External anatomy skull (
Phoca vitulina) Adult phocids vary from in length and in weight in the
ringed seal to and in the
southern elephant seal, the largest member of the order
Carnivora. Phocids have fewer teeth than land-based members of the Carnivora, although they retain powerful
canines. Some species lack
molars altogether. The
dental formula is: 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 in a sideways motion with their bodies; their fore flippers are primarily used for steering, whereas their hind flippers are used for underwater propulsion. While phocids' streamlined bodies make them better long-distance swimmers than otariids, unlike
fur seals, their hind flippers are bound to the
pelvis and unable to be rotated forward for walking. As a result, they are clumsy on land, having to wriggle with their front flippers and abdominal
muscles. Phocid respiratory and circulatory systems are adapted to facilitate diving into considerable depths, allowing them to spend long periods of time underwater. When a seal dives, air is forced from the
lungs and into the upper respiratory passages, where gases cannot easily be absorbed into the bloodstream, protecting it from
the bends. The
middle ear is also lined with blood sinuses that inflate during diving, helping to maintain a constant pressure. and an internal
penile sheath. Phocids are able to divert
blood flow to a smooth layer of
blubber underneath their skin to help regulate body temperature.
Communication Unlike otariids, phocine seals do not communicate by "barking". Instead, they communicate by slapping the water and grunting, with a few species having 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. 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 identify themselves. The
Weddell and
leopard seals are well known for their underwater singing.
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
galumphing; the polar explorer
Edward Wilson referred to the motion as
lolloping. It is also referred to as "worm-style" locomotion.
Reproduction ,
Finland,
Saimaa ringed seals, a subspecies of
ringed seal, are among the most
endangered seals in the world, having a total population of only about 400 individuals. 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
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
lactating. 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
humpback whales' 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). Phocids are known to deliberately nurse young that are not their own, this is particularly seen in individuals that have lost their own pup."Milk stealers" that suckle from unrelated, sleeping females are not uncommon; this often results in the death of the mother's own pup through starvation; another side-effect of milk stealing in some species of phocid is the formation of
"super weaners".
Growth and maturation The pup's diet is so high in
food energy 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==