Digestive system of humpback whales stretching like an accordion. They allow whales to open their mandible to 90 degrees to gulp water and prey. They expel saltwater through the baleen plates, leaving behind krill and plankton. The whale's tongue plays a vital role in filter feeding, as it is used to push the water out of its mouth while keeping the prey trapped in its baleen bristles. As humpback whales do not have teeth, their food is swallowed whole and travels down the esophagus into the preliminary stomach (known as the forestomach) to be ground. Baleen whales have a multi-chambered stomach, divided into three distinct
Baleen whale selenensis" is the earliest mysticete, dating back to 37 to 33 million years ago (mya) in the Late Eocene, and, like other early toothed mysticetes, or "archaeomysticetes", "M. selenensis" had heterodont dentition used for suction feeding. Archaeomysticetes from the Oligocene are the Mammalodontidae ("Mammalodon" and "Janjucetus") from Australia. They were small with shortened rostra, and a primitive dental formula (). In baleen whales, it is thought that enlarged mouths adapted for suction feeding evolved before specializations for bulk filter feeding. In the toothed Oligocene mammalodontid "Janjucetus", the symphysis is short and the mouth enlarged, the rostrum is wide, and the