Cetaceans

Most baleen whales first mate when they are 4-10 years of age. Many toothed whales take longer to mature, and in sexually dimorphic species, males take longer still. Sperm whales require 7-12 years, killer whales 8-10, and false killer whales (Pseudorca crassidens) need up to 14 years to mature.

Gestation in mysticetes lasts 10-13 months on average. Many odontocetes have similar gestation times but some are longer: pilot, sperm, and killer whale pregnancies last up to 16 months. The birth of a baby has rarely been witnessed by humans—typically, a formerly pregnant female simply appears one day with her new infant at her side. On rare occasions, however, a human observer has been lucky enough to see a birth. One laboring gray whale spent the last 10 minutes of her labor hanging vertical in the sea, head down, with her flukes held 6 ft (1.8 m) out of the water. As she lowered her flukes to a horizontal position, the calf's snout was seen to be emerging from her belly. The mother shifted to a belly-up position, just at the surface, as the calf continued emerging; then the mother submerged, and the calf popped up to the surface, separate from its mother. Bottlenose dolphins are thought to usually deliver their calves tail-first, but head-first deliveries have been observed in captivity. In a captive situation, the dolphin mother typically uses her tail and rostrum (beak) to guide the baby to the surface for its first breath of air.

In all species, the baby soon takes up the "infant" position below and to her side, near her mammary slits. The nipple of a nursing mother protrudes from this slit on her belly, and the milk is ejected into the baby's mouth by her mammary muscles with no effort on the part of the calf. Nursing bouts are relatively brief, since the calf must surface to breathe. Even so, the baby gains weight steadily; blue whale infants gain 200 lb (90.8 kg) per day! Blue and gray whales nurse for a period of about seven months; bottlenose dolphins nurse for three to four years or more. The record must be for pilot and sperm whales, who are occasionally observed to be nursing at 10-15 years of age. Of course, these youngsters have been eating other foods as well.