The more recently mammals have shared a common ancestor, the more closely related they are considered to be. However, for the purpose of identification, it is necessary to find some visible common features.

Since the evolution of mammalian morphology is so closely associated with dietary preference, one identification method uses characteristics related to feeding adaptations. Although this works well for many mammals, there are cases when it fails. Contrary to what their names imply, the orders Insectivora and Carnivora include animals that are not strict insect-eaters, or meat-eaters. For example, the greater panda is a strict vegetarian, eating only bamboo.

There is no order Herbivora. Since vegetation is the most plentiful food source for terrestrial mammals, herbivory has evolved within the majority of the world's 24 orders.