All clawless and river otters have a gestation period of about two months.
North American, smooth, spot-necked, and marine otters in North American zoological parks have demonstrated delayed implantation for as long as 10 months. When the fertilized egg has divided into a hollow ball of cells one layer thick (blastocyst stage) it stops developing, and floats in the uterus. After several months the blastocyst implants in the uterine wall and continues to develop. Actual embryonic development takes only about two months. Delayed implantation ensures that the pup will be born when environmental conditions are optimal for its survival. It also allows some recovery time for the mother from her last pregnancy.
The giant otter's gestation period lasts about 65 to 70 days.
Reports on sea otter gestation range from four to nine months, with an average of six months. This probably includes a period of delayed implantation of two to three months.
In the subarctic climates of Sweden and Siberia, Eurasian otters give birth in April and May; in more temperate climates there is no distinct birth season.
North American, smooth, spot-necked, and marine otters give birth in winter and spring, one year following mating.
Giant otters are born late August to early October, during the dry season. If a female loses her first litter to predators or other natural causes, she may sometimes produce a second litter between December and April.
Birth seasons for Cape clawless otters are highly variable, and depend largely on location.
Sea otters give birth throughout the year, with peaks in late May and June for Alaska otters, and March and September for California otters.
Frequency of Birth
Most freshwater otters produce one litter each year. Some Eurasian and Asian small-clawed otters produce two litters in a year.
Sea otters are capable of giving birth every year, but females usually experience much longer birth intervals.
Freshwater otters usually have litters of one to three young. River and giant otters may have as many as five young in a litter, and Asian small-clawed otters may have six.
Sea otters normally have a single offspring. About 2% of all otter births are multiple, but only one pup can be successfully cared for.
Freshwater otters give birth on land, in dens. Birthing dens are secure and undisturbed.
Most sea otter births occur in the water.
Otters at Birth
River otter pups average about 130 grams (4.6 oz.).
Giant otters weigh about 200 grams (7 oz.) at birth.
Freshwater otter pups are born blind, toothless, and practically immobile. They remain in their birthing dens, and spend their first few weeks nursing and sleeping.
Sea otter pups measure 56 to 61 cm (22-24 in.) in length and weigh 2 to 2.3 kg (4.5-5 lb.). They are somewhat precocial; born with eyes open, first teeth already emerging, and a full coat of dense fur that enables them to float. They are, however, completely dependent on their mothers for care and protection.
Care of Young
A female river otter has four nipples on her lower abdomen. River otters' milk is 24% fat. Pups nurse every three to four hours for 10 to 15 minutes at a time. They are fully weaned at 14 weeks.
A female sea otter carries and nurses her pup on her belly as she floats on her back. She holds her pup belly down so it can nurse from her two abdominal nipples. Sea otter milk is 20% to 25% fat. Sea otters nurse for six to eight months.
All female otters aggressively defend their young against intruders.
A female sea otter carries her pup on her belly and spends a great deal of time grooming it for the first three months. When she dives for food, the mother often wraps her pup in strands of kelp to keep it from drifting. If she senses danger, she grabs the pup by the loose skin of the neck with her teeth and dives until they reach safety.
River otter pups open their eyes at about one month, and most species first emerge from the den and begin to swim at two months. By four months of age, most river otters can swim and dive well enough to catch their own food.
Giant otter pups eat solid food at three to four months.
Clawless otters open their eyes at about 40 days, and first swim at nine weeks. They take their first solid food at about 80 days.
All freshwater otter pups stay with their mothers for a year, until the next litter is born.
Sea otter pups may begin to swim and take solid food at about four weeks, and dive at six weeks. Pups remain dependent on their mothers for an average of six months.
Growth rates in otters vary considerably between species, litters, sexes, and individuals.