Baleen whales (Mysticeti) are large filter-feeding marine mammals that are named for the specialized filter-like structures in their mouth (these structures are called baleen plates). Baleen plates are fibrous, flexible strips that hang down from the whale's upper jaw. On the outer side, the baleen plates are smooth, but towards the inside, the plates are fringed and tangled in such a way that they form a highly effective filter. Whales flush water through the baleen and in doing so are able to trap tiny prey such as krill in the baleen. Baleen whales feed low on the food chain and this means that some of the largest animals on the planet grow to such proportions by feeding on some of the smallest. The diet of baleen whales consists of primarily zooplankton (such as copepods and krill) as well as some small fish.
Baleen whales are one of two main groups of cetaceans, the other being the toothed whales. In general, baleen whales are larger than toothed whales (in fact, some of the largest animals alive today—the blue whale and the fin whale—are baleen whales).
Baleen whales have developed a number of characteristics for filter-feeding including baleen plates, the loss of functional teeth (some vestigal teeth remain but are lost before birth), large body size, large head, short neck and a large upper jawbones (to accomodate the attachment of the baleen plates).
Not all baleen whales filter-feed in the same manner. The rorquals feed by gulping-in large quantities of food, the right whales feed by skimming food from the water and the gray whales filter water and sediment from the sea floor.
Baleen whales have a double blowhole located at the top of their head. In most species, their blow spout is visible and is often unique enough between the species that it can be used to identify them. Other characteristics vary amoung the baleen whale subgroups. The right whales lack a dorsal fin. They have large heads (as long as one third the entire body length of the animal). Rorquals have a dorsal fin and prominant groves on their throat which enable them to expand their mouth and take in large volumes of water (and prey). Grey whales today are only represented by one living species (the grey whale). Grey whales have shorter baleen than other baleen whales and lack a dorsal fin (instead, they have a half-dozen or more raised bumps on their back).
Numerous baleen whales are classified as endangered, some of which include blue whales, North Atlantic right whales, North Pacific right whales, fin whales and sei whales. Whales have been hunted by humans for their meat and oil for thousands of years. But whereas traditional hunting methods harvested whales in a sustanable manner, modern hunting methods and commercial whaling expeditions resulted in the devastation of whale populations throught the world's oceans. Such pressures have driven many species to the brink of extinction.