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Baleens or baleen plates are triangular sheets of keratin that make up a filter-feeding system inside the mouth of baleen whales. The feeding process starts as the animal opens its mouth to take in water. The whale then pushes the water out through a rack of baleen plates, so as to retain (filter) what will serve as food for the whale. A baleen is similar to a bristle and consists of keratin, the same substance found in human fingernails, skin and hair. Some whales, such as the bowhead whale, have baleen of differing lengths. Other whales, such as the gray whale, only use one side of their baleen. These baleen bristles are arranged in plates across the upper jaw of whales.