- Harbor seals have a total gestation of about 9 to 11 months.
- Gestation includes a period of delayed implantation: when the fertilized egg divides into a hollow ball of cells one layer thick (blastocyst), it stops growing and remains free-floating in the uterus for one-and-a-half to three months. The blastocyst then implants on the uterine wall and continues to develop.
- Delayed implantation gives the mother time to recover from her last pregnancy. It also assures that the ensuing pup will be born when environmental conditions are optimal for its survival.
- Implantation of the blastocyst may be triggered by hormonal changes at the end of the molting season.
- Although most pups are born in February through July, the pupping season varies widely among the regional populations.
- Harbor seals of the northern Pacific population give birth from May to July. Farther south, the pupping season becomes progressively earlier; in Baja California, the season is February and March.
- Harbor seals inhabiting the coasts of British Columbia and Washington give birth from June to September.
Frequency of Birth
Females generally give birth to one pup each year. Multiple births are extremely rare, but twin fetuses have been documented.
Female harbor seals give birth on land, ice, or in the water near shore.
Pups at Birth
- Pups are about 75 to 100 cm (30-39 in.) and weigh 8 to 12 kg (18-26 lb.).
- Pups are well-developed at birth. Their eyes are open and they can swim and follow their mothers.
Care of Young
- Unlike most seals, which fast while nursing, harbor seal mothers leave their pups during the nursing period to forage at sea. Researchers believe they may do this because their relatively small body size cannot store enough fat to withstand a fast.
- A female harbor seal has two mammary glands on her lower abdomen.
- On average, harbor seal milk is about 45% fat, 9% protein, and 45.8% water, with traces of lactose (milk sugar). These figures may vary among individuals and may fluctuate throughout the nursing period. The extremely high fat content of the milk helps the pups more than double their weight by the time they are weaned.
- Harbor seals have been observed nursing both on land and in the water.
- A pup nurses for about one minute every three to four hours.
- Pups nurse for about four to six weeks.
- The female is an attentive parent during the nursing period. She noses the pup often. The pup may ride on her back, nip at her flippers, and chase her through the water.
- Females recognize their own pups by vocalizations and by smell.
- After her pup is weaned, a mother shows no interest in the pup.
Pup Growth and Development
- A pup learns to catch and eat shrimp and bottom-dwelling crustaceans after it is weaned. Later, it learns to catch fish.
- Pups do not wander far from adults.