Why are dolphins classified as mammals?
Mammals are warm-blooded and, most importantly, have mammary glands to produce milk for their offspring. Dolphins have a body shape similar to a fish because they have evolved to live in the same environment. To survive in the ocean as a swimmer you need a hydrodynamic body shape to reduce drag and fins to propel, stabilize and steer you through the water. Body shape isn’t particularly important from a classification standpoint. There are mammals who are adapted similarly to birds, for example (bats). Other than being vaguely similar in body shape, they really aren’t like fish in any way at all. Most importantly, however, they are mammals because they evolved from mammals. The ancient ancestors of dolphins and other whales (and yes, dolphins are toothed whales), were land mammals. Gradually they evolved into something that was similar to a modern otter (able to function well on land and in the water), then something like a seal (much less able to function on land) and then a whale (not
