Earless
Seals |
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Grey seal |
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Scientific classification |
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Genera |
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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.
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.
SUBORDER PINNIPEDIA
Wikipedia
http://en.wikipedia.org/w/index.php?title=Fur_seal&action=history
http://en.wikipedia.org/w/index.php?title=Earless_seal&action=history
http://www.gnu.org/copyleft/fdl.html