The hair lies flat so mammal can pass smoothly through the water. To keep warm in icy cold waters, marine mammals have large appetites, and rapid digestion provides the calories to fuel the high metabolic rates, which produce heat. Loss of heat is minimized by the thick layer of blubber or, in the case of the otter, with a very thick fur coat. Changes in the circulatory and respiratory systems have supplied marine mammals with sufficient levels of oxygen to withstand long dives. The livers and kidneys of marine mammals are super-filters that remove excess salts from the blood and prevent dehydration.
Fresh water is obtained from the fish eaten.
Nursing mothers conserve their own body fluids by producing a thick milk, rich in fats. The babies metabolize the fat to gain the nutrients they need. Marine mammals have excellent underwater vision for finding prey. Toothed cetaceans also have a sonartype system called echolocation. Teeth, baleen, cooperative hunting and feeding, and tool-using are only a few of the structural and behavioral adaptations that have allowed various marine mammals to successfully exploit the food resources of the sea.