Baleen whales known earlier as whalebone whales

Baleen whales (systematic name Mysticeti), known earlier as whalebone whales, form a parvorder of the infraorder Cetacea, including sperm whales, beaked whales, dolphins, and porpoises. Baleens are a widely distributed and diverse parvorder of carnivorous marine mammals. Mysticeti comprise the families Balaenidae (right whales), Balaenopteridae (rorquals), Cetotheriidae (the pygmy right whale), and Eschrichtiidae (the gray whale). There are currently 15 species of baleen whale. While cetaceans were historically thought to have descended from mesonychids, molecular evidence supports them as relatives of even-toed ungulates (Artiodactyla). Baleen whales split from toothed whales (Odontoceti) around 34 million years ago.

Baleen whales range in size from the 20 ft (6 m) and 6,600 lb (3,000 kg) pygmy right whale to the 112 ft (34 m) and 190 t (210 short tons) blue whale, which is also the largest creature on earth. They are sexually dimorphic. Baleen whales can have streamlined or large bodies, depending on the feeding behavior, and two limbs that are modified into flippers. Though not as flexible and agile as seals, baleen whales can swim very fast, with the fastest able to travel at 23 miles per hour (37 km/h). Baleen whales use their baleen plates to filter out food from the water by either lunge-feeding or gulp-feeding. Baleen whales have fused neck vertebrae, and are unable to turn their head at all. Baleen whales have two blowholes. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water.

Although baleen whales are widespread, most species prefer the colder waters around the Northern and Southern poles. Gray whales are specialized for feeding on bottom-dwelling mollusks. Rorquals are specialized at lunge-feeding, and have a streamlined body to reduce drag while accelerating. Right whales gulp-feed, meaning they use their enlarged head to effectively take in a large amount of water and sieve the slow-moving prey. Males typically mate with more than one female (polygyny), although the degree of polygyny varies with the species. Male strategies for reproductive success vary between performing ritual displays (whale song) or lek mating. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them. Mothers fast for a relatively long period of time over the period of migration, which varies between species. Baleen whales produce a number of vocalizations, notably the songs of the humpback whale.

The meat, blubber, baleen, and oil of baleen whales have traditionally been used by the indigenous peoples of the Arctic. Once relentlessly hunted by commercial industries for these products, cetaceans are now protected by international law. However, the North Atlantic right whale is ranked critically endangered by the International Union for Conservation of Nature. Besides hunting, baleen whales also face threats from marine pollution, ocean acidification, and naval sonar, which results in strandings. They have rarely been kept in captivity, and this has only been attempted with juveniles or members of one of the smallest species.