The class Mammalia is distinguished from other animal classes in a number of ways. Perhaps most notably, nearly all mammals give birth to live young as opposed to laying eggs.  (There are only two exceptions: the platypus and the echidna—more commonly known as an anteater.)  In addition, all mammals nurse their young, providing milk for as short a time as a few days or for as long as few months or years.

Another interesting characteristic is that all mammals have hair or fur. This includes dolphins, whales, and other cetaceans which, for a short period of time after birth, all sport little whiskers along their snouts. Hair or fur can serve a number of important purposes—from insulation to camouflage to tactile sensing.

Finally, all mammals are hot-blooded or “endothermic,” which means they produce body heat internally.  Endothermic heat enables mammals to move faster, avoid predators, and live in cooler climates than if they were to draw their heat primarily from the sun.

Many mammals build structures to improve their surroundings—burrows, dams, schools, and more. These creations can provide protection and food storage space, improve living conditions, and serve as a place to hibernate or raise young.

In preparation for food shortages, it is common for mammals (including humans) to store food, rather than hunt. Rodents, for instance, hide roots, mushrooms, seeds, and other items underground or in trees, and then depend on the food caches in winter months. Carnivorous mammals have a harder time storing food because meat perishes quickly (though some do stash partially eaten prey).

Originally descended from reptilian ancestors, mammals have evolved into many forms over the past 200 million years. Bats can fly at night, aided by sonar. Enormous whales can dive underwater for extended periods of time. Naked mole rats can live almost exclusively underground.

Driven by natural selection, different species of mammals have evolved specialized features tailored to their environments. When alterations occur within a species, they are called adaptations. For example, a pocket gopher’s coloring helps it blend into its surroundings. Gophers living near dark soil are much darker than those in areas with light-colored soil (a decided advantage when hiding from predators). Through adaptations like these, creatures evolve to find their own safe niches within different ecosystems.