The superfamily containing all seals is Pinnipedia (literally "winged feet). Pinnipeds comprise three families: "true" seals (Phocidae), walruses (Odobenidae), and sea lions, eared seals and fur seals (Otariidae). The three families are monophyletic, meaning they derived from a common ancestor, the closest living relatives to which are bears.
True seals breed in colonies, or "rookeries", on land or ice, often far from their feeding grounds. For this reason, mating and birthing are highly synchronized. The greatest synchronicity is shown by harp and hooded seals, with all females becoming sexually receptive within a period of just 10-15 days. They form tight clusters, vying for central positions, to catch the attention of the dominant bull while making it more difficult for subordinate males to target them.
In general male eared seals defend a territory, whereas true seals defend clusters of females, reflecting their different levels of mobility. Eared seals have opposable hind flippers that can be placed flat on the ground to aid locomotion, whilst true seals can only drag themselves along with their front flippers.
Females have a postpartum oestrus allowing them to mate soon after giving birth. Subsequent implantation of the embryo is delayed - a process known as embryonic diapause - thus removing the need to come ashore twice, once to give birth and again later to mate. After giving birth, mothers suckle their young for a variable length of time. Amongst the true seals, lactation varies from 4 to 50 days, whereas eared seals may lactate from 4 to 36 months. This reflects the fact that true seals' feeding grounds tend to be far from shore so lactation is associated with maternal fasting. To compensate for the short lactation period, the fat content of true seals' milk is higher than in any other species of marine mammal (45-60% fat). After lactation most female true seals make extensive migratory movements to feeding grounds for intensive foraging to recoup depleted energy reserves. Eared seals, in contrast, generally feed closer to shore and females go on foraging trips to maintain lactation. The fat content of their milk is thus lower (typically 25-50%) than that of true seals. Protracted nursing also leads to the formation of social bonds in eared seals.