1. Mammals are endothermic vertebrates.
2. They have hair, which varies greatly among species.
3. Most have sudoriferus (sweat) glands.
4. They have mammary (milk-secreting) glands.
5. They have sebaceous (fat-secreting) glands.
6. They have heterodont dentition (different types of teeth).
Cazadero has a lot of mammals, but they tend to live elsewhere and only move through the Redwood Forest. There is not much in the way of natural food in the redwoods, but camp provides a lot of human food, so raccoons and mice are frequent guests. The opossum and deer might wander along Austin Creek.
Deer, mice, opossums, raccoons, and humans are mammals because they are warm-blooded, have hair, and nourish their young on milk.
Bat
Bat is a member of the class Mammalia. Despite the fact that it has wings, you can tell it is not a bird because it does not have feathers or lay eggs. It must be a mammal because it has fur (fish have scales, amphibians have moist skins with fur, reptiles have scales). This also tells you that it bears live young that it feeds with milk and that it is warm-blooded.
A duck-billed platypus.
A duck-billed platypus. If you were mystified at first, you are in good company: the scientists were mystified, too. However, this is how they solved the puzzle: the animal has fur and feeds its young on milk. This makes it a mammal, but a rather unusual one. It forced the creation of a subset of mammals called monotremes.
It could not be a fish, amphibian, reptile, or bird, because it has fur. It has a backbone, so it could only be a mammal. Other characteristics of anatomy, such as arms and legs, pelvis, kidney structure, heart chambers, and other features are also shared only with mammals.
