There are several characteristics that are indicative of mammals, but only two are definitive. It is easier to understand things if you know that there are three types of mammal. Almost all mammals are eutherian, or true mammals. A much smaller group are the marsupials; all but one species of these is found only in Australia and its islands. The remaining group, the monotremes, is limited to that same area.
The relevant characteristics are : -
- Three tiny bones in the ear are absolutely definitive; all mammals have them and no other animals do. The bones are hammer, anvil and stirrup.
- Mammals feed their neonates by milk supplied through the female's skin; this also is definitive. Eutheria have nipples on which the newborns suck. Marsupials and monotremes have a skin patch which the young lick.
- Mammals are hairy. All mammals have hair, although some have only traces. Hair is not definitive - many insects have it. (Look at a honey-bee through a magnifying glass!)
- Almost all mammals give birth to live young, but the three species of monotremes lay eggs. When the egg hatches it is still fed on mother's milk.
