Dolphins are marine mammals. There are almost forty species of dolphin. They vary in size from 1.2 m (4 ft) and 40 kg (90 lb) (Maui's Dolphin), up to 9.5 m (30 ft) and 10 tonnes (9.8 LT; 11 ST) (the Orca or Killer Whale). They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating fish and squid. The family Delphinidae is the largest in the Cetacea, and relatively recent: dolphins evolved about ten million years ago, during the Miocene. Dolphins are considered to be amongst the most intelligent of animals and their often friendly appearance and seemingly playful attitude have made them popular in human culture.
Dolphins are an increasingly popular choice of animal-assisted therapy for psychological problems and developmental disabilities. For example, a 2005 study with 30 participants found it was an effective treatment for mild to moderate depression.However, this study was criticized on several grounds; for example, it is not known whether dolphins are more effective than common pets. Reviews of this and other published dolphin-assisted therapy (DAT) studies have found important methodological flaws and have concluded that there is no compelling scientific evidence that DAT is a legitimate therapy or that it affords any more than fleeting improvements in mood.
A number of militaries have employed dolphins for various purposes from finding mines to rescuing lost or trapped humans. Such military dolphins, however, drew scrutiny during the Vietnam War when rumors circulated that dolphins were being trained by the United States Navy to kill Vietnamese divers. However, no evidence to support these rumors ever surfaced, and the United States Navy denies that at any point Dolphins were trained to do harm. Dolphins are still being trained by the United States Navy as part of the U.S. Navy Marine Mammal Program. The Russian military is believed to have closed its marine mammal program in the early 1990s. In 2000 the press reported that dolphins trained to kill by the Soviet Navy had been sold to Iran.