Earless seal

Earless seals
Common Seal, Phoca vitulina
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Carnivora Suborder: Caniformia Superfamily: Pinnipedia Family: Phocidae Gray, 1821
Genera
Cystophora Erignathus Halichoerus Histriophoca Hydrurga Leptonychotes Lobodon Mirounga Monachus Ommatophoca Pagophilus Phoca Pusa

The true seals or earless seals are one of the three main groups of mammals within the seal suborder, Pinnipedia. All true seals are members of the family Phocidae. They are sometimes called crawling seals to distinguish them from the fur seals and sea lions of family Otariidae. Seals live in the oceans of both hemispheres and are mostly confined to polar, sub-polar, and temperate climes, with the exception of the more tropical monk seals.

Overview

In zoology, a seal is any pinniped other than a walrus. There are two main groups of these large, aquatic, mostly marine mammals. Earless seals (or "true seals" or "crawling seals") belong to the taxonomic family Phocidae (phocids). Eared seals (or "walking seals") are members of the family Otariidae (otarids) and include fur seals and sea lions.

As with all pinnipeds, seals have both front and hind limbs in the form of flippers and belong to the taxonomic group Pinnipedia, which is variously considered a suborder or superfamily of the order Carnivora or a separate order. Pinnipedia only includes seals and walruses. Walruses are generally considered a separate family of pinnipeds, the Obobenidae, although sometimes they are included with the phocids (McKenna and Bell 1997).

One way of differentiating between the two main groups of seals is by the presence of the pinna, a small furry earflap, found on the otarids and missing from phocids. Phocids are referred to as "earless seals" because their ears are not easily seen, while otarids are referred to as "eared seals." There are a number of other distinctions, including movement of hind flippers, swimming motion, and breeding systems. (See description section below.)

Seals spend most of their lives in the ocean, with the major exception of breeding and raising their young, which they do on land. The other major groups of marine mammals are either entirely aquatic (cetaceans and sirenians) or can breed and raise their young at sea (sea otters).

The largest seal is the male southern elephant seal, a phocid. It is over 4 m (13 ft) long and weighs up to 2,200 kg (4,850 lb, more than 2 tons).

All seals are carnivorous, eating fish, shellfish, squid, and other marine creatures. The leopard seal is probably the most carnivorous and predatory of all the pinnipeds, eating a wide variety of prey, from krill to penguins to other seals.

In addition to the presence of the pinna, there are other obvious differences between otarids and phocids. Otarids have hindflippers that can be inverted under the body, aiding their movement on land, while the hindflippers of phocids cannot be turned forward under the body, causing their movement on land to be slow and awkward (Riedman 1990). Otarids also swim using their long front flippers to move themselves through the water, while phocids swim by using their rearflippers and lower body in a side-to-side motion (Riedman 1990). There are also behavioral differences, including the breeding systems of the two groups (see below).

Of the estimated 50 million pinnipeds in the world, roughly 90 percent are phocid seals, largely because of the 30 million or so crabeater seals (Lobodon sp.) in the Antarctic (Riedman 1990).

Only earless seals live in the Antarctic and Arctic; there are no otariids in the extreme polar regions (Riedman 1990). On the other hand, only two species of phocids (the endangered Hawaiian and Mediterranean monk seals) are found in the tropics and these are small populations, while a number of fur seals and sea lions live in tropical and subtropical areas, with fur seals ranging widely into colder climates as well (Riedman 1990).

Anatomy and behavior

Phocids are more highly specialized for aquatic life than otariids, although they still return to dry land or pack ice in order to breed and give birth. They lack external ears and have sleek, streamlined bodies. To further aid streamlining, their nipples can be retracted, their testicles are internal, and the penis lies in an internal sheath. A smooth layer of blubber lies underneath the skin, and phocids are able to divert blood-flow to this layer to help control their temperature.

Their fore-flippers are used primarily for steering, while their hind flippers are bound to the pelvis in such a way that they cannot bring them under their body to walk on them. Phocids swim by sideways movements of their bodies, using their hind-flippers to their fullest effect.

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 muscles.

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

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

Adult phocids vary from 1.17 meters in length and 45kg in weight, in the Ringed Seal, to 4.9 meters and 2,400kg in the Southern Elephant Seal.

Phocids have a reduced number of teeth compared with land-based members of the Carnivora, although they retain powerful canines. Some species lack molars altogether. The dental formula is:

2-3.1.4.0-2

1-2.1.4.0-2

Evolution

The earliest 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 and odobenids from otter-like animals, such as Potamotherium, which inhabited European fresh-water lakes. Recent evidence strongly suggests a monophyletic origin for all pinnipeds from a single ancestor, possibly Enaliarctos, most closely related to the bears.

Monk seals and elephant seals are believed to have first entered the Pacific through the open straits between North and South America, which closed only in the Pliocene. The various Antarctic species may have either used the same route, or travelled down the west coast of Africa.

Feeding and reproduction

File:Argentina-Puerto Madrew-Sea Lion.jpg

Phocids in Argentina

While otariids are known for speed and maneuverability in the water, phocids are known for efficient, economical movement. This allows most phocids to make long foraging trips to exploit prey resources that are far from land, whereas otariids are tied to rich upwelling zones close to their breeding sites. A pregnant female spends a long period of time foraging at sea, building up her fat reserves and then returns to the breeding site and using her stored energy reserves to provide milk for her pup. The Common Seal, Phoca vitulina, displays a reproductive strategy similar to those 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 while she is lactating. This combination of fasting with lactation is one of the most unusual and extraordinary behaviors displayed by the Phocidae, because it requires the mother seal to provide large amounts of energy to her pup at a time when she herself is taking in no food (and often, no water) to replenish her stores. Because they must continue to burn fat reserves to supply their own metabolic needs while they are feeding their pups, phocid seals have developed an extremely thick, fat-rich milk that allows them to provide their pups with a large amount of energy in as small a period of time as possible. This allows the mother seal to maximize the efficiency of her energy transfer to the pup and then quickly return to sea to replenish her reserves. The length of lactation in phocids ranges from 28 days in the Northern Elephant Seal to just 3–5 days in the Hooded Seal. The nursing period is ended by the mother, who departs to the sea and leaves her pup at the breeding site. Pups will continue to nurse if given the opportunity, and "milk stealers" that suckle from unrelated, sleeping females are not uncommon; this often results in the death of the pup whose mother the milk was stolen from, as any single female can only produce enough milk to provision one pup.

The pup's diet is so high-calorie that the pup builds up a large store of fat. Before the pup is ready to forage on its own, the mother abandons it, and it lives on its fat for weeks or months while it develops independence. 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 fast, although some polar species have been observed to eat snow. The post-weaning 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.

Classification

In the 1980s, phylogenetic analysis of the phocids have comes to a few conclusions about the interrelatedness of the various genera. The four genera Hydrurga, Leptonychotes, Lobodon, and Ommatophoca form a monophyletic group, the tribe Lobodontini. Likewise, the Phocinae subfamily (Erignathus, Cystophora, Halichoerus, and Phoca) 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 and Mirounga is probably paraphyletic.

SUBORDER PINNIPEDIA

• Family Otariidae: fur seals and sea lions
• Family Odobenidae: Walrus
• Family Phocidae
  • Subfamily Monachinae
    • Tribe Monachini
      • †Monachopsis
      • †Pristiphoca
      • †Properiptychus
      • †Messiphoca
      • †Mesotaria
      • †Callophoca
      • †Pliophoca
      • †Pontophoca
      • Hawaiian Monk Seal, Monachus schauinslandi
      • Mediterranean Monk Seal, Monachus monachus
      • †Caribbean Monk Seal, Monachus tropicalis (probably extinct around 1950)
    • Tribe Miroungini
      • Northern Elephant Seal, Mirounga angustirostris
      • Southern Elephant Seal, Mirounga leonina
    • Tribe Lobodontini
      • †Monotherium wymani
      • Ross Seal, Ommatophoca rossi
      • Crabeater Seal, Lobodon carcinophagus
      • Leopard Seal, Hydrurga leptonyx
      • Weddell Seal, Leptonychotes weddellii
    • †Acrophoca longirostris
    • †Piscophoca pacifica
    • †Homiphoca capensis
  • Subfamily Phocinae
    • †Kawas benegasorum
    • †Leptophoca lenis
    • †Preapusa
    • †Cryptophoca
    • Bearded Seal, Erignathus barbatus
    • Hooded Seal, Cystophora cristata
    • Tribe Phocini
      • Common Seal, Phoca vitulina
      • Spotted Seal, Phoca largha
      • Ringed Seal, Pusa hispida (formerly Phoca hispida)
      • Nerpa, Pusa sibirica (formerly Phoca sibirica)
      • Caspian Seal, Pusa caspica (formerly Phoca caspica)
      • Harp Seal, Pagophilus groenlandica (formerly Phoca groenlandicus)
      • Ribbon Seal, Histriophoca fasciata (formerly Phoca fasciata)
      • †Phocanella
      • †Platyphoca
      • †Gryphoca
      • Grey Seal, Halichoerus grypus

References

ISBN links support NWE through referral fees

• Riedman, M. 1990. Pinnipeds: Seals, Sea Lions, and Walruses. Berkeley: University of California Press.

External links

Wikimedia Commons has media related to:

Phocidae

• Elephant Seal Research Group

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