Earless seal
Encyclopedia
The true seals or earless seals are one of the three main groups of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s within the seal superfamily, Pinniped
Pinniped
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

ia. All true seals are members of the family Phocidae (icon). They are sometimes called crawling seals to distinguish them from the fur seal
Fur seal
Fur seals are any of nine species of pinnipeds in the Otariidae family. One species, the northern fur seal inhabits the North Pacific, while seven species in the Arctocephalus genus are found primarily in the Southern hemisphere...

s and sea lion
Sea Lion
Sea lions are pinnipeds characterized by external ear-flaps, long fore-flippers, the ability to walk on all fours, and short thick hair. Together with the fur seal, they comprise the family Otariidae, or eared seals. There are six extant and one extinct species in five genera...

s of the family Otariidae
Eared Seal
The eared seals or otariids are marine mammals in the family Otariidae, one of three groupings of Pinnipeds. They comprise 16 species in seven genera commonly known either as sea lions or fur seals, distinct from true seals and the Walrus...

. Seals live in the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s of both hemispheres and are mostly confined to polar
Polar region
Earth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...

, subpolar, and temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...

 climates, with the exception of the more tropical monk seals
Monachus
Monachus is a genus containing the monk seals, a group of three Pinniped species. They are the only earless seals which are found in tropical climates...

.

Anatomy

Adult phocids vary from 1.17 metres (3.8 ft) in length and 45 kilograms (99.2 lb) in weight, in the ringed seal
Ringed Seal
The ringed seal , also known as the jar seal and as netsik or nattiq by the Inuit, is an earless seal inhabiting the Arctic and sub-Arctic regions...

, to 4.9 metres (16.1 ft) and 2400 kilograms (5,291.1 lb) in the southern elephant seal
Southern Elephant Seal
The Southern Elephant Seal is one of the two extant species of elephant seal. It is both the most massive pinniped and member of the order Carnivora living today...

.

Phocids are more specialized for aquatic
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

 life than otariids. They lack external ears and have sleek, streamlined bodies. Retractable nipple
Nipple
In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...

s, internal testicle
Testicle
The testicle is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system...

s and an internal penis
Penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...

 sheath provide further streamlining. A smooth layer of blubber
Blubber
Blubber is a thick layer of vascularized adipose tissue found under the skin of all cetaceans, pinnipeds and sirenians.-Description:Lipid-rich, collagen fiber–laced blubber comprises the hypodermis and covers the whole body, except for parts of the appendages, strongly attached to the musculature...

 lies underneath the skin. Phocids are able to divert blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 flow to this layer to help control their temperature.

Limbs

Their fore flippers are used primarily for steering, while their hind flippers are bound to the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...

 in such a way that they cannot bring them under their body to walk on them.

They are more streamlined than fur seals and sea lions and can therefore swim more effectively over long distances. However, because they cannot turn their hind flippers downward, they are very clumsy on land, having to wriggle with their front flippers and abdominal muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s.

Phocids have fewer teeth than land-based members of the Carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...

, although they retain powerful canines
Canine tooth
In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, fangs, or eye teeth, are relatively long, pointed teeth...

. Some species lack molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....

s altogether. The dental formula
Dentition
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...

 is:

Respiration and circulation

Phocid respiratory
Respiratory system
The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles...

 and circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...

s are adapted to allow diving to considerable depths, and they can spend a long time underwater between breaths. Air is forced from the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

s during a dive and into the upper respiratory passages, where gases cannot easily be absorbed into the bloodstream. This helps protect the seal from the bends
Decompression sickness
Decompression sickness describes a condition arising from dissolved gases coming out of solution into bubbles inside the body on depressurization...

. The middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...

 is also lined with blood sinuses that inflate during diving, helping to maintain a constant pressure.

Swimming

While otariids are known for speed and maneuverability, phocids are known for efficient, economical movement. This allows most phocids to forage far from land to exploit prey resources, while otariids are tied to rich upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...

 zones close to breeding sites.

Phocids swim by sideways movements of their bodies, using their hind flippers to fullest effect.

Communication

True seals do not communicate by "barking" like otariids. Instead, they communicate by slapping the water and grunting.

Reproduction

Phocids spend most of their time at sea, although they return to land or pack ice to breed and give birth.

Pregnant females spend long periods foraging at sea, building up fat reserves, and then return to the breeding site to use their stored energy to nurse pups. The common seal
Harbor Seal
The harbor seal , also known as the common seal, is a true seal found along temperate and Arctic marine coastlines of the Northern Hemisphere...

, Phoca vitulina, also known as the harbor seal, displays a reproductive strategy similar to that of otariids in which the mother makes short foraging trips between nursing bouts.

Because a phocid mother's feeding grounds are often hundreds of kilometers from the breeding site, she must fast
Fasting
Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. An absolute fast is normally defined as abstinence from all food and liquid for a defined period, usually a single day , or several days. Other fasts may be only partially restrictive,...

 while lactating
Lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...

. This combination of fasting with lactation requires the mother to provide large amounts of energy to her pup at a time when she is not eating (and often, not drinking.) Mothers must supply their own metabolic needs while nursing. This is a miniature version of the humpback whale
Humpback Whale
The humpback whale is a species of baleen whale. One of the larger rorqual species, adults range in length from and weigh approximately . The humpback has a distinctive body shape, with unusually long pectoral fins and a knobbly head. It is an acrobatic animal, often breaching and slapping the...

's strategy. They fast during their months-long migration from arctic feeding areas to tropical breeding/nursing areas and back.

Phocids produce thick, fat-rich milk that allows them to provide their pups with large amounts of energy in a short period. This allows the mother to return to the sea in time to replenish her reserves. Lactation ranges from 28 days in the northern elephant seal
Northern Elephant Seal
The northern elephant seal is one of two species of elephant seal . It is a member of the family Phocidae . Elephant seals derive their name from their great size and from the male's large proboscis, which is used in making extraordinarily loud roaring noises, especially during the mating...

 to just three to five days in the hooded seal
Hooded Seal
The hooded seal is an arctic pinniped found only in the central and western North Atlantic ranging from Svalbard in the east to the Gulf of St...

. The mother ends nursing by leaving her pup at the breeding site to search for food (pups continue to nurse if given the opportunity). "Milk stealers" that suckle from unrelated, sleeping females are not uncommon; this often results in the death of the mother's pup, since a single female can only feed one pup.

Growth and maturation

The pup's diet is so high in calories, it builds up a fat store. Before the pup is ready to forage, the mother abandons it, and the pup consumes its own fat for weeks or months while it matures. Seals, like all marine mammals, need time to develop the oxygen stores, swimming muscles, and neural pathways necessary for effective diving and foraging. Seal pups typically eat no food and drink no water during the period, although some polar species eat snow. The postweaning fast ranges from two weeks in the hooded seal to 9–12 weeks in the northern elephant seal. The physiological and behavioral adaptations that allow phocid pups to endure these remarkable fasts, which are among the longest for any mammal, remain an area of active study and research.

Taxonomy

In the 1980s, phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...

 analysis of the phocids has led to a few conclusions about the interrelatedness of the various genera. The four genera Hydrurga
Leopard Seal
The leopard seal , also referred to as the sea leopard, is the second largest species of seal in the Antarctic...

, Leptonychotes
Weddell Seal
The Weddell seal , is a relatively large and abundant true seal with a circumpolar distribution surrounding Antarctica. Weddell seals have the most southerly distribution of any mammal, with a habitat that extends as far south as McMurdo Sound...

, Lobodon
Crabeater Seal
The crabeater seal, Lobodon carcinophagus, is a true seal with a circumpolar distribution around the coast of Antarctica. They are medium to large-sized , relatively slender and pale-colored, found primarily on the free floating pack ice that extends seasonally out from the Antarctic coast, which...

, and Ommatophoca
Ross Seal
The Ross seal is a true seal with a range confined entirely to the pack ice of Antarctica. It is the only species of the genus Ommatophoca. First described during James Clark Ross' British Antarctic Expedition in 1841, it is the smallest, least abundant and least well known of the Antarctic...

form a monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 group, the tribe
Tribe (biology)
In biology, a tribe is a taxonomic rank between family and genus. It is sometimes subdivided into subtribes.Some examples include the tribes: Canini, Acalypheae, Hominini, Bombini, and Antidesmeae.-See also:* Biological classification* Rank...

 Lobodontini
Lobodontini
The Lobodontini tribe of true seals , collectively known as the lobodontine seals, consist of four species of seals in four genera: the crabeater seal , the leopard seal , the Weddell seal and the Ross seal...

. Likewise, the Phocinae subfamily (Erignathus
Bearded Seal
The bearded seal , also called the square flipper seal, is a medium-sized pinniped that is found in and near to the Arctic Ocean. It gets its generic name from two Greek words that refer to its heavy jaw...

, Cystophora
Hooded Seal
The hooded seal is an arctic pinniped found only in the central and western North Atlantic ranging from Svalbard in the east to the Gulf of St...

, Halichoerus, and Phoca
Phoca
Phoca is a genus of the earless seals, within the Family Phocidae. It now contains just two species, the Common Seal and the Spotted Seal...

) is also monophyletic. More recently, five species have been split off from Phoca, forming three additional genera. However, the family Monachinae (the lobodonts plus Monachus
Monachus
Monachus is a genus containing the monk seals, a group of three Pinniped species. They are the only earless seals which are found in tropical climates...

and Mirounga
Elephant seal
Elephant seals are large, oceangoing seals in the genus Mirounga. There are two species: the northern elephant seal and the southern elephant seal . Both were hunted to the brink of extinction by the end of the 19th century, but numbers have since recovered...

) is probably paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...

.

SUPERFAMILY PINNIPEDIA
Pinniped
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

  • Family Otariidae
    Eared Seal
    The eared seals or otariids are marine mammals in the family Otariidae, one of three groupings of Pinnipeds. They comprise 16 species in seven genera commonly known either as sea lions or fur seals, distinct from true seals and the Walrus...

    : Fur seals and sea lions
  • Family Odobenidae
    Walrus
    The walrus is a large flippered marine mammal with a discontinuous circumpolar distribution in the Arctic Ocean and sub-Arctic seas of the Northern Hemisphere. The walrus is the only living species in the Odobenidae family and Odobenus genus. It is subdivided into three subspecies: the Atlantic...

    : Walrus
  • Family Phocidae
    • Subfamily Monachinae
      • Tribe Monachini
        • Monachopsis
        • Pristiphoca
        • Properiptychus
        • Messiphoca
        • Mesotaria
        • Callophoca
        • Pliophoca
        • Pontophoca
        • Hawaiian monk seal
          Hawaiian Monk Seal
          The Hawaiian monk seal, Monachus schauinslandi, is an endangered species of earless seal in the Phocidae family that is endemic to the Hawaiian Islands....

          , Monachus schauinslandi
        • Mediterranean monk seal
          Mediterranean Monk Seal
          The Mediterranean monk seal is a pinniped belonging to the Phocidae family. At some 450-510 remaining individuals, it is believed to be the world's second-rarest pinniped , and one of the most endangered mammals in the world.It is present in parts of the Mediterranean Sea and the eastern Atlantic...

          , Monachus monachus
        • Caribbean monk seal
          Caribbean Monk Seal
          The Caribbean monk seal or West Indian monk seal is an extinct species of seal. It is the only seal ever known to be native to the Caribbean Sea and the Gulf of Mexico. The last verified recorded sighting occurred in 1952 at Serranilla Bank...

          , Monachus tropicalis (probably extinct around 1950)
      • Tribe Miroungini
        • Northern elephant seal
          Northern Elephant Seal
          The northern elephant seal is one of two species of elephant seal . It is a member of the family Phocidae . Elephant seals derive their name from their great size and from the male's large proboscis, which is used in making extraordinarily loud roaring noises, especially during the mating...

          , Mirounga angustirostris
        • Southern elephant seal
          Southern Elephant Seal
          The Southern Elephant Seal is one of the two extant species of elephant seal. It is both the most massive pinniped and member of the order Carnivora living today...

          , Mirounga leonina
      • Tribe Lobodontini
        Lobodontini
        The Lobodontini tribe of true seals , collectively known as the lobodontine seals, consist of four species of seals in four genera: the crabeater seal , the leopard seal , the Weddell seal and the Ross seal...

        • Monotherium wymani
        • Ross seal
          Ross Seal
          The Ross seal is a true seal with a range confined entirely to the pack ice of Antarctica. It is the only species of the genus Ommatophoca. First described during James Clark Ross' British Antarctic Expedition in 1841, it is the smallest, least abundant and least well known of the Antarctic...

          , Ommatophoca rossi
        • Crabeater seal
          Crabeater Seal
          The crabeater seal, Lobodon carcinophagus, is a true seal with a circumpolar distribution around the coast of Antarctica. They are medium to large-sized , relatively slender and pale-colored, found primarily on the free floating pack ice that extends seasonally out from the Antarctic coast, which...

          , Lobodon carcinophagus
        • Leopard seal
          Leopard Seal
          The leopard seal , also referred to as the sea leopard, is the second largest species of seal in the Antarctic...

          , Hydrurga leptonyx
        • Weddell seal
          Weddell Seal
          The Weddell seal , is a relatively large and abundant true seal with a circumpolar distribution surrounding Antarctica. Weddell seals have the most southerly distribution of any mammal, with a habitat that extends as far south as McMurdo Sound...

          , Leptonychotes weddellii
      • Acrophoca longirostris
        Acrophoca
        Acrophoca longirostris is an extinct species of pinniped whose fossils have been discovered in Peru and Chile. It is thought to have been the ancestor of the modern leopard seal....

      • Piscophoca pacifica
      • Homiphoca capensis
    • Subfamily Phocinae
      • Kawas benegasorum
      • Leptophoca lenis
      • Preapusa
      • Cryptophoca
      • Bearded seal
        Bearded Seal
        The bearded seal , also called the square flipper seal, is a medium-sized pinniped that is found in and near to the Arctic Ocean. It gets its generic name from two Greek words that refer to its heavy jaw...

        , Erignathus barbatus
      • Hooded seal
        Hooded Seal
        The hooded seal is an arctic pinniped found only in the central and western North Atlantic ranging from Svalbard in the east to the Gulf of St...

        , Cystophora cristata
      • Tribe Phocini
        • Common seal or harbor seal, Phoca vitulina
        • Spotted seal
          Spotted Seal
          The spotted seal , also known as the larga or largha seal, is a member of the family Phocidae, and is considered a "true seal". It inhabits ice floes and waters of the north Pacific Ocean and adjacent seas...

          , Phoca largha
        • Ringed seal
          Ringed Seal
          The ringed seal , also known as the jar seal and as netsik or nattiq by the Inuit, is an earless seal inhabiting the Arctic and sub-Arctic regions...

          , Pusa hispida (formerly Phoca hispida)
        • Baikal seal, Pusa sibirica (formerly Phoca sibirica)
        • Caspian seal
          Caspian Seal
          Caspian seals , one of the smallest members of the earless seal family, are unique in that they are found exclusively in the brackish Caspian Sea. They can be found not only along the shorelines, but also on the many rocky islands and floating blocks of ice that dot the Caspian Sea...

          , Pusa caspica (formerly Phoca caspica)
        • Harp seal
          Harp Seal
          The harp seal or saddleback seal is a species of earless seal native to the northernmost Atlantic Ocean and adjacent parts of the Arctic Ocean. It now belongs to the monotypic genus Pagophilus. Its scientific name, Pagophilus groenlandicus, means "ice-lover from Greenland", and its synonym, Phoca...

          , Pagophilus groenlandicus (formerly Phoca groenlandica)
        • Ribbon seal
          Ribbon Seal
          The ribbon seal is a medium-sized pinniped from the true seal family . A seasonally ice-bound species, it is found in the Arctic and Subarctic regions of the North Pacific Ocean, notably in the Bering Sea and Sea of Okhotsk. It is distinguished by its striking coloration, with two wide white...

          , Histriophoca fasciata (formerly Phoca fasciata)
        • Phocanella
        • Platyphoca
        • Gryphoca
        • Grey seal
          Grey Seal
          The grey seal is found on both shores of the North Atlantic Ocean. It is a large seal of the family Phocidae or "true seals". It is the only species classified in the genus Halichoerus...

          , Halichoerus grypus

Evolution

The earliest fossil phocids date from the mid-Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

, 15 million years ago in the north Atlantic. Until recently, many researchers believed phocids evolved separately from otariids and odobenids
Walrus
The walrus is a large flippered marine mammal with a discontinuous circumpolar distribution in the Arctic Ocean and sub-Arctic seas of the Northern Hemisphere. The walrus is the only living species in the Odobenidae family and Odobenus genus. It is subdivided into three subspecies: the Atlantic...

 from otter
Otter
The Otters are twelve species of semi-aquatic mammals which feed on fish and shellfish, and also other invertebrates, amphibians, birds and small mammals....

-like animals, such as Potamotherium
Potamotherium
Potamotherium an extinct genus from the Miocene period, which has been assigned both to the mustelids and to the pinnipeds....

, which inhabited European fresh-water lakes. Recent evidence strongly suggests a monophyletic origin for all pinnipeds from a single ancestor, possibly Enaliarctos
Enaliarctos
Enaliarctos is an extinct genus of pinniped.Prior to the discovery of Puijila, the five species in the genus Enaliarctos represented the oldest known pinniped fossils, having been recovered from late Oligocene and early Miocene strata of California and Oregon.It had a short tail and developed...

, most closely related to the bear
Bear
Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...

s.

Monk seal
Monachus
Monachus is a genus containing the monk seals, a group of three Pinniped species. They are the only earless seals which are found in tropical climates...

s and elephant seal
Elephant seal
Elephant seals are large, oceangoing seals in the genus Mirounga. There are two species: the northern elephant seal and the southern elephant seal . Both were hunted to the brink of extinction by the end of the 19th century, but numbers have since recovered...

s are believed to have first entered the Pacific through the open straits between North and South America, which closed only in the Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

. The various Antarctic species may have either used the same route, or traveled down the west coast of Africa.

External links

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