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Dolphins, Orcas, and Other Small Cetaceans

There are about 30 species of marine dolphins—all belonging to the family Delphinidae—including the common dolphin, the spinner dolphin, Risso's dolphin, and the orca (also known as the killer whale). Orca? Yes, the orca and other small whales, such as the pilot whale and the melon-headed whale, are actually the largest members of the Delphinidae family.

Delphinids are found in all oceans of the world. Some species are found exclusively in tropical waters, others in temperate regions, and a rare few in sub-polar waters. Some species are found exclusively in one small area of the ocean, such as the endangered Hector's dolphin, which is found only in the waters around New Zealand. Some live in coastal habitats, while others inhabit the open ocean. Still others are considered "cosmopolitan" in distribution, meaning they may be found virtually anywhere. Orcas fall into this category; they can be seen in Arctic and Antarctic waters, as well as in the tropics and temperate zones, near the shore and in the open sea.

The delphinids are distinguished from the six porpoise species, or Phocoenidae, primarily by the shape of their teeth, which are conical. Phocoenids have flattened, spade-shaped teeth and tend to be smaller, less social, and harder to spot than delphinids. The five species of river dolphins are distinguished by their primitive eyes, prominent snouts, flexible necks, and the fact that they are found in fresh water and estuaries.

There are only two species in the small whale family of Monodontidae: the beluga, or white, whale and the narwhal (the unicorn whale), both of whom are found exclusively in Arctic waters.

The 20 or so species of beaked whales, or Ziphiidae, are the most curious of the small cetaceans. Most people have never heard of them, and even scientists know little about them. They range in size from 10 to 40 feet. The males have two to four teeth, which usually emerge only in the lower jaw (in some species of beaked whales, the teeth actually wrap around the upper jaw, preventing the whale from being able to open its mouth fully). Female beaked whales tend to have no teeth. Beaked whales live in deep, usually open, ocean water and spend little time at the surface. Some species of beaked whale have never been observed alive and are known because their carcasses have been found.

Together, the delphinids, phocoenids, monodontids, and ziphiids make up what scientists commonly refer to as small cetaceans. Small cetaceans usually give birth to only one calf at a time (with gestation lasting approximately a year), have long intervals between births, and nurse and care for their young for several years. Socially, the group varies: Some species, such as the dolphins, are tremendously social, forming large, inter-related, cohesive groups, while others appear relatively solitary. Their longevity also varies. While some species live an average of 20 years, others live as long as humans do (and the longevity of some species is entirely unknown).

One thing that remains constant throughout the group is hunting. All are efficient predators, even the odd beaked whales (males of these species may suck their prey into their muzzled mouths with a vacuum-like action). Some small cetacean species are threatened or endangered (the Baiji River dolphin of China may be near extinction) or simply rare, while others number in the millions. But all are vulnerable to habitat destruction, chemical and noise pollution, plastic and other debris, and in some places are hunted for meat and other products.