All non-mammals are hairless and lack mammary glands. Non-mammals with jawbones have lower jaws composed of multiple bones; mammals have a lower jaw composed of a single bone. A non-mammal with teeth loses and replaces teeth multiple times during its lifespan, while a mammal goes through this process only once.
The lack of mammary glands means that non-mammals cannot feed their young with milk. Non-mammal species that care for their young, such as birds, must utilize outside sources of food instead. The inability to produce milk and lack of hair are the only two traits shared across every single species of non-mammal animal.
Because of the wide variety of complexity among non-mammals, other traits are not applicable to every animal. Non-mammal animals range from the simplest sponges and single-celled animals to complex animals that share many traits with mammals, such as birds.
Traits shared by non-mammals are largely defined by the shared traits of mammals possessed by no non-mammal species, rather than specific qualities shared by all non-mammals. Examples of these traits are jawbone and tooth-related traits; not all non-mammals have jawbones or teeth. While these traits are shared across all non-mammals they are applicable to, they do not apply to every non-mammal.