Glossary of Useful TermsBipedalism: using two feet to stand and move on land (by walking, hopping, or running). Relatively few modern mammal species are bipeds.
Carnivore: an animal that eats primarily meat, whether scavenged or hunted. Most living species of the mammal order Carnivora (dogs, bears, cats, seals, and their relatives are carnivores like their common ancestor, but some have evolved to eat primarily fish, fruit, or even leaves.
Cladogram: a tree-like diagram that depicts the evolutionary history of a group of organisms. Branching points are where new, advanced features appeared, and species diverged from common ancestors.
Characteristic: any feature or trait of an organism that can be measured, counted, or otherwise assessed, such as hair color or number of limbs. Characteristics help biologists distinguish one species from another.
Echolocation: the process of emitting sound waves and listening to the echoes to navigate and locate food—or "biological sonar." Echolocation evolved independently in bats, cetaceans (dolphins and whales), shrews, and some other mammals.
Evolution: the process by which populations accumulate genetic changes over time that are passed on from ancestors to subsequent generations, or descent with modification. (See "Useful Concepts.")
Extinction: the death of every member of a biological species or all species in a bigger group of organisms. The vast majority of species that have lived on Earth are now extinct.
Fossil: Typically preserved in rocks, fossils are any remains or traces of ancient life. Examples include bones, teeth, shells, leaf impressions, nests, and footprints. Fossils document how organisms changed over time, and how they're related to one another.
Herbivore: an animal that eats plants, like horses and mice. In the food web, herbivores link primary producers (plants) and consumers such as predators (carnivores).
Keratin: a fibrous protein that is the main component of structures that grow from the skin, such as hair, hooves, nails, claws, and horns.
Mammal: a class of vertebrate animals descended from the common ancestor of living placentals, marsupials, and monotremes. Almost all mammals share certain physical characteristics: they have hair, they're warm-blooded, and they produce milk to nurse their young.
Marsupial: a group of a few hundred living (and many more fossil) mammal species named for a distinctive pouch (called the marsupium) in which many species carry their young. Born tiny and very undeveloped, the young feed on their mother's milk for an extended period of time, often in a pouch.
Monotreme: a group of living mammals that lay eggs instead of giving birth to live young, as the vast majority of living mammals do.
Placental: Placental mammals bear live young, which are nourished before birth through a specialized organ (a placenta) that attaches to the mother's uterus as an embryo grows. Although most living placental species are rodents and bats, this diverse group also includes whales, elephants, shrews, armadillos, and humans, among many other species.
Prehensile: adapted for grasping or holding. This adaptation confers many advantages, in feeding (the giraffe's tongue), locomotion (the monkey's tail), drinking and defense (the elephant's trunk), and other activities.
Quadrupedalism: using four limbs or legs to move. Most land mammals are quadrupeds.
Species: a basic unit of biological classification. A species is often defined as a group of organisms that share ancestry and characteristics, and can interbreed and produce fertile offspring.