Dolphins are highly intelligent and can be found all over the world in oceans, rivers and estuaries. As mammals, dolphins don"t have gills so they can"t breathe underwater. They must surface to inhale and exhale stale air, which they do through blowholes and not their mouths. How long they can hold their breath depends on the species; some can go for 3 to 7 minutes, while others can stay submerged for up to 20 minutes. Because they must always surface, half of dolphins" brains are always active, meaning they never completely fall asleep. Despite being able to exist in an array of environments and water temperatures, dolphins are threatened by a variety of human activities, such as hunting, pollution and fishing.