seconds, before the enemy realizes what happened.[133] Organizations like PETA believe that such operations put the animals in danger.[134] However, the Navy insists that the sea lions are removed once their mission is complete.[135] The Marine Mammal Center in California cares for sick or injured pinnipeds and releases them back into the wild.[131]

Conservation and management issues

As of 2013, International Union for Conservation of Nature (IUCN) recognizes 34 pinniped species. Two species–the Japanese sea lion and the Caribbean monk seal–are recently extinct, and ten more are considered at risk, as they are ranked as "Critically Endangered" (the Mediterranean and Hawaiian monk seals), "Endangered" (Galápagos fur seal, Australian sea lion, Caspian seal, Galápagos sea lion), and "Vulnerable" (northern fur seal, hooded seal, New Zealand sea lion). Three species, the walrus, the ribbon seal, and the spotted seal, have a "Data Deficient" ranking.[136] These species, which depend on sea ice habitats in the Northern Hemisphere, are vulnerable to the effects of recent and ongoing climate change, particularly declines in sea ice.[137] The decline of Steller sea lions in Alaska since the 1970s has been subject to much debate.[138]

Men killing northern fur seals on Saint Paul Island, Alaska, in the 1890s

Humans have hunted pinnipeds since the Stone Age. Originally, seals were hit with clubs during haul-out. Eventually, seal hunters used harpoons to spear the animals from boats out at sea, and hooks for killing pups on ice or land. They were also trapped in nets. The use of firearms in seal hunting during the modern era drastically increased the number of killings. Pinnipeds are typically hunted for their meat and blubber. The skins of fur seals and phocids are made into coats and the tusks of walruses continue to be used for carvings or as ornaments.[139] There is a distinction between the subsistence hunting of seals by indigenous peoples of the Arctic and commercial hunting: subsistence hunters typically use seal products for themselves and depend on them for survival.[140] National and international authorities have given special treatment to aboriginal hunters since their methods of killing are seen as less destructive and wasteful. This distinction is being questioned, however, as indigenous people are using more modern weaponry and mechanized transport to hunt with, and are selling seal products in the marketplace. As such, some anthropologists argue that the term "subsistence" should also apply to these cash-based exchanges as long as they take place within local production and consumption. More than 100,000 phocids (especially ringed seals) as well as around 10,000 walruses are harvested annually by native hunters.[139]

Protests of Canada's seal hunts

Commercial sealing was historically just as important of an industry as whaling. Exploited species included harp seals, hooded seals, Caspian seals, elephant seals, walruses and all species of fur seal. The scale of seal harvesting decreased substantially after the 1960s. Several species that were commercially exploited have rebounded in numbers; for example, Antarctic fur seals may be as numerous as they were prior to harvesting. The northern elephant seal was hunted to near extinction in the late 19th century, with only a small population remaining on Guadalupe Island. It has since recolonized much of its former range but has a population bottleneck. Several species of pinniped continue to be harvested. The Convention for the Conservation of Antarctic Seals allows limited hunting of crabeater seals, leopard seals and Weddell seals. However, Weddell seal hunting is prohibited if the animal is over one year of age between September and February to ensure breeding stocks are healthy. Other species protected are southern elephant seals, Ross seals and Antarctic fur seals.[140] The Government of Canada has permitted the hunting of harp seals. This has been met with controversy and debate. Proponents of seal hunts insist that the animals are killed humanely and the white-coated pups are not taken, while opponents argue that it is irresponsible to kill harp seals as they are already threatened by declining habitat.[141][142]

The Caribbean monk seal was killed and exploited by Europeans settlers and their descendants since 1494, starting with Christopher Columbus himself. The seals were easy targets for organized sealers, fishermen, turtle hunters and buccaneers because they evolved with little pressure from terrestrial predators and were thus "genetically tame". In the Bahamas, as many as 100 seals were slaughtered in one night. In the mid-nineteenth century, the species was thought to have gone extinct until a small colony was found near the Yucatán Peninsula in 1886. Seal killings continued and the last reliable report of the animal alive was in 1952. The IUCN declared it extinct in 1996.[143] The Japanese sea lion was common around the Japanese islands but overexploitation and competition from fisheries drastically decreased the population in the 1930s. The last recorded individual was a juvenile in 1974.[144]

Grey seal on beach occupied by humans

Some species have become so numerous that they conflict with local people. In the United States, pinnipeds and other marine mammals are protected under the Marine Mammal Protection Act of 1972 (MMPA). Since that year, California sea lion populations have risen to 250,000. These animals began exploiting more man-made environments like docks for haul-out sites. Many docks are not designed to withstand the weight of several resting sea lions, which causes major tilting and other problems. Wildlife managers have used various methods to control the animals and some city officials have redesigned docks so they can better withstand them.[145][146] Sea lions also conflict with fisherman as they often steal their catches.[147] In 2007, the MMPA was amended to permit their lethal removal from salmon runs at Bonneville Dam.[148] The 2007 law seeks to relieve pressure on the crashing Pacific Northwest salmon populations.[149] Wildlife officials have unsuccessfully attempted to ward off the sea lions using bombs, rubber bullets and bean bags.[150] Efforts to chase sea lions away from the area have also proven ineffective.[151] Critics like the Humane Society object to the killing of the sea lions, claiming that hydroelectric dams pose a greater threat to the salmon.[150] Similar conflicts have existed in South Africa with brown fur seals. In the 1980s and 1990s, South African politicians and fisherman demanded that the animals be culled, believing the animals competed with commercial fisheries. However, scientific studies found that culling fur seals would actually have a negative effect on the fishing industry and the culling option was dropped in 1993.[152]

Pinnipeds can also be threatened by humans in more indirect ways. They are unintentionally caught in fishing nets by commercial fisheries and accidentally shallow fishing hooks. Gillnetting and Seine netting is a considerable cause of mortality in seals and other marine mammals. Species commonly entangled include California sea lions, Hawaiian monk seals, northern fur seals and brown fur seals.[140] Pinnipeds are also affected by marine pollution. High levels of organic chemicals accumulate in these animals since they are near the top of food chains and have large reserves of blubber. Lactating mothers can pass the toxins on to their young. These pollutants can cause gastrointestinal cancers, decreased reproductivity and greater vulnerability to infectious diseases.[153] Other man-made threats include habitat destruction by oil and gas exploitation as well as encroachment by boats.[140]

Notes

  1. ^ This term typically excludes the walrus in everyday English.

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Bibliography

  • Berta, A. (2012). Return to the Sea: The Life and Evolutionary Times of Marine Mammals. University of California Press. ISBN 978-0-520-27057-2.
  • Berta, A.; Sumich, J. L.; Kovacs, K. K. (2006). Marine Mammals: Evolutionary Biology (2nd ed.). Academic Press. ISBN 978-0-12-369499-7.{{cite book}}: CS1 maint: multiple names: authors list (link)
  • Perrin, W. F.; Würsig, B.; Thewissen, J. G. M., ed. (2009). Encyclopedia of Marine Mammals (2nd ed.). Academic Press. ISBN 978-0-12-373553-9.{{cite book}}: CS1 maint: multiple names: editors list (link)
  • Renouf, D., ed. (1991). Behaviour of Pinnipeds. Chapman and Hall. ISBN 978-0-412-30540-5.
  • Riedman, M. (1990). The Pinnipeds: Seals, Sea Lions, and Walruses. University of California Press. ISBN 978-0-520-06497-3.
  • Scheffer, V. B. (1958). Seals, Sea Lions, and Walruses: A Review of the Pinnipedia. Stanford University Press. ISBN 978-0-8047-0544-8.

External links

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