Dolphins are warm-blooded, not cold-blooded. This is because dolphins are classified as marine mammals, rather than fish. Dolphins maintain consistent body temperature just like other warm-blooded animals.
Dolphins, whales and porpoises are all members of the cetacean family which are warm-blooded marine mammals. In order to stay warm in the cold ocean, cetaceans like dolphins grow blubber in a thick layer of fat inside their bodies. This layer insulates dolphins and allows them to keep their body temperatures up. Dolphins can also increase their internal metabolic rate to help stay warm. This allows dolphins to quickly transfer heat to their organs in colder waters by burning extra calories. If the water is particularly cold, dolphins can shiver just like other land mammals. Dolphins tend to have less blubber than other marine mammals like whales. As a result, they often stay in warmer climates and swim in warmer waters such as those near the equator. Dolphins also tend to spend more time closer to the surface of oceans for this reason as well because the water is warmer nearer the surface where sunlight has more of a warming effect. The small data known about dolphins shows that many don’t go below 150 feet.