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 body to walk on them. Phocids swim by sideways movements of their bodies, using their hind-flippers to their fullest effect.
They are more streamlined than fur seals and sea lions and can therefore 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 muscles.
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 lungs 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.
True seals do not communicate by "barking" like otariids. Instead, they communicate by slapping the water and grunting.
Adult phocids vary from 1.17 meters in length and 45kg in weight, in the Ringed Seal, to 4.9 meters and 2,400kg in the Southern Elephant Seal.
Monk seals and elephant seals are believed to have first entered the Pacific through the open straits between North and South America, which closed only in the Pliocene. The various Antarctic species may have either used the same route, or travelled down the west coast of Africa.
While otariids are known for speed and maneuverability in the water, phocids are known 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. 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. The Common Seal, Phoca vitulina, 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, 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 the 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.
The pup's diet is so high-calorie that the pup builds up a large store of fat. Before the pup is ready to forage on its own, the mother abandons it, and it lives on its fat for weeks or months while it develops independence. 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 fast, although some polar species have been observed to eat snow. The post-weaning 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.