Are Penguins Mammals
Are penguins mammals? No. Penguins are birds and birds are not mammals. They are animals of course, but mammals in general do not lay eggs, they nurse their young, and they have fur, not feathers. The duck-billed platypus, which seems to be stuck somewhere in middle, lays eggs, but has fur and does nurse its young.
It could be argued than mammals lay eggs as well, but the eggs "hatch", or rather metamorphose before live birth. Except for the platypus, female mammals could be said to produce eggs, but not lay them.
Watching penguins move about makes the question "are penguins mammals?" almost reasonable. From a distance it would appear they have fur, they don't fly, and walk or march from place to place almost like humans would. Observe them in the water and you could almost be tempted to ask "are penguins fish?", as they are as graceful in the water as most any fish, and faster than most as well.
Built For The Cold - One thing the Antarctic penguin does share with mammals living in the region, primarily seals, is the way they are constructed to deal with the bitter cold. They have the necessary size and body shape to keep heat loss to a minimum. In fact the closer to the pole one gets, the larger the penguins are, although they don't venture all that far inland. Most birds would be simply too small to survive an Antarctic winter and would not have the dense feathers and stores of fat needed. Smaller birds would have to keep moving about and very active to generate body heat, whereas the penguin is build such that it can remain relatively motionless in the severest of winter storms, relying on its internal resources plus the warmth gained by huddling together with hundreds and even thousands of other penguins to survive.
The penguin does have a layer of down under its external feather, which has somewhat the appearance of fur but it's fine feathers, not fur. The external feathers are rather short and overlapping, and while not in themselves providing warmth, they do function as a windbreaker when on land, keeping body heat from radiating away. In the water, the outer feathers serve as a waterproof covering, again helping to keep heat from radiating into the water and at the same allowing the bird to emerge from the water perfectly dry.
They Act More Like Mammals Than Birds In Some Respects - Are penguins mammals? You might be tempted to say yes if you've seen some of the smaller penguins, coming from more temperate zones, in the zoo. They walk around, study their surroundings, including the people watching them, and really seem more mammal-like that bird-like. Besides the beak, the dead giveaway is when they flap their wings. You don't see paws or fingers, just stubs. The penguin's wings aren't completely useless. They extend them to lose body heat and cool down when it gets too warm. Another mammal-like characteristic of the penguin is that it will pant like a dog if it gets overheated. Whether their tongue actually hangs out of their beak is doubtful, but by panting they can eliminate a good deal of excess body heat. The penguin also sheds excess body heat by blushing, just as we do. Blood is moved towards the skin where it can cool down, and the feathers are ruffled to help the process along. The feathers are also ruffled at times to keep warm, which just goes to show you how efficiently the penguin's heat exchange system works.
Are penguins mammals? No, but it's an amazing bird, and in many ways every bit as interesting as most mammals.