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18.1: Characteristics of Mammalia

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    Learning Objectives

    By the end of this section, you will be able to do the following:

    • Name and describe the distinguishing features of mammals
    • List some derived features that may have arisen in response to mammals’ need for constant, high-level metabolism

     

    Mammals, comprising about 5,200 species, are vertebrates that possess hair and mammary glands. Several other characteristics are distinctive to mammals, including certain features of the jaw, skeleton, integument, and internal anatomy. Modern mammals belong to three clades: monotremes, marsupials, and eutherians (or placental mammals).

     

    Hair/Fur and Milk

    The presence of hair, composed of the protein keratin, is one of the most obvious characteristics of mammals. Although it is not very extensive or obvious on some species (such as whales), hair has many important functions for most mammals. Mammals are endothermic, and hair traps a boundary layer of air close to the body, retaining heat generated by metabolic activity. Along with insulation, hair can serve as a sensory mechanism via specialized hairs called vibrissae, better known as whiskers. Vibrissae attach to nerves that transmit information about tactile vibration produced by sound sensation, which is particularly useful to nocturnal or burrowing mammals. Hair can also provide protective coloration or be part of social signaling, such as when an animal’s hair stands “on end” to warn enemies, or possibly to make the mammal “look bigger” to predators.

    Unlike the skin of birds, the integument (skin) of mammals, includes a number of different types of secretory glands. Sebaceous glands produce a lipid mixture called sebum that is secreted onto the hair and skin, providing water resistance and lubrication for hair. Sebaceous glands are located over most of the body. Eccrine glands produce sweat, or perspiration, which is mainly composed of water, but also contains metabolic waste products, and sometimes compounds with antibiotic activity. In most mammals, eccrine glands are limited to certain areas of the body, and some mammals do not possess them at all. However, in primates, especially humans, sweat glands are located over most of the body surface and figure prominently in regulating the body temperature through evaporative cooling. Apocrine glands, or scent glands, secrete substances that are used for chemical communication, such as in skunks. Mammary glands produce milk that is used to feed newborns. In both monotremes and eutherians, both males and females possess mammary glands, while in some marsupials, mammary glands are found only in females, with exception of some opossums. Mammary glands likely are modified sebaceous or eccrine glands, but their evolutionary origin is not entirely clear.

     


    Skeletal Modifications

    The skeletal system of mammals possesses many unique features. Unlike birds, the skulls of mammals have two occipital condyles, bones at the base of the skull that articulate with the first vertebra, as well as a secondary palate at the rear of the pharynx that helps to separate the pathway of swallowing from that of breathing. Turbinate bones (conchae in humans) are located along the sides of the nasal cavity, and help warm and moisten air as it is inhaled. The pelvic bones are fused in mammals, and there are typically seven cervical vertebrae (except for some edentates and manatees).

    The lower jaw of mammals consists of only one bone, the dentary, and the jaw hinge connects the dentary to the squamosal (flat) part of the temporal bone in the skull. The jaws of other vertebrates are composed of several bones, including the quadrate bone at the back of the skull and the articular bone at the back of the jaw, with the jaw connected between the quadrate and articular bones. In the ear of other vertebrates, vibrations are transmitted to the inner ear by a single bone, the stapes. In mammals, the quadrate and articular bones have moved into the middle ear (Figure 29.37). The malleus is derived from the articular bone, whereas the incus originated from the quadrate bone. This arrangement of jaw and ear bones aids in distinguishing fossil mammals from fossils of other synapsids.

    The illustration shows the three bones of the inner ear, the malleus, the incus, and the stapes, which are connected together inside the ear canal.
    Figure 29.37 Mammalian ear bones. Bones of the mammalian middle ear are modified from bones of the jaw and skull in reptiles. The stapes is found in other vertebrates (e.g., the columella of birds) whereas in mammals, the malleus and incus are derived from the articular and quadrate bones, respectively. (credit: NCI)

     


    Mammalian Dentition

    The adductor muscles that close the jaw comprise two major muscles in mammals: the temporalis and the masseter. Working together, these muscles permit up-and-down and side-to-side movements of the jaw, making chewing possible—which is unique to mammals. Most mammals have heterodont teeth, meaning that they have different types and shapes of teeth (incisors, canines, premolars, and molars) rather than just one type and shape of tooth. Most other vertebrates are homodont, meaning all the teeth are relatively the same shape and designed for the same function.

    Incisors are chisel-shaped and located toward the front of the mouth, and they are often used to bite into food. On either side of the incisors are the canines, which are pointed, fang-like teeth that are superb for piercing. Behind the canines are the premolars and molars, which are often very similar functionally; they generally have an overall flatter shape with rounded cusps useful for mashing foods. In humans, the wisdom teeth are the last set of molars. Mammals vary in the presence and number of each different type of tooth, and information about diet can be obtained just by looking at the dentition of a mammal. Carnivores tend to have very pronounced canines and premolars/molars that are sharpened for shearing flesh. Herbivores often have very broad, flattened premolars/molars and reduced or absent canines. Mammals specialized for gnawing and chewing, like rodents, have enlarged incisors that grow continuously through life.

    Most mammals are also diphyodonts, meaning that they have two sets of teeth in their lifetime: deciduous or “baby” teeth, and permanent teeth. Most other vertebrates with teeth are polyphyodonts, that is, their teeth are replaced throughout their entire life.

     


    Mammalian Digestive Systems

    Monogastric: Single-chambered Stomach

    As the word monogastric suggests, this type of digestive system consists of one (“mono”) stomach chamber (“gastric”). Humans and many animals have a monogastric digestive system as illustrated in Figure 34.6ab. The process of digestion begins with the mouth and the intake of food. The teeth play an important role in masticating (chewing) or physically breaking down food into smaller particles. The enzymes present in saliva also begin to chemically breakdown food. The esophagus is a long tube that connects the mouth to the stomach. Using peristalsis, or wave-like smooth muscle contractions, the muscles of the esophagus push the food towards the stomach. In order to speed up the actions of enzymes in the stomach, the stomach is an extremely acidic environment, with a pH between 1.5 and 2.5. The gastric juices, which include enzymes in the stomach, act on the food particles and continue the process of digestion. Further breakdown of food takes place in the small intestine where enzymes produced by the liver, the small intestine, and the pancreas continue the process of digestion. The nutrients are absorbed into the bloodstream across the epithelial cells lining the walls of the small intestines. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces; it is stored until it is excreted through the rectum.

    The basic components of the human and rabbit digestive system are the same: each begins at the mouth. Food is swallowed through the esophagus and into the kidney-shaped stomach. The liver is located on top of the stomach, and the pancreas is underneath. Food passes from the stomach to the long, winding small intestine. From there it enters the wide large intestine before passing out the anus. At the junction of the small and large intestine is a pouch called the cecum. The small and large intestines are much longer in rabbits than in humans, and the cecum is much longer as well.
    Figure 34.6 (a) Humans and herbivores, such as the (b) rabbit, have a monogastric digestive system. However, in the rabbit the small intestine and cecum are enlarged to allow more time to digest plant material. The enlarged organ provides more surface area for absorption of nutrients. Rabbits digest their food twice: the first time food passes through the digestive system, it collects in the cecum, and then it passes as soft feces called cecotrophes. The rabbit re-ingests these cecotrophes to further digest them.

     

    Ruminants

    Ruminants are mainly herbivores like cows, sheep, and goats, whose entire diet consists of eating large amounts of roughage or fiber. They have evolved digestive systems that help them digest vast amounts of cellulose. An interesting feature of the ruminants’ mouth is that they do not have upper incisor teeth. They use their lower teeth, tongue and lips to tear and chew their food. From the mouth, the food travels to the esophagus and on to the stomach.

    To help digest the large amount of plant material, the stomach of the ruminants is a multi-chambered organ, as illustrated in Figure 34.8. The four compartments of the stomach are called the rumen, reticulum, omasum, and abomasum. These chambers contain many microbes that breakdown cellulose and ferment ingested food. The abomasum is the “true” stomach and is the equivalent of the monogastric stomach chamber where gastric juices are secreted. The four-compartment gastric chamber provides larger space and the microbial support necessary to digest plant material in ruminants. The fermentation process produces large amounts of gas in the stomach chamber, which must be eliminated. As in other animals, the small intestine plays an important role in nutrient absorption, and the large intestine helps in the elimination of waste.

    Illustration shows the digestive system of a goat. Food passes from the mouth, through the esophagus and into the rumen. It circulates clockwise through the rumen, then moves forward, and down into the small, pouch-shaped reticulum. From the reticulum the food, which is now cud, is regurgitated. The animal chews the cud, and then swallows it into the coiled omasum, which sits between the reticulum and the rumen. After circulating through the omasum the food enters the small intestine, then the large intestine. Waste is excreted through the anus.
    Figure 34.8 Ruminant animals, such as goats and cows, have four stomachs. The first two stomachs, the rumen and the reticulum, contain prokaryotes and protists that are able to digest cellulose fiber. The ruminant regurgitates cud from the reticulum, chews it, and swallows it into a third stomach, the omasum, which removes water. The cud then passes onto the fourth stomach, the abomasum, where it is digested by enzymes produced by the ruminant.

     

    Pseudo-ruminants

    Some animals, such as camels and alpacas, are pseudo-ruminants. They eat a lot of plant material and roughage. Digesting plant material is not easy because plant cell walls contain the polymeric sugar molecule cellulose. The digestive enzymes of these animals cannot breakdown cellulose, but microorganisms present in the digestive system can. Therefore, the digestive system must be able to handle large amounts of roughage and breakdown the cellulose. Pseudo-ruminants have a three-chamber stomach in the digestive system. However, their cecum—a pouched organ at the beginning of the large intestine containing many microorganisms that are necessary for the digestion of plant materials—is large and is the site where the roughage is fermented and digested. These animals do not have a rumen but have an omasum, abomasum, and reticulum.

     


    The Mammalian Heart

    Mammals, like birds, possess a four-chambered heart; however, the hearts of birds and mammals are an example of convergent evolution, since mammals clearly arose independently from different groups of tetrapod ancestors. Mammals also have a specialized group of cardiac cells (fibers) located in the walls of their right atrium called the sinoatrial node, or pacemaker, which determines the rate at which the heart beats. Mammalian erythrocytes (red blood cells) do not have nuclei, whereas the erythrocytes of other vertebrates are nucleated.

    The heart muscle is asymmetrical as a result of the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit it is smaller than the left side which must send blood out to the whole body in the systemic circuit, as shown in Figure 40.11. In humans, the heart is about the size of a clenched fist; it is divided into four chambers: two atria and two ventricles. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the bicuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the pulmonary arteries, bypassing the semilunar valve (or pulmonic valve) to the lungs for re-oxygenation. After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through the aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals.

    Visual Connection

    Illustration A shows the parts of the heart. Blood enters the right atrium through an upper, superior vena cava and a lower, inferior vena cava. From the right atrium, blood flows through the funnel-shaped tricuspid valve into the right ventricle. Blood then travels up and through the pulmonary valve into the pulmonary artery. Blood re-enters the heart through the pulmonary veins, and travels down from the left atrium, through the mitral valve, into the right ventricle. Blood then travels up through the aortic valve, into the aorta. The tricuspid and mitral valves are atrioventricular and funnel-shaped. The pulmonary and aortic valves are semilunar and slightly curved. An inset shows a cross section of the heart. The myocardium is the thick muscle layer. The inside of the heart is protected by the endocardium, and the outside is protected by the pericardium. Illustration B shows the outside of the heart. Coronary arteries and coronary veins run from the top down along the right and left sides.
    Figure 40.11 (a) The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers. (b) Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart musculature oxygenated.

     


    Mammalian Excretory System

    Mammals excrete urea to get rid of nitrogenous waste. Recall that ammonia is toxic to the body and must be converted to urea or uric acid in terrestrial organisms. Urea is less energetically expensive to produce from ammonia than uric acid is, but it utilizes more water than uric acid conversion. The kidneys of mammals have a portion of the nephron called the loop of Henle or nephritic loop, which allows mammals to reabsorb a larger quantity of water in the kidneys and produce urine with a high concentration of solutes—higher than that of the blood. Some mammals specialized for desert habitats, like kangaroo rats, have particularly long loops of Henle. Mammals lack a renal portal system, which is a system of veins that moves blood from the hind or lower limbs and region of the tail to the kidneys. Renal portal systems are present in all other vertebrates except jawless fishes. A urinary bladder is present in all mammals.

     


    Respiration

    In mammals, pulmonary ventilation occurs via inhalation (breathing). Mammals have a muscular diaphragm that is lacking in birds. During inhalation, air enters the body through the nasal cavity located just inside the nose (Figure 39.7). As air passes through the nasal cavity, the air is warmed to body temperature and humidified. The respiratory tract is coated with mucus to seal the tissues from direct contact with air. Mucus is high in water. As air crosses these surfaces of the mucous membranes, it picks up water. These processes help equilibrate the air to the body conditions, reducing any damage that cold, dry air can cause. Particulate matter that is floating in the air is removed in the nasal passages via mucus and cilia. The processes of warming, humidifying, and removing particles are important protective mechanisms that prevent damage to the trachea and lungs. Thus, inhalation serves several purposes in addition to bringing oxygen into the respiratory system.

    Visual Connection

    The illustration shows the flow of air through the human respiratory system. The nasal cavity is a wide cavity above and behind the nostrils, and the pharynx is the passageway behind the mouth. The nasal cavity and pharynx join and enter the trachea through the larynx. The larynx is somewhat wider than the trachea and flat. The trachea has concentric, ring-like grooves, giving it a bumpy appearance. The trachea bifurcates into two primary bronchi, which are also grooved. The primary bronchi enter the lungs, and branch into secondary bronchi. The secondary bronchi in turn branch into many tertiary bronchi. The tertiary bronchi branch into bronchioles, which branch into terminal bronchioles. Each terminal bronchiole ends in an alveolar sac. Each alveolar sac contains many alveoli clustered together, like bunches of grapes. The alveolar duct is the air passage into the alveolar sac. The alveoli are hollow, and air empties into them. Pulmonary arteries bring deoxygenated blood to the alveolar sac (and thus appear blue), and pulmonary veins return oxygenated blood (and thus appear red) to the heart. Capillaries form a web around each alveolus. The diaphragm is a membrane that pushes up against the lungs.
    Figure 39.7 Air enters the respiratory system through the nasal cavity and pharynx, and then passes through the trachea and into the bronchi, which bring air into the lungs. (credit: modification of work by NCI)

     

    From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box), as it makes its way to the trachea. The main function of the trachea is to funnel the inhaled air to the lungs and the exhaled air back out of the body. The human trachea is a cylinder about 10 to 12 cm long and 2 cm in diameter that sits in front of the esophagus and extends from the larynx into the chest cavity where it divides into the two primary bronchi at the midthorax. The end of the trachea bifurcates (divides) to the right and left lungs. In humans, the lungs are not identical. The right lung is larger and contains three lobes, whereas the smaller left lung contains two lobes (Figure 39.9). The muscular diaphragm, which facilitates breathing, is inferior to (below) the lungs and marks the end of the thoracic cavity.

    The illustration shows the trachea, which starts at the top of the neck and continues down into the chest, where it branches into the bronchi, which enter the lungs. The left lung has two lobes. The upper lobe is located in front of and above the lower lobe. The right lung has three lobes. The upper lobe is on the top, the lower lobe is on the bottom, and the middle lobe is sandwiched between them. The diaphragm presses against the bottom of the lungs and has the appearance of skin stretched over the top of a drum. Wide flaps of the diaphragm extend downward on the front left and right sides of the body. On the back, thin flaps of diaphragm stretch downward on either side of the spine.
    Figure 39.9 The trachea bifurcates into the right and left bronchi in the lungs. The right lung is made of three lobes and is larger. To accommodate the heart, the left lung is smaller and has only two lobes.

     

    In the lungs, air is diverted into smaller and smaller passages, or bronchi. Bronchi continue to branch into even smaller structures called bronchioles. The terminal bronchioles subdivide into microscopic branches called respiratory bronchioles. The respiratory bronchioles subdivide into several alveolar ducts. Numerous alveoli and alveolar sacs surround the alveolar ducts. The alveolar sacs resemble bunches of grapes tethered to the end of the bronchioles (Figure 39.10). Each alveolar sac contains 20 to 30 alveoli that are 200 to 300 microns in diameter. Gas exchange occurs only in alveoli. Alveoli are made of thin-walled cells, typically one-cell thick, that look like tiny bubbles within the sacs. Alveoli are in direct contact with capillaries (one-cell thick) of the circulatory system. Such intimate contact ensures that oxygen will diffuse from alveoli into the blood and be distributed to the cells of the body. In addition, the carbon dioxide that was produced by cells as a waste product will diffuse from the blood into alveoli to be exhaled.

    The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems. Because there are so many alveoli (~300 million per lung) within each alveolar sac and so many sacs at the end of each alveolar duct, the lungs have a sponge-like consistency. This organization produces a very large surface area that is available for gas exchange. The surface area of alveoli in the lungs is approximately 75 m2. This large surface area, combined with the thin-walled nature of the alveolar parenchymal cells, allows gases to easily diffuse across the cells.

    The illustration shows a terminal bronchial tube branching into three alveolar ducts. At the end of each duct is an alveolar sac made up of 20 to 30 alveoli clustered together, like grapes. The airspace in the middle of the alveolar sac, called the atrium, is continuous with the air space inside the alveolus so that air can circulate from the atrium to the alveolus. Capillaries surround each alveolus, and this is where gas exchange occurs. A pulmonary artery (shown in blue) runs along the terminal bronchiole, bringing deoxygenated blood from the heart to the alveoli. A pulmonary vein (shown in red) running along the bronchiole brings oxygenated blood back to the heart. Small, flat mucous glands are associated with the outside of the bronchial tubes.
    Figure 39.10 Terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolar sacs. Each alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes. Air flows into the atrium of the alveolar sac, then circulates into alveoli where gas exchange occurs with the capillaries. Mucous glands secrete mucous into the airways, keeping them moist and flexible. (credit: modification of work by Mariana Ruiz Villareal)

     


    Mammalian Nervous & Sensory System

    Mammals have movable eyelids and fleshy external ears (pinnae), quite unlike the naked external auditory openings of birds. Mammalian brains also have certain characteristics that differ from the brains of other vertebrates. In some, but not all mammals, the cerebral cortex, the outermost part of the cerebrum, is highly convoluted and folded, allowing for a greater surface area than is possible with a smooth cortex. The optic lobes, located in the midbrain, are divided into two parts in mammals, while other vertebrates possess a single, undivided lobe. Eutherian mammals also possess a specialized structure, the corpus callosum, which links the two cerebral hemispheres together. The corpus callosum functions to integrate motor, sensory, and cognitive functions between the left and right cerebral cortexes.

     


    This page titled 18.1: Characteristics of Mammalia is a derivative of Biology 2e by OpenStax that is licensed under a CC BY 4.0 license and a derivative of Anatomy & Physiology 2e by OpenStax that is licensed under a CC BY 4.0 license


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