Mammals characteristics include numerous adaptations that enable them to survive in a wide range of environments. They live in nearly every habitat around the globe, from frigid polar regions, to turbulent seas, to dense tropical forests. Modern mammals range in stature from tiny field mice to massive whales and although various species may look drastically different, all mammals still share a unifying set of characteristics.
Some mammal characteristics—such as their hair, mammary glands, and three specialized middle-ear bones—are shared by no other groups of animals. Here we'll explore key facts and information about mammals so we can better understand important mammal characteristics.
Hair is one of the characteristics of a mammal that is unique to mammals—no other animals have true hair and all mammals have hair covering at least part of their body at some time during their life. An individual hair consists of a rod of cells that are reinforced by a protein known as keratin. Hair grows from skin cells called follicles. Hair can take on several different forms including thick fur, long whiskers, defensive quills or fearsome horns. Hair serves a variety of functions for mammals. It provides insulation, protects the skin, serves as camouflage and provides sensory feedback.
Some mammals have thick coats of fur that consist of two layers, a soft underfur and a coarse protective outer fur. Sea otters, for example, have this type of two-layered fur. In fact, sea otters have one the thicket coats of fur of all mammals, with more than 100,000 individual hairs per square centimeter. Since sea otters lack a layer of insulating blubber, they must compensate by having fur with superior insulation power. Cetaceans, in contrast, have a thick layer of insulating blubber and therefore have lost most of their hair over the course of their evolution. Some whales only have hair during their early development, while others retain sparse patches of hair on their chin or upper lip.
Mammals nurse their young with milk produced by mammary glands. Mammary glands, like hair, are a uniquely mammalian trait. Though present in both males and females, in most mammal species mammary glands only fully develop in females. The exception to this rule is the male Dayak fruit bat, which has mammary glands that produce milk to feed its young.
Mammary glands are modified and enlarged sweat glands that consist of ducts and glandular tissues that secrete milk through nipples. Young mammals obtain milk from their mother by feeding from her nipples. The milk provides the young with much needed protein, sugars, fat, vitamins and salts.
Not all mammals have nipples. Monotremes, which include echidnas and the platypus, diverged from other mammals early in their evolution. Monotremes lack nipples and instead secrete the milk produced by their mammary glands through ducts in their abdomen.
Lower Jaw Made of a Single Bone
Mammals differ from other vertebrates such as reptiles, birds and amphibians in that they have a single lower jaw bone that attaches directly to the skull. This bone is referred to as the dentary, due to the fact that it holds the teeth of the lower jaw. In other vertebrates, the dentary is one of several bones in the lower jaw and does not attach directly to the skull.
The structure of the lower jaw and the muscles that control it provides mammals with a powerful bite and enables them to use their teeth to cut and chew their prey. Mammal species have specialized teeth adapted to their particular diet. Cats, for example, have sharp teeth that enable them to tear meat while herbivores such as bison have broad teeth, well-suited for grinding plant material.
Diphyodonty is a pattern of tooth replacement in animals in which the teeth are replaced only once throughout the lifetime. Young mammals have a set of teeth that are smaller and weaker than their adult teeth. This first set of teeth, also known as the deciduous teeth, fall out and are later replaced by a set of larger, permanent teeth.
Polyphydonty, in contrast to diphyodonty, is a term used to describe the pattern of tooth replacement in which teeth are continuously replaced throughout the lifetime of an animal. Toothed fishes, reptiles and many other non-mammalian vertebrates are polyphydonts.
Three Middle Ear Bones
Mammals have a unique arrangement of three bones in the middle ear. These bones—the incus, malleus and stapes, commonly referred to as the hammer, anvil and stirrup—are unique to mammals, no other animal group has them. The middle ear bones transmit sound vibrations from the tympanic membrane or eardrum to the inner ear and transforms them into neural impulses. The malleus and incus are derived bones that were once part of the lower jaw in mammal ancestors.
Mammals are endothermic which means they are capable of regulating their own body temperature so that it remains at a relatively constant temperature regardless of the temperature of the surrounding environment.
The diaphram is a layer of muscle located at the base of the ribcage that separates the thoracic cavity from the adominal cavity in mammals. Mammals are not the only vertebrates to posess a diaphram, amphibians and reptiles also have diaphragms or diaphragm-like structures. It should be noted that the anatomy of the diaphram and its position vary among the different classes of vertebrates.
Like all vertebrates (and even all animals with a circulatory system), mammals have a muscular heart that contracts repeatedly to pump blood throughout the body's blood vessels. The heart serves to deliver oxygen and nutrients throughout the body and remove waste products. In general, the heart consists of multiple chambers (the number of chambers differs for the various animal groups). Two to four chambers may be present and there are two types of chambers, the atrium and ventricle (the atrium receives the blood returning to the heart while the ventricle pumps blood from the heart to the rest of the body).
The structural details of the heart differs among the various animal groups. Fish have the simplest heart structure of all vertebrates which consists of two chambers (one atrium and one ventricle). Amphibians and most reptiles have a three-chambered heart (two atria and one ventricle). Birds and mammals have a four-chambered heart (two atria and two ventricles).
The structure of a four-chambered heart offers greater efficiency than the three- and two- chambered heart structures. A four-chambered heart separates oxgenated blood coming from the lungs from the partially deoxygenated blood returning from the body to the lungs to be re-oxygenated. The prevention of mixing of these two streams of blood ensures that tissues receive oxygen-rich blood which in turn enables sustained muscle activity and helps in maintaining constant body temperatures.