Mammals are vertebrate animals with hair. Vertebrates are animals with backbones. Other vertebrate animals are bony fish, sharks, amphibians, reptiles and birds. Like birds, mammals are warm-blooded. Their hearts have four chambers. That allows one channel to carry oxygen-rich blood to their brains, muscles and tissues, and a separate channel to carry oxygen-poor blood back to the lungs for more oxygen.
Mammals are warm-blooded animals. That means they maintain a high, constant body temperature even though outside temperatures change. Insulation, such as hair or fat, helps keep mammals warm. Mammals arose from reptiles about 240 million years ago. They began to become really successful and important in ecosystems after dinosaurs became extinct about 65 million years ago. Mammals are different from reptiles in a number of ways.
Reptiles have a lower jaw made up of several bones. Reptilian jaws are attached loosely to their skulls. A snake, for example, can loosen its jaw and swallow a mouse fatter than the snake’s own body. Mammals have one bone in their lower jaws. Their jawbones are firmly attached to their skulls. In mammals, the extra jaw bones of reptiles evolved into ear bones. In short, reptiles have three jaw bones and one ear bone. Mammals have one jaw bone and three ear bones.
Mammals’ jaws are less flexible than reptilian jaws. But, the jaws of mammals are stronger, which help them use their fancy teeth better. Mammals’ teeth have three distinct features. First, their teeth are specialized from front to back. Second, their teeth are set firmly in sockets. Third, mammals have two sets of teeth —“baby teeth” and permanent ones. Mammals have four kinds of teeth: incisors for nipping, canines for grasping and puncturing prey, and premolars and molars for crushing, shearing, or grinding food. Molars are present only in permanent teeth.
These special kinds of teeth help mammals chew their food better. Once the small bits of chewed food enter the digestive tract, they mix with digestive juices and are digested rapidly. That gives mammals fuel they need to stay active regardless of the temperature outside.
We might think of mammals as “reptiles glorified with hair.” Many basic features of mammals are associated with their hair. Originally, their hair helped insulate them. But, their hair began serving other purposes as well, such as helping them achieve camouflage, sense their environment, warn them of dangers, and recognize members of the opposite sex.
Attached to each hair is a tiny muscle that can raise the hair to trap warm air. In humans, these muscles contract and raise “goose bumps” when we are cold. Each hair has a gland that secretes oil to waterproof hair.
Sweat glands release water and salts that evaporate from the skin and hair, helping cool the body. Cats have few sweat glands, so they lick their fur to help cool off. Dogs pant, and evaporation off their tongues helps them cool. Many mammals also have hair glands that release musky odors. The odors serve as sexual attractions or as ways of recognizing each other.
Mammary glands produce milk—a mixture of water, sugars, fats, proteins and salts. Milk gives newborn, nursing mammals a nutritional “jumpstart.” Nursing also lets newborns interact with their mothers and littermates, helping them become trained and socialized.
The platypus and echidnas of Australia lay eggs like their reptilian ancestors did. In most other mammals, embryos grow within their mothers and are given birth. An intimate relationship develops between mothers and embryos. What allows that to happen is the placenta, made of tissues that feed and remove waste from the developing fetus inside. This doesn’t happen, however, with marsupials, which are mammals with external pouches for their young, such as kangaroos.
An advanced brain is another key to the success of mammals. The basic parts of mammals’ brains were present in the ancestral reptiles and even in fish. Mammals, however, have developed the parts of their brains used for learning. Mammals depend much more on learning than most other animals do. Mammals don’t rely much on instinctive ways of solving problems. Instead they piece together solutions based on experience. The cerebral cortex is the center of learning in their brains and is larger than in other animals.
There are about 5,400 species of living mammals, many fewer than species of birds, reptiles, fish or invertebrate groups, such as insects and snails. Mammals impress us not by their number of species, but by the number of body types. The common ancestor of placental mammals and marsupials (which carry their young in a pouch) was a rat- to opossum-sized mammal. That mammal was four-legged, omnivorous (eating plants as well as meat), and lived on land.
From those beginnings, many different kinds of mammals evolved, from bats and shrews the length of your little finger and the weight of a penny to blue whales as long as a tennis court and as heavy as three loaded railway freight cars.