Pinniped
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 Cape fur seal (Arctocephalus pusillus pusillus)
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Pinnipeds (Latin, pinna, "feather or wing," and pedis, "foot," meaning "winged feet" or "fin-feet") are large marine mammals belonging to the taxonomic group Pinnipedia. Pinnipedia is usually considered a suborder of the Order Carnivora, but is sometimes considered a separate order, and more recently is regarded as a superfamily of the Suborder Caniformia of Carnivora.
Pinnipeds comprise the families Odobenidae (walruses), Otariidae (sea lions, eared seals, and fur seals), and Phocidae (true seals). They are one of four groups of marine mammals , the others being cetaceans (whales, dolphins and porpoises), sirenians ( manatees and dugongs), and sea otters.
Taxonomy
The placement of Pinnipedia in taxonomic schemes has been a source of controversy. Some taxonomists regard them as a separate order based on their uniqueness as a group (Riedman 1990). Generally, however, they are placed within the Order Carnivora.
Carnivora is an order of placental animals that includes such familiar placental animals as bears, foxes, cats, hyenas, skunks, otters, racoons, and so forth. Older classification schemes divided this order into Pinnipedia (marine carnivores) and Fissipedia (primarily land carnivores). However, pinnipeds are considered to have derived from land-based Carnivora ancestors and thus this division is **artificial. Thus, pinnipeds are placed as a subdivision of Carnivoria, generally within the arctoid group ("bear-like") of carnivores. Newer classification schemes divide the Carnivora into the suborders Caniformia (dog-like) and Feliformia (cat-like). Pinnipeds are placed in the Caniformia group, generally as a superfamily.
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.
One very visible difference between the otariids and the phocids is the pinna, a small furry earflap, found on the otarids. Phocids are thus referred to as "earless seals," since their ears are not easily seen, while otarids are referred to as "eared seals." 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 different, 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 (Riedman 1990).
There are 33 or 34 recognized species of pinnipeds. These include 1 odobenid, 14 otariids, and 18 or 19 phocids (one, the Caribbean monk seal, may be extinct).
Location and description
Among the pinnipeds, only phocids live in the Antarctic and Artic, while walruses inhabit the northern circumpolar waters; there are no otariids in the extreme polar regions (Riedman 1990). ONly two species of phocids are fouond in the tropics and these are small and endangered populations, but 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, sea lions are common to both hemispheres. No pinnipeds are found in Asia (Riedman 1990).
Pinnipeds 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.
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, with the male and female of equal size, while others form dominance hierarchies, with larger males competing for females, and the breeding takes place in large groups with hundreds of animals.
In general male otarids (sea lions) defend a territory, whereas phocids defend clusters of females. These strategies reflect the different levels of mobility on land between otarids and phocids. 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.
After giving birth 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
The pinnipeds come ashore to breed (haul-out), and this often necessitates travelling long distances from their feeding grounds to suitable mating grounds (either on land or ice). Because of these constraints, pinnipeds 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 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.
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).
Evolution
Pinnipeds appear to have diverged from their bear-like ancestors during the Latest Oligocene. The earliest fossil pinniped that has been found is Enaliarctos, which lived 24–22 million years ago[citation needed], at the boundary between the Oligocene and Miocene periods. It is believed to have been a good swimmer, but to have been able to move on land as well as in water, more like an otter than like modern pinnipeds. DNA evidence suggests that all modern pinnipeds descend from a common ancestor that lived sometime in the earliest Miocene, possibly an Enaliarctos-like mammal.
History
Recent molecular analysis reveals that the closest living relatives of the pinnipeds are the bears, which was already suspected for some time.[1] An alternative hypothesis held that pinnipeds are polyphyletic, with the true seals derived from otterlike creatures and the walruses and eared seals from bearlike creatures, but the aforementioned molecular study established that the pinnipeds are indeed monophyletic (derived from a common ancestor).
See also
- Cetaceans
- Sirenians
ReferencesISBN links support NWE through referral fees
- ↑ John J. Flynn et al (2005). Molecular Phylogeny of the Carnivora. Systematic Biology 54: 317–337.
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