Toothed whales that echolocate, socialize, and span river to sea
Forty extant dolphin species belong to oceanic, river, and brackish families within toothed whale parvorder Odontoceti. Sizes run from 1.7-meter Maui's dolphin to ten-tonne orca apex predators. Streamlined bodies, pod societies, and click-whistle echolocation define the group.
Dolphins sit among the toothed whales, a branch they share with porpoises, belugas, narwhals, sperm whales and beaked whales. The oceanic family is by far the largest, joined by river dolphins of South Asia and South America, the La Plata dolphin of brackish estuaries and the Yangtze's baiji, which is probably extinct. Six members, including the orca, the false killer whale and two pilot whales, are popularly called whales and grouped as blackfish, while in casual speech dolphin often just means the bottlenose. The name goes back to Greek delphis, tied to delphus, womb, hinting at a fish that carries its young inside.
Their ancestors were even-toed hoofed mammals; the line parted from Indohyus, a small extinct ungulate, about 48 million years ago. Early whales entered the sea around 49 million years ago and were fully aquatic within another 5 to 10 million years. Ambulocetus already channelled sound from jaw to ear about 49 million years ago, and Protocetus had a streamlined body and tail flukes by around 43 million years ago. Hippos are their closest living kin, the two groups sharing a semi-aquatic forebear from some 60 million years ago.
The body is a torpedo with a stiff neck, flippers for forelimbs, only a vestigial pelvis where hind legs once were, and a horizontal tail fluke. The skull has small eye sockets and no external ear flaps, and the teeth are conical, unlike the spade-shaped teeth of porpoises. Some species can briefly hit 29 kilometres per hour or leap about 9 metres. Blubber chiefly insulates but also aids buoyancy and stores energy, and the skin is smooth and hairless apart from a row of hairs along a newborn's snout.
Most species favour warm tropical seas, though the right whale dolphin prefers cold water. Fish and squid dominate the menu, while orcas also take seals, sharks, bluefin tuna and even seabirds. Pods split and merge constantly, and members click and whistle, relying heavily on echolocation. Males mate with several females each year, but females breed only every two to three years and raise calves alone. Threats include drive hunts in Japan and parts of Latin America, bycatch, ghost nets and pollution, and around 60 orcas live in captivity.
Source: Dolphin
