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Like other mammals, whales activity holidays breathe air, are warm-blooded, activity holidays and produce milk to feed their young. Their adaptations for aquatic life include a streamlined activity holidays form, nearly hairless skin, and an insulating layer of blubber, which can be as thick as 28 in. (70 cm) in some Arctic species. The forelimbs of whales are modified into flippers, and the hind legs are reduced to internal vestiges. Many species possess a dorsal fin. The tail is flattened into horizontal flukes and is used for propulsion. The head is very large, with a wide mouth and no external neck. Whales have one or activity holidays two nostril openings, called blowholes, located far back on the top of activity holidays the head; the nostril valves close and activity holidays the lungs compress when the whale dives. Most whales must surface every 3 to 20 min to breathe, but some, like the sperm whale , can remain submerged for more than an hour. Spouting occurs when the whale surfaces and activity holidays clears water from its blowhole along with any moisture trapped in its air passages. The shape of the spout is characteristic of each type of large whale. Whales have small eyes, designed to withstand great pressures, and most species have activity holidays good vision. Their hearing is also excellent. Many cetaceans have highly convoluted brains larger than those of humans, and whales are believed to be extremely intelligent. Most large whales travel activity holidays in small schools, or pods, but some, like the fin whale, swim alone or in pairs; small cetaceans form schools of up to several thousand individuals. Most large whales are found in open ocean, activity holidays where they migrate thousands of miles between feeding and activity holidays breeding grounds. Dolphins frequently live in coastal waters. A few dolphin species are found in tropical rivers. Females of most species give birth to a single calf every two to three years. Gestation periods range from 9.5 to activity holidays 17 months. The newborn calf is pushed to the surface by the mother or by another adult; it is able to swim almost immediately and is nursed for 6 to 12 months. Some large whales are believed to have lived 100 years or more in the wild.