earless seal
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The earless seals, phocids, or true seals are one of the three main groups of
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s within the seal lineage,
Pinniped Pinnipeds (pronounced ), commonly known as seals, are a widely range (biology), distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant taxon, extant families Odobenidae (whose onl ...
ia. All true seals are members of the family Phocidae (). They are sometimes called crawling seals to distinguish them from the
fur seal Fur seals are any of nine species of pinnipeds belonging to the subfamily Arctocephalinae in the family Otariidae. They are much more closely related to sea lions than Earless seal, true seals, and share with them external ears (Pinna (anatomy ...
s and
sea lion Sea lions are pinnipeds characterized by external ear flaps, long foreflippers, the ability to walk on all fours, short and thick hair, and a big chest and belly. Together with the fur seals, they make up the family Otariidae, eared seals. ...
s of the family Otariidae. Seals live in the
ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...
s of both hemispheres and, with the exception of the more
tropical The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...
monk seals, are mostly confined to polar, subpolar, and
temperate In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ran ...
climates. The
Baikal seal The Baikal seal (''Pusa sibirica''), also known as Lake Baikal seal or Baikal nerpa (), is a species of earless seal endemic to Lake Baikal in Siberia, Russia. The Baikal seal is one of the smallest earless/true seals, and the only exclusively ...
is the only species of exclusively freshwater seal.


Taxonomy and evolution


Evolution

The earliest known fossil earless seal is '' Noriphoca gaudini'' from the late
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
or earliest
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
( Aquitanian) of
Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...
. Other early fossil phocids date from the mid-Miocene, 15 million years ago in the north Atlantic. Until recently, many researchers believed that phocids evolved separately from
otariids An eared seal, otariid, or otary is any member of the marine mammal family Otariidae, one of three groupings of pinnipeds. They comprise 15 extant taxon, extant species in seven genus, genera (another species became extinct in the 1950s) and are ...
and odobenids; and that they evolved from
otter Otters are carnivorous mammals in the subfamily Lutrinae. The 13 extant otter species are all semiaquatic, aquatic, or marine. Lutrinae is a branch of the Mustelidae family, which includes weasels, badgers, mink, and wolverines, among ...
-like animals, such as '' Potamotherium'', which inhabited European freshwater lakes. Recent evidence strongly suggests a monophyletic origin for all pinnipeds from a single ancestor, possibly '' Enaliarctos'', most closely related to the mustelids and
bear Bears are carnivoran mammals of the family (biology), family Ursidae (). They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats ...
s. Monk seals and elephant seals were previously believed to have first entered the Pacific through the open straits between North and South America, with the Antarctic true seals either using the same route or travelled down the west coast of Africa. It is now thought that the monk seals, elephant seals, and Antarctic seals all evolved in the southern hemisphere, and likely dispersed to their current distributions from more southern latitudes.


Taxonomy

In the 1980s and 1990s, morphological
phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
analysis of the phocids led to new conclusions about the interrelatedness of the various genera. More recent molecular phylogenetic analyses have confirmed the
monophyly In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent comm ...
of the two phocid subfamilies ( Phocinae and Monachinae). The Monachinae (known as the "southern" seals), is composed of three tribes; the Lobodontini, Miroungini, and Monachini. The four
Antarctic The Antarctic (, ; commonly ) is the polar regions of Earth, polar region of Earth that surrounds the South Pole, lying within the Antarctic Circle. It is antipodes, diametrically opposite of the Arctic region around the North Pole. The Antar ...
genera '' Hydrurga'', '' Leptonychotes'', '' Lobodon'', and '' Ommatophoca'' are part of the
tribe The term tribe is used in many different contexts to refer to a category of human social group. The predominant worldwide use of the term in English is in the discipline of anthropology. The definition is contested, in part due to conflict ...
Lobodontini. Tribe Miroungini is composed of the elephant seals. The Monk seals ('' Monachus'' and '' Neomonachus'') are all part of the tribe Monachini. Likewise, subfamily Phocinae (the "northern" seals) also includes three tribes; Erignathini ('' Erignathus)'', Cystophorini ('' Cystophora)'', and Phocini (all other phocines). More recently, five species have been split off from ''Phoca'', forming three additional genera. Alternatively the three monachine tribes have been evaluated to familiar status, which elephant seals and the Antarctic seals are more closely related to the phocines.


Extant genera


Biology


External anatomy

Adult phocids vary from in length and in weight in the ringed seal to and in the
southern elephant seal The southern elephant seal (''Mirounga leonina'') is one of two species of elephant seals. It is the largest member of the clade Pinnipedia and the order Carnivora, as well as the largest extant marine mammal that is not a cetacean. It gets its ...
, which is the largest member of the order
Carnivora Carnivora ( ) is an order of placental mammals specialized primarily in eating flesh, whose members are formally referred to as carnivorans. The order Carnivora is the sixth largest order of mammals, comprising at least 279 species. Carnivor ...
. Phocids have fewer teeth than land-based members of the
Carnivora Carnivora ( ) is an order of placental mammals specialized primarily in eating flesh, whose members are formally referred to as carnivorans. The order Carnivora is the sixth largest order of mammals, comprising at least 279 species. Carnivor ...
, although they retain powerful canines. Some species lack molars altogether. The
dental formula Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiology ...
is: 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 is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted sur ...
zones close to breeding sites. Phocids swim by sideways movements of their bodies, using their hind flippers to fullest effect. Their fore flippers are used primarily for steering, while their hind flippers are bound to the
pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...
in such a way that they cannot bring them under their bodies to walk on them. They are more streamlined than fur seals and sea lions, so they can 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 is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...
s. Phocid respiratory and circulatory systems are adapted to allow diving to considerable depths, and they can spend a long time underwater between breaths. Air is forced from the
lung The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...
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. The
middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...
is also lined with blood sinuses that inflate during diving, helping to maintain a constant pressure. Phocids are more specialized for aquatic life than otariids. They lack external ears and have sleek, streamlined bodies. Retractable
nipple The nipple is a raised region of tissue on the surface of the breast from which, in lactating females, breast milk, milk from the mammary gland leaves the body through the lactiferous ducts to Breastfeeding, nurse an infant. The milk can flow th ...
s, internal
testicle A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is Homology (biology), homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of Androgen, androgens, p ...
s, and an internal
penile sheath Almost all mammal penises have foreskins or prepuces. In non-human mammals, the prepuce is sometimes called the penile sheath or preputial sheath. In koalas, the foreskin contains naturally occurring bacteria that play an important role in fer ...
provide further streamlining. A smooth layer of
blubber Blubber is a thick layer of Blood vessel, vascularized adipose tissue under the skin of all cetaceans, pinnipeds, penguins, and sirenians. It was present in many marine reptiles, such as Ichthyosauria, ichthyosaurs and plesiosaurs. Description ...
lies underneath the skin. Phocids are able to divert
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...
flow to this layer to help control their temperatures.


Communication

Unlike otariids, true seals do not communicate by "barking". Instead, they communicate by slapping the water and grunting. True seals have also been documented to clap at each other underwater. One study published in ''
Animal Behaviour Ethology is a branch of zoology that studies the behavior, behaviour of non-human animals. It has its scientific roots in the work of Charles Darwin and of American and German ornithology, ornithologists of the late 19th and early 20th cen ...
'' found that seals use rhythmic
percussive A percussion instrument is a musical instrument that is sounded by being struck or scraped by a beater including attached or enclosed beaters or rattles struck, scraped or rubbed by hand or struck against another similar instrument. Ex ...
signalling in both "agonistic interactions" and display behavior.


Movements

With short front flippers and because their rear flipper is unable to rotate, true seals cannot walk when out of the water like sea lions. As a result, they bounce themselves forward in a motion called ''
galumphing "Jabberwocky" is a Nonsense verse, nonsense poem written by Lewis Carroll about the killing of a creature named "the Jabberwock". It was included in his 1871 novel ''Through the Looking-Glass'', the sequel to ''Alice's Adventures in Wonderland' ...
''; the polar explorer Edward Wilson referred to the motion as ''lolloping''.


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. However, the common seal displays a reproductive strategy similar to that used by
otariids An eared seal, otariid, or otary is any member of the marine mammal family Otariidae, one of three groupings of pinnipeds. They comprise 15 extant taxon, extant species in seven genus, genera (another species became extinct in the 1950s) and are ...
, 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 while
lactating 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 naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...
. 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 The humpback whale (''Megaptera novaeangliae'') is a species of baleen whale. It is a rorqual (a member of the family Balaenopteridae) and is the monotypic taxon, only species in the genus ''Megaptera''. Adults range in length from and weigh u ...
s' strategy, which involves fasting 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 five to seven weeks in the monk seal to just three to five days in the hooded seal. 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 female can only feed one pup.


Growth and maturation

The pup's diet is so high in
calorie The calorie is a unit of energy that originated from the caloric theory of heat. The large calorie, food calorie, dietary calorie, kilocalorie, or kilogram calorie is defined as the amount of heat needed to raise the temperature of one liter o ...
s that 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 even 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.


Feeding strategy

Phocids make use of at least four different feeding strategies: suction feeding, grip and tear feeding, filter feeding, and pierce feeding. Each of these feeding strategies is aided by a specialized skull, mandible, and tooth morphology. However, despite morphological specialization, most phocids are opportunistic and employ multiple strategies to capture and eat prey. For example, the leopard seal, ''Hydrurga leptonyx'', uses grip and tear feeding to prey on penguins, suction feeding to consume small fish, and filter feeding to catch krill.


See also

*
Marine mammals as food Marine mammals are a food source in many countries around the world. Historically, they were hunted by coastal people, and in the case of aboriginal whaling, still are. This sort of subsistence hunting was on a small scale and produced only local ...


References

{{Authority control Extant Miocene first appearances Pinnipeds Taxa named by John Edward Gray Non-human celestial navigation Taxa described in 1821