Theearless seals,phocids, ortrue seals are one of the three main groups ofmammals within the seal lineage,Pinnipedia. All true seals are members of the familyPhocidae (/ˈfoʊsɪdiː/). They are sometimes calledcrawling seals to distinguish them from thefur seals andsea lions of the familyOtariidae. Seals live in theoceans of both hemispheres and, with the exception of the moretropicalmonk seals, are mostly confined topolar, subpolar, andtemperate climates. TheBaikal seal is the only species of exclusivelyfreshwater seal.
The earliest known fossil earless seal isNoriphoca gaudini from the lateOligocene or earliestMiocene (Aquitanian) ofItaly.[1] Other early fossil phocids date from the mid-Miocene, 15 million years ago in the north Atlantic.[1][3] Until recently, many researchers believed that phocids evolved separately fromotariids andodobenids; and that they evolved fromotter-like animals, such asPotamotherium, which inhabited European freshwater lakes. Recent evidence strongly suggests a monophyletic origin for all pinnipeds from a single ancestor, possiblyEnaliarctos, most closely related to themustelids andbears.[4]
Monk seals andelephant seals were previously believed to have first entered the Pacific through the open straits between North and South America,[5] with the Antarctic true seals either using the same route or travelled down the west coast of Africa.[6] It is now thought that themonk seals,elephant seals, andAntarctic seals all evolved in the southern hemisphere, and likely dispersed to their current distributions from more southern latitudes.[7]
Cladogram showing relationships among the phocids, combining several phylogenetic analyses.[8]
In the 1980s and 1990s, morphologicalphylogenetic analysis of the phocids led to new conclusions about the interrelatedness of the various genera. More recent molecular phylogenetic analyses have confirmed themonophyly of the two phocid subfamilies (Phocinae and Monachinae).[9][10][11][5] The Monachinae (known as the "southern" seals), is composed of three tribes; the Lobodontini, Miroungini, and Monachini. The fourAntarctic generaHydrurga,Leptonychotes,Lobodon, andOmmatophoca are part of thetribeLobodontini. Tribe Miroungini is composed of theelephant seals. TheMonk seals (Monachus andNeomonachus) are all part of the tribe Monachini.[12] Likewise, subfamily Phocinae (the "northern" seals) also includes three tribes; Erignathini (Erignathus), Cystophorini (Cystophora), andPhocini (all other phocines). More recently, five species have been split off fromPhoca, forming three additional genera.[13]
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.[14]
Skeletal anatomy of a harbor seal. 1. Skull. 2. Spine. 3. Tail. 4. Hindlimb. 5. Forelimb. 6. Shoulder. 7. Pelvis. 8. Rib cage.Harbor seal skull (Phoca vitulina)
Adult phocids vary from 1.17 m (3.8 ft) in length and 45 kg (99 lb) in weight in theringed seal to 5.8 m (19 ft) and 4,000 kg (8,800 lb) in thesouthern elephant seal, which is the largest member of the orderCarnivora. Phocids have fewer teeth than land-based members of theCarnivora, although they retain powerfulcanines. Some species lackmolars altogether. Thedental formula is:2–3.1.4.0–21–2.1.4.0–2
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 richupwelling zones close to breeding sites. Phocids swim by sideways movements of their bodies, using their hind flippers to fullest effect.[15] Their fore flippers are used primarily for steering, while their hind flippers are bound to thepelvis 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 abdominalmuscles.
Seal kidney
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 thelungs during a dive and into the upper respiratory passages, where gases cannot easily be absorbed into the bloodstream. This helps protect the seal fromthe bends. Themiddle ear is also lined with blood sinuses that inflate during diving, helping to maintain a constant pressure.[15]
Phocids are more specialized foraquatic life than otariids. They lack external ears and have sleek, streamlined bodies. Retractablenipples, internaltesticles,[16] and an internalpenile sheath provide further streamlining. A smooth layer ofblubber lies underneath the skin. Phocids are able to divertblood flow to this layer to help control their temperatures.[17]
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 byotariids, 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 mustfast whilelactating. 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 thehumpback whales' 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 themonk seal to just three to five days in thehooded 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.
The pup's diet is so high incalories 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.[19] 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.
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.[20]
^abFulton, Tara L.; Strobeck, Curtis (2010). "Multiple fossil calibrations, nuclear loci and mitochondrial genomes provide new insight into biogeography and divergence timing for true seals (Phocidae, Pinnipedia)".Journal of Biogeography.37 (5):814–829.Bibcode:2010JBiog..37..814F.doi:10.1111/j.1365-2699.2010.02271.x.S2CID59436229.
^Árnason, Úlfur; Bodin, Kristina; Gullberg, Anette; Ledje, Christina; Suzette, Mouchaty (1995). "A molecular view of pinniped relationships with particular emphasis on the true seals".Journal of Molecular Evolution.40 (1):78–85.Bibcode:1995JMolE..40...78A.doi:10.1007/BF00166598.PMID7714914.S2CID7537924.
^Arnason, Ulfur; Gullberg, Anette; Janke, Axel; Kullberg, Morgan; Lehman, Niles; Petrov, Evgeny A.; Väinölä, Risto (2006-11-01). "Pinniped phylogeny and a new hypothesis for their origin and dispersal".Molecular Phylogenetics and Evolution.41 (2):345–354.doi:10.1016/j.ympev.2006.05.022.PMID16815048.
^Berta, Annalisa; Churchill, Morgan (2012). "Pinniped taxonomy: review of currently recognized species and subspecies, and evidence used for their description".Mammal Review.42 (3):207–234.doi:10.1111/j.1365-2907.2011.00193.x.
^Bonner, N. (1994).Seals and Sea Lions of the World. United Kingdom: Blandford. pp. 1–224.ISBN9780816057177.
^Costa, D. P.; Boeuf, B. J. Le; Huntley, A. C.; Ortiz, C. L. (1986). "The energetics of lactation in the Northern elephant seal,Mirounga angustirostris".Journal of Zoology.209 (1):21–33.doi:10.1111/j.1469-7998.1986.tb03563.x.
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