It was heavily hunted in the past. The rest of the larger whales belong to the suborder Mysticeti, the baleen whales. The plates number 160 to 360 on each side, are frayed into bristles on their inner edges, and are used to capture the plankton or krill on which the animals subsist. When feeding, a baleen whale swims with its mouth open in order to engulf plankton and seawater by the ton. Then, shutting its cavernous mouth and pressing its tongue against the back of the baleen bristles, the whale forces the water out of its mouth, trapping the plankton on a mat of overlapping baleen.
Baleen whales tend to spend the summer in polar seas, where plankton blooms provide abundant food.
After months of heavy feeding they migrate to temperate or tropical zones, often fasting there over the winter. The dramatic streamlining of whales in the course of their evolution resulted in an animal that appears remarkably fishlike. Thus, the front limbs became modified as paddle-shaped flippers, the bones of which are still reminiscent of jointed limbs and digits, but the hind limbs were lost. They contain no bone, and owe their firm and yet flexible shape to underlying fibrous and elastic tissue. The body is enveloped in a thick layer of blubber that aids in buoyancy, helps to preserve body heat, and is a source of stored energy.