Mammals (Mammalia) are a group of vertebrates that have hair, a four-chambered heart, a diaphram, and mammary glands. Mammals are classified into about 21 groups, some of which include bats, carnivores, cetaceans, elephants, hoofed mammals, primates, and rodents. There are about 5,400 species of mammals alive today.
Mammals are remarkably diverse in their size, appearance, form, and adaptations. They inhabit every continent and have colonized a wide variety of niches such as grasslands, wetlands, scrublands, seas, oceans, below-ground, forests, mountaintops, polar regions, and deserts.
Mammals first appeared about 200 million years ago during the Jurassic Period. The earliest of mammals were small, shrew-like creatures that hunted for insects under the cover of darkness. For 130 million years, mammals remained small and survived at the margins of a world ruled by reptiles, most notably, the dinosaurs. Then, about 65 million years ago, a cataclysmic shift took place. The climate changed and the dinosaurs died-out, as did nearly two-thirds of all animal life. This mass extinction left vast habitats available for colonization. Mammals—who survived the climate shift partly because of their ability to regulate their own body temperature—multiplied, diversified, and expanded their range.
Mammals vary in size from the tiny bumblebee bat, which measures a mere three centimeters in length, to the magnificent blue whale, which can measure 108 feet from head to tail—at such immense dimension, it is without doubt, the largest animal alive today.
The key characteristics of mammals include:
- four-chambered heart
- mammary glands
- lower jaw made of a single bone
- three middle ear bones
Mammals are classified within the following taxonomic hierarchy:
Mammals are divided into the following taxonomic groups:
- Aardvark (Tubulidentata) - There is one species of aardvark alive today. Aardvarks have a bulky body, an arched back, long ears, and a long snout. Aardvarks are solitary, nocturnal mammals that feed primarily on termites and ants.
- Armadillos, sloths, and anteaters (Xenarthra) - There are about 29 species of armadillos, sloths, and anteaters alive today. Members of this group are known for the unique joints they have in their backbone. Armadillos, sloths, and anteaters have few or no teeth and a small brain.
- Bats (Chiroptera) - There are 997 species of bats alive today. Members of this group include megabats and microbats. Bats are the only group of mammals that have wings and are capable of true flight.
- Carnivores (Carnivora) - There are about 250 species of carnivores alive today. Members of this group include dogs, wolves, foxes, cats, hyenas, aardwolves, mustleids, mongooses, racoons, bears, and many others. Carnivores are skilled hunters with many adaptations that make them fierce predators including sharp teeth and claws, a keen sense of smell, and superb eyesight.
- Cetaceans (Cetacea) - There are about 88 species of cetaceans alive today. Members of this group include baleen whales and toothed whales. Cetaceans are marine mammals that are well-adapted for living in water. The largest cetacean, the blue whale, is the largest living animal.
- Colugos (Dermoptera) - There are two species of colugos alive today. Members of this group are arboreal mammals that live in the forest of Southeast Asia. Colugos are skilled gliders—they have a flap of skin that extends between their limbs and body that enables them to glide through the air.
- Elephant shrews (Macroscelidea) - There are about 12 species of elephant shrews alive today. Members of this group include the checkered elephant shrew, four-toed elephant shrew, short-eared elephant shrew, dusky elephant shrew, and several others. Elephant shrews are small mammals that have a long nose and feed on insects.
- Elephants (Proboscidea) - There are three species of elephants alive today. Members of this group include the African elephant, the African savanna elephant, and the Asian elephant. Elephants are the largest land animals alive today.
- Hares, rabbits, and pikas (Lagomorpha) - There are about 87 species of hares, rabbits, and pikas alive today. Members of this group, also known as lagomorphs, include cottontails, pikas, hares, jacrabbits, and rabbits. Lagomorphs have long ears, a short tail, and wide-set eyes. They often serve as an important prey species in the ecosystems they inhabit.
- Hoofed mammals - even-toed (Artiodactyla) - There are about 225 species of even-toed hoofed mammals alive today. Members of this group include cattle, deer, goats, sheep, antelope, llamas, pigs, hippoptamuses, and many others. Even-toed hoofed mammals have feet that are structured such that the animal's weight is borne by the third and fourth toes.
- Hoofed mammals - odd-toed (Perissodactyla) - There are about 19 species of odd-toed hoofed mammals alive today. Members of this group include horses, rhinoceroses, and tapirs. Odd-toed hoofed mammals have feet that are structured such that the animal's weight is borne by the third (middle) toe.
- Hyraxes (Hyracoidea) - There are eight species of hyraxes alive today. Members of this group include the yellow-spotted hyrax, the rock hyrax, the western tree hyrax, and several others. Hyraxes are small herbivores that have multi-chambered stomachs that enable them to digest the tough plant material they eat.
- Insectivores (Insectivora) - There are 365 species of insectivores alive today. Members of this group include hedgehogs, moles, shrews, and moonrats. Insectivores are small nocturnal mammals that feed on insects.
- Marsupials (Marsupialia) - There are about 250 species of marsupials alive today. Members of this group include koalas, kangaroos, wallabies, wombats, opossums, bandicoots, numbats, and many others. Marsupials give birth to their young early in their development. After they are born, the young marsupials continue their development in the mother's marsupium, a pouch located on the mother's abdomen.
- Monotremes (Monotremata) - There are five species of monotremes alive today. Members of this group include the platypus and four species of echidnas. Monotremes are unique among mammals in that they lay eggs instead of giving birth to live young.
- Pangolins (Pholidota) - There are eight species of pangolins alive today. Members of this group include the giant pangolin, tree pangolin, Chinese pangolin, Indian pangolin, and several others. Pangolins have large plate-like scales that cover their body.
- Primates (Primates) - There are about 356 species of primates alive today. Members of this group include prosimians, monkeys, and apes. Primates form complex social units and many species are well-adapted for life in the trees. The primates is the group of mammals to which humans belong.
- Rodents (Rodentia) - There are more than 2,000 species of rodents alive today. Members of this group include mice, rats, hamsters, guinea pigs, beavers, squirrels, and porcupines. Rodents are a remarkably diverse—they account for almost 40 percent of all living mammals and inhabit every continent except Antarctica.
- Seals and sea lions (Pinnipedia) - There are about 33 species of seals and sea lions alive today. Members of this group include fur seals, true seals, sea lions, and the walrus. Seals and sea lions are well-adapted to aquatic habitats. They have streamlined bodies, broad flippers, and a thick layer of blubber that provides insulation from cold water.
- Sirenians (Sirenia) - There are four species of sirenians alive today. Members of this group include the dugong and three species of manatees. Sirenians are large aquatic mammals that inhabit coastal marine waters as well as rivers, estuaries and swamps.
- Treeshrews (Scandentia) - There are about 19 species of treeshrews alive today. Members of this group include the ruddy treeshrew, large treeshrew, painted treeshrew, pen-tailed treeshrew, and many others. Treeshrews are medium-sized mammals that have a slender body and a long tail.
Hickman C, Roberts L, Keen S, Larson A, l'Anson H, Eisenhour D. Animal Diversity. 6th ed. Boston MA: McGraw-Hill; 2012. 479 p.
Hickman C, Roberts L, Keen S, Larson A, l'Anson H, Eisenhour D. Integrated Principles of Zoology. 14th ed. Boston MA: McGraw-Hill; 2006. 910 p.