A mammal is a warm blooded animal that has several features that set it apart from other types of animal. It has fur covering its skin. This can be thick and heavy, as in the case of a lion or a pet schnauzer, or it can be so fine it is almost invisible. The human body has thick hair on the head and males have more body hair than women but many people have very fine hair on the arms and legs.
Another feature of a mammal that is of great importance, and is where mammals get their name, is the possession of mammary glands. Female mammals have glands that produce milk. Milk is expressed when a young of the species is born and the baby mammal depends on this milk supply as its only source of food for the first stage of its life.
Mammals also give birth to live young; they do not lay eggs like birds. Mammals mate and use internal fertilisation to start off embryos in the mother's body. The uterus, a specialised organ in the female, protects the growing embryo and the placenta, a rich blood supply, nourishes it from the blood system of the mother. The embryo is protected in its mother's body until it is at a fairly advanced stage of development.
Those characteristics are hair and milk. All mammals are nursed by a mother with mammary (milk) glands. All mammals have hair, as well, at some stage of their life. A few only have hair while they are in the womb or as young, but most keep their hair after they are born and through out their lives, yet they all have it at some point. Dolphins and whales are born with some hair in the area of their lips, but I think it is sparse.
This is a picture of a Fin whale calf, showing remnant hair folicles on his rostrum (beak/mouth).
Although it doesn't show up well enough to see it in this picture, the author of the article from which this picture was taken, said there were a few hairs sticking out of the folicles.
Animals can be characterized in terms of 1. Appearance, 2. Fundamental Material Constituent, 3 Biological Condition, 4 Size, 5. Existential Condition, and (Bonus!) 6. Social Condition. Thus we find that: 1. Unlike vegetables, Animals are cute in the infant stage. 2. All animals are made up of tiny vibrating strings 3. Every animal can be dead and alive at the same time, i.e., the Schroedinger Cat Experiment is generalizable; 4. All animals travelling at relativistic speeds appear very thin. 5. Animals live short, meaningless lives. 6. The Bonus: All animals are equal, especially the Pig.
