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Archaeocetes
Early Eocene whale species, or archaeocetes, differ from modern whales. Archaeocetes retained skeletal characteristics similar to land mammals: nostrils near the tip of the nose, specialized teeth and visible hind legs. The earliest archaeocete fossilized remains, Pakicetus, were found in the Kuldana Formation in Pakistan. Another Pakistani fossil find, Rodhocetus, had well-developed hind limbs. The two modern whale lineages diverged in the Oligocene period. Aeticotus had both baleen and toothed whale characteristics, and is considered to be the earliest fossilized remains of a baleen whale. There was a clear separation between baleen and toothed whales by the Miocene period.
Mysticetes
According to the Society for Marine Mammalogy, there are 14 species of baleen whales. Species include blue, bowhead, fin, fray, humpback, minke, right and sei whales. Blue whales are the largest mammals on earth, reaching lengths of up to 100 feet and weighing up to 160 tons. Named for the baleen plates which hang from the upper jaw, baleen whales have two blowholes. Baleen plates are sheets of strong, flexible material, similar to human fingernails, which act as a sieve, filtering out small fish and zooplankton. Baleen whales communicate by sound. Male humpbacks arrange sounds into "songs" and "sing" them to females during mating season.
Odontocetes
According to the Society for Marine Mammalogy, there are 72 species of toothed whales. Species include beaked, beluga, bottlenose, narwhal, orca, pilot and sperm whales. This group comprises up to 90 percent of all whale species, and is the most intelligent animal species. Sperm whales have the largest brain. Toothed whales have small, pointed teeth, unlike baleen whales, and one blowhole. Designed for streamlined movement through the water, toothed whales chase down and capture individual prey, which includes fish and penguins, seals and squid. Toothed whales make a clicking sound to find prey. The sound bounces off prey animals and back to the whale, a process similar to echolocation.
Threats
Whales are protected under the Marine Mammal Protection Act; endangered and threatened whales also fall under the protection of the Endangered Species Act. According to the World Wildlife Federation, seven whale species are endangered or vulnerable, including the North Atlantic right whale and the North Pacific gray whale. Among the hazards facing whales are climate change, habitat degradation and toxic contamination. The biggest threat, however, remains commercial whaling. Despite a 1986 moratorium imposed by the International Whaling Commission, more than 1,000 whales are killed annually for commercial purposes.