What is a mammal? To understand this, let’s examine how whales, dolphins and porpoises (together known as cetaceans) are different to fish.

FishGills; CAPTION: Fish breathe with gills
© Arria Belli/Wikimedia Commons

For a start, dolphins and whales can’t breathe underwater.
They have to breathe air, which they do through their blowholes. Fish can breathe underwater. They take oxygen dissolved in the water using their gills.

However… birds and reptiles also breathe air, and they are not mammals. So what other traits do all mammals share?
Unlike fish and reptiles, all mammals, including whales and dolphins, are warm-blooded; they keep a steady warm body temperature. But, this is something that birds do too.  
    

 

Reptiles do not keep a warm body temperature. In this picture you can see that the human holding the snake is warm, but the snake is cold in comparison. Lizards and snakes often need to lie out in the sun before they become warm enough to move around
© Arno/Coen (www.nutscode.com)/Wikimedia Commons

Birds lay eggs, which no dolphin or whale does. Like most other mammals, cetaceans give birth to live young instead. There are, however, five mammal species that do lay eggs.  

                                                                           

    

Platypus
© John Gould [Public domain]/Wikimedia Commons

Hair is also a good way of characterizing mammals, since no other species have true hair (the “hair” of insects like bees and some spiders are actually sensory bristles which look a lot like hair). But cetaceans seem to be hairless, and many other mammals have also lost some or most of their fur.
Can you name a mammal with little or no hair? For example: elephants, rhinos, hippos, humans…

Skin of a hippopotamus
© BS Thurner Hof /Wikimedia Commons

Apart from breathing air, keeping a steady body temperature, giving birth to live young or having hair (in most cases), what is the main characteristic that defines mammals? The answer is that all female mammals produce milk to feed their young—no other group of animals does this. Breastfeeding, together with the other features we have mentioned, is what differentiates mammals from other groups of animals.