Seal
In zoology, a seal is any large marine pinniped other than a walrus. There are two main groups of seals. 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 hindflippers, swimming motion, and breeding systems. (See description section below.)
There are 32 or 33 recognized species of pinnipeds, including 14 otariids and 18 or 19 phocids (one, the Caribbean monk seal, may be extinct).
Description
Seals are typically sleek-bodied and rather large. Their bodies are well adapted to their aquatic habitat, in which they spend most of their lives. In place of hands, their forelimbs are large flippers (hence the name "fin-feet"), and their bodies narrow out into a tail.
The smallest pinniped, the Galapagos fur seal, weighs about 30 kg (66 lb) when full-grown and is 1.2 m (4 ft) long; the largest, the male southern elephant seal, is over 4 m (13 ft) long and weighs up to 2,200 kg (4,850 lb, more than 2 tons).
Pinnipeds are more streamlined, larger, and faster swimming than the sea otters, which have retained the front paws of a terrestrial carnivore, although their hind feet are flippers (Riedman 1990).
All pinnipeds 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 presense of the pinna, there are Another difference is that 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). The breeding system of the two groups is also different (see below).
Distribution
Of the estimated 50 million pinnipeds in the world, roughly 90% are phocid seals, largely because of the 30 million or so crabeater seals in the Antarctic (Riedman 1990).
Only earless seals live in the Antarctic and Artic; there are no otariids in the extreme polar regions (Riedman 1990). On the other hand, only two species of phocids are found in the tropics and these are small and endangered 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).
All but two species of fur seals are found in the Southern Hemisphere, while sea lions are common to both hemispheres. No seals are are found in Asia (Riedman 1990).
Although fewer species of seals inhabit the Antarctic than the Artic, the Antarctic seal populations are larger in number and generally Antarctic seals also have larger bodies (Riedman 1990).
Reproduction
Unlike sea otters, which, although they can crawl on land, essentially retain no ties to land, being able to mate and raise their offspring entirely at sea, the pinnipeds are still dependent on land and return to land to breed (Riedman 1990).
Otarid seals tend to have similar breeding systems, with all sea lions and fur seals being highly polygynous, adult males normally territorial and going without eating during the breeding season, and sexual dimorphism pronounced, with males two to four times the size of females (Riedman 1990). Phocid breeding behavior is more variable, with a variety of systems and variable degrees of sexual dimorphism. Some phocids breed in solitary groups, with just the male, female, and pup, and with the male and female of equal size; others form dominance hierarchies, with larger males competing for females; and there are species where the breeding takes place in large groups with hundreds of animals.
In general, male otarids defend a territory, whereas phocids with a dominance hierarchy defend clusters of females. These strategies reflect the different levels of mobility on land between otarids and phocids. As noted, the former have opposable hind flippers that can be placed flat on the ground to aid locomotion. True seals, on the other hand, cannot do this and drag themselves along using only their front flippers.
Pinnipeds come ashore to breed (haul-out), and this often necessitates traveling long distances from their feeding grounds to suitable mating grounds (either on land or ice). Because of these constraints, pinnipeds tend to mate and give birth with a high level of reproductive synchrony. The most synchronous species are the two phocids (seals)—the harp and hooded seals—in which all females are estimated to become sexually receptive during a period of 10–15 days. Males in dominance hierarchies compete for females at rookeries. Females are usually highly clustered here to reduce the level of male harassment, particularly by low-ranking males. Females compete for central positions and call out if attacked by subordinate males that are subsequently chased away by the dominant bull.
After giving birth, pinniped mothers suckle their young for a variable length of time. Unlike otariids, which nurse the pups for several months while alternating feeding at sea, most phocid females nurse for short periods while fasting or feeding little (Riedman 1990). Among the phocids, lactation varies from 4 to 50 days, whereas the otarids may lactate from 4 to 36 months. This reflects the fact that phocid feeding grounds tend to be a long way off-shore so lactation is associated with maternal fasting. To compensate for the short lactation period, the fat content of phocid milk is higher than in any other species of marine mammal (45–60% fat). After lactation, most female phocids make extensive migratory movements to feeding grounds for intensive foraging to recoup depleted energy reserves. On the other hand, otarid feeding grounds are generally closer to shore and females go on foraging trips to maintain lactation. Fat content of otarid milk is lower than that of the phocids owing to the protracted lactatory period (typically 25–50%). Protracted nursing also leads to the formation of social bonds.
Females have a postpartum oestrus allowing them to mate soon after giving birth. Subsequent implantation of the embryo is delayed (embryonic diapause) thus removing the need to come ashore (haul-out) twice, once to give birth and again later to mate. All otarids copulate soon after birth, while most phocids mate during late lactation or after weaning their pup (Riedman 1990).
Earless seals
| Earless Seals | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
 Grey seal
| ||||||||||||
| Scientific classification | ||||||||||||
| ||||||||||||
|
Monachus (Monk Seals) |
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.
Phocids are the more highly specialized for aquatic life of the two groups and, unlike otariids, lack external ears and cannot bring their hind flippers 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 than those can. 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; this method of locomotion is called galumphing.
Additionally, true seals do not communicate by "barking" like the fur seals and sea lions of family Otariidae. They communicate by slapping the water and grunting.
Feeding and reproduction
While otariids are built for speed and maneuverability in the water, phocids are built 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. The phocid reproductive cycle is characterized by temporal and spatial separation between feeding and maternal investment; in other words, 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 uses her stored energy reserves to provide milk for her pup. It should be noted that the common seal (harbor seal in the U.S.), Phoca vitulina, does not separate foraging and maternal investment; instead, it 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, this means that 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 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.
Because the pup receives the milk energy from its mother so quickly, its development is typically not complete enough for it to begin foraging on its own as soon as the nursing period is complete. Seals, like all marine mammals, need time to develop the oxygen stores, swimming muscles and neural pathways necessary for effective diving and foraging. Because of this, most phocids undergo a postweaning fast, in which they remain on or near the breeding site and live off of the fat stores they acquired from their mothers until they are ready to begin foraging on their own. These pups typically eat no food and drink no water during the fast, although some polar species have been observed to eat snow. The postweaning fast ranges from 2 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
There is greater consensus on the division of pinnipeds into three families. These are:
- Odobenidae - walruses.
- Otariidae - sea lions, eared seals, and fur seals.
- Phocidae - true (earless) seals.
Odobenidae and Otariidae are generally placed together in the superfamily Otarioidea, while Phocidae is in the superfamily Phocoidea (Riedman 1990). However, some consider walruses more closely related to the true seals. McKenna and Bell (1997) place walruses with the true seals in the family Phocidae, a practice that is followed by the Tree of Life Web Project (2000), yielding the following two families:
- Otariidae - sea lions, eared seals, and fur seals.
- Phocidae - true (earless) seals and walruses.
SUBORDER PINNIPEDIA
- Family Otariidae: fur seals and sea lions
- Family Odobenidae: Walrus
- Family Phocidae
- Subfamily Monachinae
- Tribe Monachini
- Monachopsis (extinct)
- Pristiphoca (extinct)
- Properiptychus (extinct)
- Messiphoca (extinct)
- Mesotaria (extinct)
- Callophoca (extinct)
- Pliophoca (extinct)
- Pontophoca (extinct)
- 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 (extinct)
- Ross Seal, Ommatophoca rossi
- Crabeater Seal, Lobodon carcinophagus
- Leopard Seal, Hydrurga leptonyx
- Weddell Seal, Leptonychotes weddellii
- Swan-necked Seal, Acrophoca longirostris (extinct)
- Piscophoca pacifica (extinct)
- Homiphoca capensis (extinct)
- Tribe Monachini
- Subfamily Phocinae
- Kawas benegasorum (extinct)
- Leptophoca lenis (extinct)
- Preapusa (extinct)
- Cryptophoca (extinct)
- Bearded Seal, Erignathus barbatus
- Hooded Seal, Cystophora cristata
- Tribe Phocini
- Common Seal or Harbor Seal, Phoca vitulina
- Spotted Seal or Larga Seal, Phoca largha
- Ringed Seal, Phoca hispida
- Nerpa or Baikal Seal, Phoca sibirica
- Caspian Seal, Phoca caspica
- Harp Seal, Phoca groenlandica (or Pagophilus groenlandicus)
- Ribbon Seal, Phoca fasciata
- Phocanella (extinct)
- Platyphoca (extinct)
- Gryphoca (extinct)
- Grey Seal, Halichoerus grypus
- Subfamily Monachinae
Eared seals
| Eared seals | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| An Australian Sea Lion An Australian Sea Lion
| ||||||||||||
| Scientific classification | ||||||||||||
| ||||||||||||
|
Arctocephalus |
The eared seals (or walking seals), family Otariidae, are the fur seals and the sea lions. These are barrel shaped marine mammals, adapted to a semi-aquatic lifestyle. They feed and travel in the water but breed and largely rest on land (or, in some cases, on ice). They are slightly less adapted to the aquatic lifestyle than are the true seals.
- SUBORDER PINNIPEDIA
- Family Otariidae
- Subfamily Arctocephalinae: fur seals
- Northern Fur Seal, Callorhinus ursinus
- Antarctic Fur Seal, Arctocephalus gazella
- Guadalupe Fur Seal, Arctocephalus townsendi
- Juan Fernandez Fur Seal, Arctocephalus philippii
- Galapagos Fur Seal, Arctocephalus galapagoensis
- Cape Fur Seal or Australian Fur Seal, Arctocephalus pusillus
- New Zealand Fur Seal or Southern Fur Seal, Arctocephalus forsteri
- Subantarctic Fur Seal, Arctocephalus tropicalis
- South American Fur Seal, Arctocephalus australis
- Subfamily Otariinae: sea lions
- Steller's Sea Lion, Eumetopias jubatus
- California Sea Lion, Zalophus californianus
- South American Sea Lion, Otaria flavescens
- Australian Sea Lion, Neophoca cinerea
- New Zealand Sea Lion, Phocarctos hookeri
- Subfamily Arctocephalinae: fur seals
- Family Phocidae: true seals
- Family Odobenidae: Walrus
Fur seals
Fur seals make up one of the two distinct groups of mammals called "seals". Both the fur seals and the true seals are members of the Pinnipedia, which is usually regarded as a suborder of the order Carnivora but sometimes as an independent order. However, the fur seals, like their close relatives the sea lions, retain some ability to walk on land as their hind limbs can be brought forward under the body to bear the animal's weight, and retain small but visible external ears.
The fur seals and the sea lions as a group make up the family Otariidae, and are called eared seals or walking seals to distinguish them from the earless true seals of the family Phocidae. The fur seals alone make up the Arctocephalinae subfamily.
- SUBORDER PINNIPEDIA
- Family Otariidae
- Subfamily Arctocephalinae
- Northern Fur Seal, Callorhinus ursinus
- Antarctic Fur Seal, Arctocephalus gazella
- Guadalupe Fur Seal, Arctocephalus townsendi
- Juan Fernandez Fur Seal, Arctocephalus philippii
- Galapagos Fur Seal, Arctocephalus galapagoensis
- Cape Fur Seal or Australian Fur Seal, Arctocephalus pusillus
- New Zealand Fur Seal, Arctocephalus forsteri
- Subantarctic Fur Seal, Arctocephalus tropicalis
- South American Fur Seal, Arctocephalus australis
- Subfamily Arctocephalinae
- Subfamily Otariinae: sea lions
- Family Phocidae: true seals
- Family Odobenidae: Walrus
Sea lions
A sea lion is any of several marine mammals of the family Otariidae. Sea lions are characterized by the presence of external ear pinnae or flaps, long front flippers, and the ability to walk on four flippers on land. Sea lions are generally found in coastal waters of the temperate to subpolar regions of both northern and southern hemispheres.
Sea lions are often a popular attraction at zoos and aquariums, performing tricks such as throwing and catching balls on their noses and clapping.
Sea lions of many species have seen a severe and rapid decrease in numbers in recent years. Many factors including overfishing of other species, shooting by fishermen and pollution have probably contributed to the decline. The population of Steller's sea lions in Alaska has declined as much as 80% since the 1970s. [1] [2]
Taxonomy
- SUBORDER PINNIPEDIA
- Family Otariidae
- Subfamily Arctocephalinae: fur seals
- Subfamily Otariinae
- Steller's Sea Lion, Eumetopias jubatus
- California Sea Lion, Zalophus californianus
- South American Sea Lion, Otaria flavescens
- Australian Sea Lion, Neophoca cinerea
- New Zealand Sea Lion, Phocarctos hookeri
- Family Phocidae: true seals
- Family Odobenidae: Walrus
Hybridization
A hybrid sea lion from a cross between the California sea lion (Zalophus californianus) and the South American sea lion (Otaria byronia) has occurred.
ReferencesISBN links support NWE through referral fees
- ↑ Clover, Charles. 2004. The End of the Line: How overfishing is changing the world and what we eat. Ebury Press, London. ISBN 0-09-189780-7
- ↑ Dalton, Rex. 2005. "Is this any way to save a species?." Nature 436, 7 July 2005, 14-16.
References
- McKenna, M. C., and S. K. Bell. 1997. Classification of Mammals Above the Species Level. New York: Columbia University Press.
- Myers, P., and A. Poor. 2007. Carnivora. Animal Diversity Web. Accessed January 23, 2007.
- Nowak, R. M. 1999. Walker's Mammals of the World, 6th Edition. Baltimore: The John's Hopkins Press.
- Riedman, M. 1990. Pinnipeds: Seals, Sea Lions, and Walruses. Berkeley: University of California Press.
- Tree of Life Web Project. 2000. Carnivora. Dogs, cats, bears, raccoons, weasels, mongooses, hyenas, seals, walruses, etc. Version 01 January 2000. Tree of Life Web Project. Retrieved January 23, 2007.
- Wozencraft, W. C. 1992. Order Carnivora. In D. E. Wilson and D. M. Reeder (eds.), Mammal Species of the World: A Taxonomic and Geographic Reference, 2nd. edition. Washington: Smithsonian Institution Press.
{{credit5|Seal|102942659|Earless_seal|99734630|Eared_Seal|99215085|Fur_seal|93945371|Sea_Lion|102204910]]
