Whale than males. Baleen whales have no teeth; instead they have plates of baleen, a fringe-like structure used to expel water while retaining the krill and plankton which they feed on. They use their throat pleats to expand the mouth to take in huge gulps of water. Balaenids have heads that can make up 40% of their body mass to take in water. Toothed whales, on the other hand, have conical teeth adapted to catching fish or squid. Baleen whales have a well developed sense of "smell", whereas toothed whales have well-developed hearing − their hearing, that is adapted for both
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