Chordata—MammaliaMelanie Tabroff
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Kingdom: AnimaliaPhylum: ChordataClass: Mammalia

What are mammals and where did they come from?
Mammals are a group of vertebrates that have evolved from reptiles around 220 million years ago, during the Triassic period. Their ancestors are among the Therapsids, part of the synapsid branch of reptilian phylogeny. During the period of dinosaur’s existence, the therapsids disappeared but left Mesozoic mammalian descendants. As the Cretaceous period ended and the Cenozoic era began, the fluctuations in the environment contributed to mammalian adaptations, which are reflected in the diversification of mammals today.

How many types of mammals exist?
There are three major groups of mammals that exist today:
· Monotremes: egg-laying.
There are only 5 known species currently existing. {The name monotreme refers to the term cloaca, a single posterior opening that serves the urinary tract, anus, and reproductive tract in the monotremes (AC)(18). They are very primitive compared to other mammals due to their egg laying.The most famous is the duck-billed platypus. Interestingly, the platypus is also unique for being on of the only poisonous mammals.(KL)(1)[Monotremes also have highly modified snouts or beaks, and adults have no teeth. "Monotremes have a single bone in their lower jaw, three middle ear bones, high metabolic rates, hair, and they produce milk to nourish the young." All monotremes are found in Australia and New Guinea. (MP) 15]
· Marsupials: have pouches
Marsupials are characterized by a distinctive pouch (called the marsupium), in which females carry their young through early infancy. Most species are found in Australia, New Guinea, and nearby islands, but there are also many species in the Americas.(6)(ZS) The placenta of marsupials is a yolk-type, which accounts for the very early birth of marsupial young. The carrying of young in their pouches allows the young animal to finish developing after an early birth.Time in the pouch ranges from weeks to months.(ORS 12)
After the offspring is born, the newborn continues developing within the pouch. The young marsupial attaches itself to a nipple within the pouch in order to receive the necessary nourishment in order to continue its development. (CW)(16)
· Eutherian mammals: placental
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos). They are distinguished from non-eutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and non-placental eutherians is that placentals lack epipubic bones, which are a pair of bones that project forward from the pelvic bones of modern marsupials and of some fossil mammals to support the mother’s pouch, which are present in all other fossil mammals and living mammals. (7)(ZS)

external image evolution2.jpg(MC 11)

The diagnostic characteristics
There are about 4,500 different species of mammals on Earth today. Yet, all of these species share the same diagnostic characteristics. The principle mammalian characteristic is having mammary glands, which are organs that produce milk. Mothers nourish their young with the milk from the mammary gland, which is rich in vitamins, minerals, fats, sugars, and proteins. In addition, most mammals are born rather than hatched. Another distinctively mammalian characteristic is having hair, which is made of keratin. The hair, along with a layer of fat, is beneficial for species of mammals because it helps their bodies retain metabolic heat. Mammals also have differentiation of teeth, which enables them to chew foods that vary in size and texture. Along with having a diversified range of teeth, mammals have jaws, which were modified during the evolution of mammals from reptiles. In general, most mammals are proficient learners and have larger brains than other vertebrates of equivalent size. Mammals invest a long time in parental care so that their offspring can learn from their parent’s behavior and be able to eventually survive on its own.
All mammals have six things in common that separates them from other animals. First, all mammals have mammary glands that produce milk. Second, they all have hair at some point in their lives. Third, their lower jaw is only one bone on either side. In all other animals there is more than one bone on either side of the jaw. Fourth, they all have 3 middle ear bones. These three bones are the hammer, anvil, and stirrup. Fifth, all mammals have one main artery that leaves the heart that curve to the left. In all other animals there are multiple arties or the artery curves to the right. Finally, all mammals possess a diaphragm. The diaphragm is a sheet of tendons and muscles that are able to divide the body cavity into two sections. (9)(MF)
Mammals are also the only animals to have a neo-cortex in their brains. The neo-cortex is responsible for high-level functions such as spatial reasoning, language, and conscious thought. (JS) (17)

Acquiring and digesting food
The process of physical and chemical digestion begins in the mouth. Mammals have differentiation of teeth, which helps them chew different kinds of foods to increases its surface area to make it easy to swallow. The presence of food in the oral cavity triggers the deliverance of saliva from the salivary glands. The saliva contains salivary amylase, an enzyme that helps break down carbohydrates. When the tongue senses the food, it pushes the ball of food (or bolus) to the back of the oral cavity into the pharynx. As the mammal swallows, the food is able to travel from the pharynx down the esophagus by peristalsis, waves of involuntary muscle contractions, and then into the stomach. At this stage, the stomach releases an acid fluid called gastric juice, which mixes with the food to further break it down into its chemical components. Then the food travels to the small intestine, where it is broken down even further and its nutrients are absorbed into the bloodstream. Next, the food moves into the large intestine, where water is extracted, and the leftover is released through the anus.
The mammalian digestive system functions with the help of accessory glands that secrete digestive juices into the alimentary canal, the pathway that the food travels, through ducts. The accessory glands include the pancreas, the liver, the gallbladder, and three pairs of salivary glands, which store digestive juices.

The human digestive system. (SI) (19)

Sensing the environment
Mammals can sense different stimuli in the environment with the help of sensory receptors. They have pain receptors, thermoreceptors (sense temperature), chemoreceptors (sense odor), mechanoreceptors (sense pressure, touch, motion, and sound), and electromagnetic receptors (sense visible light and electricity). In general, mammals use the five main senses, which include sight, hearing, touch, smell, and taste, to identify the environment. The ability of mammals to sense and understand their environment is due to the inner workings of the brain.
While we are used to thinking of mammalian sensory organs are simply the eyes, nose, tongue, skin and ears, since that is what humans use, many mammals have specialized versions of these and other sensory organs. Many mammals, especially those who are nocturnal, use whiskers and hairs, or vibrissae, tactilely. Mammals such as bats and shrews utilize echolocation, in which animals with heightened senses of hearing are able to sense locations based on the echo of a sound they emit. The hearing of mammals is especially developed due to the development of three middle ear bones. Porpoises and dolphins do not have olfactory lobes, though many carnivores have enlarged lobes. There is some evidence proving that bears coming out of hibernation can smell carrion from ten miles away! though that is not confirmed. (CSR 4, 5)

Mammals have various modes of locomotion. Mammals exist in diversified environments, such as land, water, and air, and so mammals have adapted different modes of movement based on their habitat. Some adaptations include gliding, jumping, walking, or running. Bats are the only mammals that have adapted active flight as a mode of locomotion. Certain mammals have adapted to run quickly (the cheetah can top 60 mph) or do other tasks such as climb trees. Whales for example, are mammals that over time have adapted from a land mammal and developed the ability to swim and communicate underwater. (GR)(8)
Mammals can move faster than others because of the way thier limbs connect to their body and their warm blood. In opposition to reptiles, mammals generally have their legs located underneath their body, making it easier to jump, run, or walk a long distance in a short amount of time. A reptile, however, has a more difficult time because its legs are positioned to the side of the body, creating more friction when the animal is forced to drag its body. With each stride a cheetah makes, it dislocates and relocates its back. Some mammals are able to fly, such as bats and the flying squirrel. Others live in the water and make their way by swimming. Mammals that can live in and out of water have two mechanisms for locomotion. They can both swim, and effectively crawl. (NG)

Mammals have lungs, located in the thoracic (chest) cavity, where cellular respiration occurs. The lungs have a spongy texture and are honeycombed with a moist respiratory surface. Air travels to lungs through a series of branching ducts. Air first enters the nostrils, where it is filtered by hairs, and then travels through the nasal cavity to the pharynx and then to the larynx. From the larynx, air passes into the trachea (windpipe), which forks into two bronchi, leading to each lung. The bronchi branch into smaller and smaller tubes called bronchioles, which eventually leads to the alveoli, the cluster of air sacs where gas exchange occurs. Mammals ventilate their lungs by negative pressure breathing, changing the air pressure within its lungs relative to the pressure of the outside atmosphere. During inhalation, the rib muscles and diaphragm contact (moves down). This process expands the rib cage and increases the lung’s volume. The pressure gradient that results allows air to enter through the nostrils to the alveoli. During exhalation, the rib muscles and diaphragm relax which restores the lungs to its normal volume and increases air pressure within the alveoli. The increased air pressure inside the alveoli forces air to rush out via the nostrils.
Gas exchange of carbon dioxide and oxygen occurs in the alveoli. The oxygen diffuses through the wall of the alveoli into red blood cells, which carry the oxygen to body tissues. (AK) (20)

A typical set of mammal lungs. (CC) (14

Metabolic waste removal
Mammals, like all animals, remove wastes by excretion. The kidneys of mammals filter out nitrogenous wastes, in the form of urea, from the bloodstream and excrete it during urination. Urea is produced in the liver and carried to the kidneys by the circulatory system. It is not as toxic as ammonia and it can be concentrated to help mammals conserve water during excretion.
The kidney of an animal regulate the amount of water that is in an animal'a urine. Deprnding on whether an animal is dehydrated or over hydrated, the kidneys will either conserve or excrete water. (KS)
Every human has two kidneys. Each kidney is made up of three sections: the renal cortex, the renal medulla, and the renal pelvis. The blood arrives at the kidney via the renal artery, which splits into may afferent arterioles. These arterioles go to the Bowman's Capsule of nephrons, where wastes are taken out of the blood by pressure filtration. Peritubular capillaries also surround the nephron so substances can be added and removed from the blood. The renal pelvis takes away the waste, or urine, through the ureter. The ureters lead the urine into the urinary bladder. From there urine is expelled through the urethra and removed from the body. (LW) (3)

Mammals have a four-chambered heart, an efficient respiratory and circulatory system to support their endothermic way of life (ability to maintain their own heat without the help of the environment). The four-chambered heart consists of the right and left atrium, which are chambers the receive blood returning to the heart, and the right and left ventricle, which are chambers that pump blood out of the heart. Arteries function to carry blood away from the heart to organs throughout the body, while veins return blood to the heart. As the right ventricle pumps blood to the lungs via the pulmonary arteries, oxygen and carbon dioxide are exchanged with the air in the lungs. The oxygen-rich blood from the pulmonary veins flows from the lungs to the left atrium of the heart. From there, then the oxygen-rich blood flows into the left ventricle. Blood leaves the left ventricle from the aorta, the major artery that is responsible for supplying blood to the body. This continuous cycle ensures that mammals have a constant flow of oxygenated blood to essential organs.

A typical 4-chambered mammalian heart. (ZXU)

Self protection
Mammals have different mechanisms to protect themselves. Their immune system is an involuntary mechanism of self protection. All mammals have complex immune systems that control and eliminate pathogens, like bacteria or viruses. A voluntary method of protection is passive or active defense against predators. Mammals have various ways to defend themselves by using their teeth, claws, fangs, or poisons. In addition, mammals use their senses that help locate and indentify predators or keep out of trouble. For example, mammals use pain receptors to determine when they are in danger.

An atypical characteristic pertaining to mammals is fur/hair. Besides temperature balance, fur plays a major role in the self-protection of many mammals. Fur is primarily used by mammals as a method of camouflage; a way to blend into the environment so predators will not see them. This can range from the monochrome coat of a rat to the striped pattern of a lemur. Fur is also especially useful as it falls out and gets replaced. This allows some mammals to go through seasonal changes- such as the arctic fox, who can change from brown in the summer to snow white in the winter, so it can blend in at all times. Fur also can play a role as a warning signal- the bright coloration of the skunk warns predators of the mammal's foul odor gland. Finally, in rare cases, fur can act as a physical protector for certain mammals, such as the hedgehog or porcupine, where thick spikes develop to hurt any predators who try to harm them. (TM)(10)

Osmotic balance
The mammalian kidney functions to conserve water. The nephron, the functional unit of the kidney, consists of a ball of capillaries, called the glomerulus, and a long tubule that ends with a cup-shape swelling called the Bowman’s capsule. One of the regions of the nephron, the proximal tubule, functions primarily to reabsorb salt and water from the filtrate. As salt diffuses into the cells, the cellular membrane actively transports the sodium ions into the interstitial fluid (the fluid between the cells). Simultaneously, the chloride ions passively transport out of the tubule. As the salt moves into the fluid, water moves out of the filtrate and into the interstitial fluid due to osmosis (diffusion of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration). Next, the salt and water diffuse from the interstitial fluid into the capillaries. More reabsorption of water takes place as the filtrate moves into the descending limb of the loop of Henle, another region of the nephron. The filtrate continues to lose water do to the increasing osmolarity as it travels from the cortex of the nephron to the medulla. Next, the collecting duct carries the filtrate through the medulla to therenal pelvis, where more water is reabsorbed into the cells due the high osmolarity.
The kidneys have the ability to conserve or excrete water depending on the amount of water the mammal intakes. Due to the high levels of osmolarity, kidneys are able to regulate the amount of water through the excretion of urine.

Temperature balance
Mammals generally maintain body temperatures within a narrow range of 36-38 °C. Mammals are endothermic, which means that their bodies are warmed by heat generated by metabolism. Heat production is increased by muscular activities such as moving or shivering. In some mammals, certain hormones trigger mitochondria to increase their metabolic activity to produce heat instead of ATP, called nonshivering thermogenesis (NST). Some mammals also have a tissue called brown fat that is specialized for rapid heat production. Other adaptations include insulation from hair and fat layers, which help mammals retain metabolic heat by reducing the flow of heat and lowering the energy cost of keeping warm. Heat loss occurs faster in water than in air, therefore marine mammals need blubber, a very thick layer of fat, to insulate themselves. When mammals are in warm environments above their body temperature, evaporation prevents their body temperature from rising. Most terrestrial mammals have sweat glands and other mechanisms to cool themselves, like panting, bathing, and spreading saliva and urine on their body surfaces. The several efficient ways that help regulate body temperature enable mammals to perform a variety of high-energy, intense activities in a range of temperatures and environments.
Hibernation is done in order to conserve energy. When animals hibernate, their metabolism slows down, decreasing the overall body temperature. Small mammals such as rodents and squirrels hibernate during the winter to prevent a shortage of food and harsh winter temperatures. (SR) (21)

Review Questions
1. If the climate all of a sudden changed, and the temperature decreased 20 degrees below room temperature, how would a mammal's body react, in contrast to that of a cnidaria? (YA)
2. List the parts of the nephron in order and explain what each part does. (SM)
3. In what ways do humans differ from mammals? [MS]
4. Why is fur a mechanism for self protection? How else are mammals able to protect and preserve themselves? (SD)
5. Describe the Heart, its chambers and the pathway the blood takes through it.(MLK)
6. What are the major differences between the three major types of mammals. (LJ)
7. How does the nervous system provide self protection for mammals? (RG)
8. Explain the process of food uptake and digestion in the digestive system of mammals. Mention the anatomical structures and their functions as they contribute to digestion. (DB)
Reece, Jane B., and Neil A. Campbell. Campbell Biology. San Francisco: Pearson Benjamin Cummings, 2009. Print.
2. (ZXU)
4. (CSR)
5. (")
6. (ZS)
7. (ZS)
8. (GR)
9. (MF)
10. (TM)
11. (MC)
12. (ORS)
13. (NG)
15. (MP)
18. (AC)
20. (AK)
21. (SR)