Whales mature sexually between their third and seventh years. Toothed
cetaceans attain sexual maturity later than filter-feeding whales. A
blue whale is sexually mature at 5 years, whereas porpoises require at
least 7 years. Most filtering whales are sexually mature in 2 or 3
years. Whales are not fully grown at sexual maturity, but they continue
to grow for years. In most mammals growth stops with sexual maturity.
Female whales generally can be expected to produce a baby every other
year, for the gestation period is approximately one year. Babies are
nursed for about 9 months. At birth the baby is completely formed and
active, but lacking baleen, must nurse. A blue whale baby at birth
weighs approximately 8 tons, about 1/12th of the weight of the mother.
The mother provides the baby with 50 gallons of milk a day. Since the
nursing is done under water, and the baby must surface frequently to
breathe, the act of nursing is very brief. Muscles in the breasts of the
mother force the milk into the baby's mouth in large amounts. The baby
will double its length in 7 months, which averages to a daily weight
gain of 220 pounds. During all this time the mother must fatten for the
winter ahead, and perhaps continue to grow herself.