Baleen Baleen Baleen is a filter-feeder system inside the mouths of baleen whales. The baleen system works by a whale opening its mouth underwater and taking in water. The whale then pushes the water out, and animals such as krill are filtered by the baleen and remain as food source for the whale. Baleen is similar to bristles and consists of keratin, the same substance found in human fingernails and hair. Baleen is a skin derivative. Some whales, such as the bowhead whale, have longer baleen than others. Other whales, such as the gray whale, only use one side of their
Baleen whale mysticete, yielded a total length of , indicating filter feeding was not a driving feature in mysticete evolution. The discovery of "Janjucetus" and others like it suggests that baleen evolution went through several transitional phases. Species like "Mammalodon colliveri" had little to no baleen, while later species like "Aetiocetus weltoni" had both baleen and teeth, suggesting they had limited filter feeding capabilities; later genera like "Cetotherium" had no teeth in their mouth, meaning they were fully dependent on baleen and could only filter feed. However, the 2018 discovery of the toothless "Maiabalaena" indicates some lineages evolved toothlessness before baleen. "Mystacodon