Product Code Database
The earless seals, phocids, or true seals are one of the three main groups of within the seal lineage, . All true seals are members of the family Phocidae (). They are sometimes called crawling seals to distinguish them from the and of the family Eared seal. Seals live in the of both hemispheres and, with the exception of the more tropical , are mostly confined to Polar region, subpolar, and temperate climates. The Baikal seal is the only species of exclusively freshwater seal.
and were previously believed to have first entered the Pacific through the open straits between North and South America, with the Antarctic true seals either using the same route or travelled down the west coast of Africa. It is now thought that the , , and Lobodontini all evolved in the southern hemisphere, and likely dispersed to their current distributions from more southern latitudes.
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. (1994). 9780816057177, Blandford. ISBN 9780816057177
|
|
|
|
|
|
|
|
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 rich upwelling zones close to breeding sites. Phocids swim by sideways movements of their bodies, using their hind flippers to fullest effect. 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 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 abdominal .
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 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.
Phocids are more specialized for Aquatic animal life than otariids. They lack external ears and have sleek, streamlined bodies. Retractable , internal , and an internal penile sheath provide further streamlining. A smooth layer of blubber lies underneath the skin. Phocids are able to divert blood flow to this layer to help control their temperatures.
Conversely, monachine seals are well known to vocalize: elephant seals often vocalize on land, with the bulls being well known for roaring to establish dominance and identifying themselves. The Weddell and leopard seals are well known for their underwater singing.
Because a phocid mother's feeding grounds are often hundreds of kilometers from the breeding site, she must fasting while lactation. 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 the ' 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 the monk seal to just three to five days in the hooded 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.
Communication
Unlike otariids, phocine seals do not communicate by "barking". Instead, they communicate by slapping the water and grunting, with a few species have been documented to clap at each other underwater. One study published in Animal Behaviour found that seals use rhythmic percussive signalling (clapping) in both "agonistic interactions" and display behavior.
Movements
With short front flippers and because their rear flipper is unable to rotate, true seals cannot walk when out of the water like sea lions. As a result, they bounce themselves forward in a motion called ; the polar explorer Edward Wilson referred to the motion as lolloping.
Reproduction
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 by Eared seal, in which the mother makes short foraging trips between nursing bouts.
Growth and maturation
The pup's diet is so high in 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. 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.
Feeding strategy
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.
See also
|
|
