Question

Why are dolphins and whales called mammals even they swim like fishes​

Answer 1

They are not fish, they are cetaceans! Unlike fish, they don't have gills, they are viviparous, which means they don't lay eggs and they produce milk for their babies. Also, they are warm-blooded.

Answer 2

Both Whales and Dolphins are classified as mammals since they produce milk to feed to their young. Unlike other creatures in the sea, they also have warm blood, not cold.