Mammals are endothermic, air-breathing, vertebrate tetrapods, with hair and 3 middle ear bones. Female mammals have mammary glands to nurse their young.
4 limbs are ubiquitous among terrestrial mammals. Marine mammals (e.g., dolphins, dugongs) descended from tetrapods, but lost their hind limbs. Most land mammals ambulate on all 4, though there are several exceptions. Habitual bipedalism independently evolved numerous times in mammals: macropods (e.g., kangaroos), kangaroo mice, dipodids (e.g., jerboas, jumping mice), hopping mice, pangolins, and hominids.
There are 29 mammalian orders, 153 families, 1,229 genera, and 5,702 known species. Mammals range in size from the 35-mm, 2-gram bumblebee bat to the 30-meter, 180-tonne blue whale.
Mammals were classified early on by their dietary habits as well as appearance. The inner logic to this is that a mammal’s nutritional needs shape its body.
Teeth and feet are exemplary. Carnivores have sharp blades for teeth, and claws for catching prey.
Herbivores have molars for grinding plant matter, and limbs designed for foraging treks. Many have especial digestive systems to handle large volumes of low-nutrition food.
Rodents have pronounced incisors that gnaw well through tough materials, such as nut shells. Many have front paws which allow them to grasp their food.