Baleen is a filter-feeder system inside the mouths of baleen whales. The system works when a whale opens its mouth underwater and then water pours into the whale’s mouth. The whale then pushes the water out, and food such as krill are filtered through. Baleen is similar to bristles and is made of keratin, the same substance found in human fingernails and hair. Baleen whales include: blue whale, bowhead whale, humpback whale, and pygmy right whale.