Life-long pair-bonding was the rule, quite opposite to today's manatees and dugongs. Copulation in general took place in early spring, usually in the evening at
calm weather "... after protracted preludes. The female flees slowly before the male with continual turns about, but the male pursues her without cessation. When, however, the female is finally weary of this mock coyness, she turns on her back and the male completes the mating in the human manner"*)Steller, description of Bering Island
The young were born at any time of year, but most frequently in early autumn, as Steller judged from the growth stage of the little ones during his stay. Gestation period was therefore about 15 month. Presumably they only had one calf at a time, as Steller had never seen more than one with a mother. Parents cared for their offspring more than two years, so they were usually accompanied by a baby and a half grown of the previous litter.
Nothing is known about the age of sexual maturity and the life expectancy of the species.
Seacows are long-lived:
Manatees live for about 60 years, are sexually mature at 6-10 years, gestation period is about 13
months, cows give birth to a calf about every 3 years (twins are the exception), who stay with the mother for up to 2 years.
Dugongs live for about 70 years, are sexually mature at 10-15 years, gestation period is about 12
months, cows give birth to a calf about every 3-7 years (twins are the exception), who stay with the mother for about 1 year.
Life expectancy for Steller's Seacow was estimated at 90 years,
based on extrapolation from living sirenians (Turvey & Risley, 2005). Sexual maturity unknown, gestation period about 15 months, cows gave birth to a calf probably every 3-4 years (no twins), who stayed with the parents for over 2 years.