Pinniped
Birth and parenting

Except for the walrus, which has five- to six-year gaps between births, female pinnipeds enter estrus shortly after they give birth, and can thus produce pups every year. All species have delayed implantation, in which the embryo does not enter the uterus for weeks or months. Delayed implantation allows the female to wait until conditions are right for birthing. Gestation in seals (including delayed implantation) typically lasts a year. For most species, birthing takes place in spring and summer. Usually, single pups are born; twins are rare and have high mortality rates. Pups of most species are born relatively developed and precocial.

Pinniped milk has "little to no lactose ". Mother pinnipeds have different strategies for maternal care and lactation. Phocids such as elephant seals, grey seals and hooded seals have a lactation period that lasts days or weeks, during which they fast and nurse their pups on land or ice. The milk of these species consists of up to 60% fat, allowing the young to grow quickly. Each day until they are weaned, northern elephant seal pups gain 4 kg (9 lb). Some pups gain weight more quickly than others by stealing extra milk from other mothers. Alloparenting occurs in these fasting species; while most northern elephant seal mothers nurse their own pups and reject nursings from alien pups, some do accept alien pups with their own.

For otariids and some phocids like the harbor seal, mothers fast and nurse their pups for a few days at a time. In between nursing bouts, the females forage at sea while the young stay behind onshore. If there is enough food close to shore, a female can be gone for as little as a day, but otherwise may be at sea for as long as three weeks. Lactation in otariids may last 6–11 months; in the Galápagos fur seal it can last up to 3 years. Pups of these species are weaned at heavier weights than their phocid counterparts, but the latter grow quicker. Walruses are unique in that mothers nurse their young at sea. Young pinnipeds typically start swimming on their own and some species can even swim as newborns. Young may wait days or weeks before entering the water; elephant seals start swimming weeks after weaning.

Male pinnipeds generally play little role in raising the young. Male walruses may help inexperienced young as they learn to swim, and have even been recorded caring for orphans. When a group is threatened, all the adults may protect the young. Male California sea lions have been observed to help shield swimming pups from predators. Males can also pose threats to the safety of pups, particularly during fights. Pups of some species may be abducted, assaulted and killed by males.
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