Baleen is a filter-feeding system inside the mouths of baleen whales. The closest living relatives to baleen whales are toothed whales both from the infraorder Cetacea. Killer whales have been known to help whalers hunt baleen whales. Whalebone was baleen plates from the mouths of the baleen whales. Whaling is the practice of hunting whales, mainly baleen and sperm whales. The sei whale looks similar to other large baleen whales. Krill are also the main prey of baleen whales, including the blue whale. The minke whale , or lesser rorqual, is a species complex of baleen whale. The baleen of baleen whales are keratinous plates. The humpback has the most diverse hunting repertoire of all baleen whales. Some whales, such as the bowhead whale, have longer baleen than others. The baleen of baleen whales consists of long, fibrous strands of keratin. The humpback whale ("Megaptera novaeangliae") is a species of baleen whale. The bowhead whale has been hunted for blubber, meat, oil, bones, and baleen. Mysticetes are also known as baleen whales.