With the exception of the walrus, which has five- to six-year-long inter-birth intervals, female pinnipeds enter estrous shortly after they give birth.
All species go through delayed implantation, wherein the embryo remains in suspended development for weeks or months before it is implanted in the uterus.
Delayed implantation postpones the birth of young until the female hauls-out on land or until conditions for birthing are favorable.
Gestation in seals (including delayed implantation) typically lasts a year.
For most species, birthing takes place in the spring and summer months.
Typically, single pups are born; twins are uncommon and have high mortality rates.
Pups of most species are born precocial.

Unlike terrestrial mammals, 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 remain on land or ice and fast during their relatively short lactation period–four days for the hooded seal and five weeks for elephant seals.
The milk of these species consist of up to 60% fat, allowing the young to grow fairly quickly.
In particular, northern elephant seal pups gain 4 kg (9 lb) each day before they are weaned.
Some pups may try to steal extra milk from other nursing mothers and gain weight more quickly than others.
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 leave their young onshore to forage at sea.
These foraging trips may last anywhere between a day and two weeks, depending on the abundance of food and the distance of foraging sites.
While their mothers are away, the pups will fast.
Lactation in otariids may last 6–11 months; in the Galápagos fur seal it can last as long as 3 years.
Pups of these species are weaned at lower weights than their phocid counterparts.
Walruses are unique in that mothers nurse their young at sea.
The female rests at the surface with its head held up, and the young suckle upside down.
Young pinnipeds typically learn to swim on their own and some species can even swim at birth.
Other species may wait days or weeks before entering the water.
Elephant seals do not swim until weeks after they are weaned.

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.
Male California sea lions have been observed to help shield swimming pups from predators.
Males can also pose threats to the safety of pups.
In terrestrially breeding species, pups may get crushed by fighting males.
Subadult male South America sea lions sometimes abduct pups from their mothers and treat them like adult males treat females.
This helps them gain experience in controlling females.
Pups can get severely injured or killed during abductions.
With the exception of the walrus, which has five- to six-year-long inter-birth intervals, female pinnipeds enter estrous shortly after they give birth. All species go through delayed implantation, wherein the embryo remains in suspended development for weeks or months before it is implanted in the uterus. Delayed implantation postpones the birth of young until the female hauls-out on land or until conditions for birthing are favorable. Gestation in seals (including delayed implantation) typically lasts a year. For most species, birthing takes place in the spring and summer months. Typically, single pups are born; twins are uncommon and have high mortality rates. Pups of most species are born precocial.
Unlike terrestrial mammals, 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 remain on land or ice and fast during their relatively short lactation period–four days for the hooded seal and five weeks for elephant seals. The milk of these species consist of up to 60% fat, allowing the young to grow fairly quickly. In particular, northern elephant seal pups gain 4 kg (9 lb) each day before they are weaned. Some pups may try to steal extra milk from other nursing mothers and gain weight more quickly than others. 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 leave their young onshore to forage at sea. These foraging trips may last anywhere between a day and two weeks, depending on the abundance of food and the distance of foraging sites. While their mothers are away, the pups will fast. Lactation in otariids may last 6–11 months; in the Galápagos fur seal it can last as long as 3 years. Pups of these species are weaned at lower weights than their phocid counterparts. Walruses are unique in that mothers nurse their young at sea. The female rests at the surface with its head held up, and the young suckle upside down. Young pinnipeds typically learn to swim on their own and some species can even swim at birth. Other species may wait days or weeks before entering the water. Elephant seals do not swim until weeks after they are weaned.
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. Male California sea lions have been observed to help shield swimming pups from predators. Males can also pose threats to the safety of pups. In terrestrially breeding species, pups may get crushed by fighting males. Subadult male South America sea lions sometimes abduct pups from their mothers and treat them like adult males treat females. This helps them gain experience in controlling females. Pups can get severely injured or killed during abductions.
Pinniped