Question: hi my names jemima. what are mammals? why are dolphins counted as mammal but some fishes arent

Jason King answered on 11 Mar 2013:

Hi Jemima, mammals have many things in common that make them different from other animals like fish, or reptiles. Basically all mammals feed their young with milk (made by their mammary glands i.e. breasts) . Even dolphins breast-feed their young, which seems weird underwater. Unlike fish, they also need to breathe air so have to come up to the surface to breath, and don’t have gills. Although they live in the sea, dolphins have much more in common with you and me, than fish.