Chordata Mammalia are vertabrae. There are currently over 4,500 known species in the world today.


  • The Characteristics of Mammals

Mammals are warm-blooded vertebrates characterized by mammary glands in the female. The word “mammal” comes from the Latin word mamma, which means breast. All mammals have hair, which is made from keratin, and are endothermic. Because of this, most mammals have a high metabolism. All mammals have four-chambered hearts and breathe air (cannot obtain oxygen from water). They have three middle ear bones, the malleus, incus, and stapes, which helps conduct sound from the outer ear to the inner ear.

Mammals have seven cervical vertebrae, and an extended gestation period of uterine development after fertilization is common in placental mammals. Mammals have a well developed brain, a diaphragm to seperate the thoracic cavity from the abdominal cavity, and teeth embedded in the jaw bone. They evolved from the therapsid ancestors from the Triassic period. (JP- Source 4)

All mammals have hair that covers the entire body. Mammals are milk producers that provide milk for their young. Milk is full of nutrients essential for the growth of large organisms. Mammals have modifications in their skull, for example the secondary pallets, that allow breathing from the nose while the mammal is eating. (AR)

There are three sub-classes of mammals:
Subclass Eutheria – These are placental mammals, or mammals who complete their embryonic development within the uterus that is joined to the mother via the placenta. Most mammals fall under the classification of Eutheria, including humans.

Subclass Marsupials: These are mammals that are born before finishing embryonic development, and instead complete development in a pouch while nursing. An example of this would be the kangaroo.

Subclass Monotremes: These are the only mammals that lay eggs. Although they legs eggs, they still produce milk and have hair. The only monotreme mammals are the platypuses and the echidnas.

Within the subclasses are about 26 orders. These orders are used to classify each species.

There are three main groups of Mammalia:

Prototheria: Members of this group have females that lay eggs rather than give birth to live young, a common mammalian characteristic. This group is all in the order, Monotremeta containing the families: Tachyglossidae, spiny anteaters and echidnas found in Australia and New Guinea, and Ornithorhynchidae, containing only one species, the platypus.

Metatheria: These mammals give birth to altricial, undeveloped, young that grow in the pouch on the female enclosed in mammary glands. Mammary glands are the organ in mammals that produces milk. The young can drink the mother's milk in a safe environment, hinting that these animals produce few young and give great care to their young. The most common member of this group is the marsupial that is only found in the Australian and American region of the world.

Placentalia: These mammals give birth to live young. The mother is connected to the young through an umbilical cord before it is born. This group has the most species of animals including all mammals found in Africa. (AR) Placental mammals are nourished before birth in the mother's uterus through a specialized embryonic organ attached to the uterus wall, the placenta. The placenta is derived from the same membranes that surround the embryos in the amniote eggs of reptiles, birds, and monotreme mammals. (RW)

  • Acquiring and Digesting Food

All mammals are consumers. The digestive system consists of the alimentary canal and various glands that secrete digestive juices into the canal through ducts. A rhythmic movement of contractions pushes the nourishment along the tract. Valves, called sphincters, regulate the passage of material between chambers of the canal. The accessory glands consist of the salivary glands, the pancreas, the liver, and the gallbladder.


After the food enters the stomach through the oral cavity, the process to break it down is started through both chemical and physical changes. Teeth, which include canines, incisors, and molars, break apart the food simultaneously with the salivary glands, which are at work chemically breaking down carbohydrates. From the oral cavity the food goes to the pharynx, the junction between the esophagus, which leads to the stomach, and the wind pipe (trachea), which leads to the lungs. A part called the epiglottis blocks the trachea so that no food goes into the lungs. After going down the esophagus, the food enters the stomach, which performs preliminary digestion and stores some nutrients. From there the food goes through the pyloric sphincter to the small intestine. Here, most digestion and nutrient absorption happens. The remaining food passes another sphincter into the large intestine, which absorbs water. From there the remaining product goes to the rectum. The rectum stores waste and is able to expel it from the body as feces.

·Sensing the environment

All mammals have five senses; touch, taste, smell, hearing and sight. Senses are transmitted to the nervous system through nerve impulsive, or active potential. Mammals interpret touch as pressure, stretching, and motion. Taste is interpreted by the tongue, which can distinguish between four different impulses (flavors); bitter, salty, sweet and sour. The sense of smell is found in the nose. Olfactory receptors, the nerves that transmit messages to the brain, are found on the upper lining of the nasal cavity. Sound is able to be interpreted via the ears, which have three main parts: the outer, the middle, and the inner ear. Within the middle ear are the three bones that is one of the three major defining characteristics of mammals. The stimuli taken from the sound waves is transduced, or translated from physical sound into chemical patterns the brain can understand in the inner ear. Sight is located in the eyes. The eyes capture light, which is then sent to photoreceptors to translate into messages the brain can interpret.

Some mammals have specialized hair, whiskers, that give the animal a sensory ability which allows them to know its external environment. These hairs are extremely sensitive and have muscles at their bases which control their position. (TB)

The mammalian nervous system is divided up into two parts, the central nervous system and the peripheral nervous system. The Central Nervous system includes the brain and spinal cord, and sends motor signals to various parts of the body. The peripheral nervous system includes all neurons outside of the brain and spinal cord, and sends signals about the environment to the brain. (PS Source 12)

  • Locomotion
Crawling, swimming, running, walking, hopping, and any combination of the examples given achieve locomotion. One species of mammal, the bat, even has the ability to fly. Locomotion occurs by the mammal being able expend enough energy to overcome the force of friction and gravity. Movement is made possible by muscles contracting against the skeleton. Calcium and regulatory proteins controls the muscle contractions and therefore movement. Because of their general build, mammals are able to travel much greater distances than most other organisms.

Terrestrial mammals can travel more often and greater distances because of the mechanics of how they walk. Mammals move their legs backwards and forwards underneath their bodies. Mammals also use their tendons and backs to store and release energy for running. Cheetahs even dislocate and relocate their backs with every stride. Mammals have the most efficient methods of walking and running. (JS)

  • Respiration
The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. Breathing is largely driven by the muscular diaphragm, which divides the thorax from the abdominal cavity and forms a dome with its convexity towards the thorax. Contraction of the diaphragm flattens the dome increasing the volume of the cavity in which the lung is enclosed. Air enters through the oral and nasal cavities; it flows through the larynx, trachea and bronchi, and expands the alveoli. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing Mammals take oxygen into their lungs, and discard carbon dioxide. The oxygen is absorbed through alveoli. The alveoli are wrapped in capillaries, or tiny blood vessels, and therefore can easily absorb the oxygen into the blood stream and dispel carbon dioxide.

To further explain, alveoli are gas sacs that form the functional gas-exchange surface area of the lungs. (Human lungs have approximately 300,000,000 alveoli.) Therefore, the elevated metabolic rate of mammals can supported by sufficient amounts of gas exchange. (MB)

·Metabolic waste removal

Waste is removed from the mammalian body in the form of a liquid nitrogenous waster compound (urine) and a harder waste called feces. Both urine and feces are the body’s way of removing products that cannot be used. Another waste product of the body is carbon dioxide, a product of respiration.

Carbon dioxide is the waste gas that is produced when carbon is combined with oxygen as part of respiration. The lungs and respiratory system within a mammal allow respiration to occur by exchanging oxygen taken in from the environment for carbon dioxide being breathed out as waste. (CP source 3)

  • Circulation

The Mammalian Circulatory System. Note that red blood is oxygen rich, while blue blood is oxygen poor. (ZJ)
The circulatory system works only with that assistance of other systems within an animals body. It is based off the mammals four-chambered hear system. The heart is divided into two different types of heart chambers. The first is the atrium, which receives blood returning to the heart through the veins. The right atrium pumps blood to the right ventricle, and the left atrium pumps blood into the left ventricle. This blood is then pumped from the atrium into the second chamber called the ventricle. The ventricles are much larger than the atria and their thick, muscular walls are used to forcefully pump the blood from the heart to the body and lungs. The circulation system transports materials throughout the body to allow it to function properly, such as oxygen to the lungs, and CO2 transported out of the lungs as waste. This system also helps transport nutrients throughout the body. One example of this is ammonia, a product of digestion, is transported to the liver, where it is made less toxic urea, which is then transported to the kidneys and is excreted as urine. ATP is also a product of the circulatory system, when glucose is added to the body. As soon as glucose is digested it is transported to the liver and by this process the body is able to maintain a stable level of glucose in the blood.
  • Self protection

Hair covers the body that provides insulation from cold temperatures. It allows polar bears to live in Antarctica. (AR)
Some mammals, such as skunks, are defended by chemical feeding deterrents, which prevent predators from approaching or harming them. Many mammals have sharp teeth, antlers, hooves, or or horns to fend off predators. (SP)
Other mammals have distinctive colorings on their fur, which - when running in groups - breaks up their shape and makes them harder to recognize. (AM)

Mammals have developed a variety of adaptations in order to protect themselves in the varying environments they live in. Mammals in cold climates, such as bears and whales, have insulating layers such as thick coats of fur or layers of fat (blubber). These layers helps mammals retain body heat and keep body temperatures constant. Some mammals living in deserts survive by special adaptations in their kidneys and sweat glands which allow survival even when their are very small amounts of water available. Other adaptations include hibernation (sleeping during the winter) or estivation (summer sleeping) in order to help mammals conserve energy as food supplies become scarce. (VN)

  • Osmotic balance
The ability of a mammal’s body to maintain a balance within itself is done using many highly complex interactions between systems within the body that must function perfectly for everything to work. This delicate balance is essential to survival. Systems within the body such as the liver, kidneys, and other parts of the endocrine system help to maintain this constant level of homeostasis. The liver takes any toxic substance that is put into the body, and maintains carbohydrate metabolism, while the kidneys regulate the blood water levels within the body, pH levels in the blood, and much more. 

The mammalian kidney is a prime example of osmoregulation and homeostasis through the excretion of waste. The mammalian kidney is made of functional units (the individual parts) called nephrons. In the circulatory system (the kidneys), blood is passed through and then filtered to remove the extraneous material. This serves as a purification method but more importantly, it serves as a way to regulate the osmotic balance through urine (which is the material that is filtered out of the blood). In addition, during this process, the urine is hypotonic relative to the blood plasma (which means that water is reabsorbed from the urine before it is excreted from the system. (RK)

  • Temperature balance

All mammals’ temperature regulation is the balance between heat production from metabolic sources, and heat lost from evaporation, and other such processes. When a mammal is in a cold environment the body heat is conserved at first by constricting the blood vessels near the surface of the body, shivering may be a result of this as the waves of constricting blood vessels travel through the deeper muscles in the body. In a warm environment, the body blood flow is increased to the surface, to try to dissipate the heat out of the body.

In harsh climates mammals have the ability to use thermoregulation which helps the body maintain its own homeostasis. All mammals have neurons that are temperature sensitive and nerve endings all around the body. The body’s hypothalmus acts like a thermostat that receives messages from all the thermoreceptors around the body to control the temperature. An example of this is the desert rat which uses thermoregulation to reduce the resting metabolic rate. This in turn helps the desert rat be more efficient in conserving its water and energy levels. (AP-source 6)

Mammals are a part of one of only two classes of vertebrates, the other being that of birds, that are able to regulate and keep constant their body temperatures through physiological instead of behavioral means. The former method of temperature regulation is also formally referred to as endothermy, while the latter method of temperature regulation is formally referred to as ectothermy. (MR; Source 17)

Methods of Thermoregulation (SP)
Methods of Thermoregulation (SP)
Description of how Hypothalamus Preserves Thermoregulation (SP)
Description of how Hypothalamus Preserves Thermoregulation (SP)

The Reproductive system

All mammalian reproduction is through sexual reproduction. The male has a phallus and testicles that produce and delivers the sperm, while the females have a uterus. Through direct insertion of the phallus into the females vagina, semen is deposited. The fertilized egg develops inside of the mother. In marsupials, after a short gestation period the young are born. Still undeveloped, they finish development in a pouch attached to the mother. In placental mammals, the young are born more developed. They go through a longer gestation period and are nourished through the presents of a placenta. The placenta acts as a median allowing nutrients and waste transfer between the mother and the fetus. Once born, the young are fed through the mother's milk, which is secreted by mammary glands that produce milk. A young mammal is eventually transitions off of the mother's milk as they begin to consume solid food. (MS 15)

Review Questions:
1) What are the three main types of mammals and how do they differ? (MM)
2) What is the main system used to in the removal of metabolic waste and what are the other function for the those organs?(RJ)
3) Please describe the circulatory system in mammals. How does this aid in cellular respiration and excretion? (Matt B)
4) Explain how a mammalian body works to create a balance or how homeostasis is maintained. (IL)
5) Describe three key characteristics of mammals. (CM)
6) Relate the peripheral and central nervous systems to thermoregulation and other tasks the mammal undergoes. (LPE)
Sources (TB) (MB)

3. "Lungs and Respiratory System." KidsHealth - the Web's Most Visited Site about Children's Health. Web. 29 Oct. 2011. <>. (CP)


5. (AR)

6. Goli, Roberta. "Thermoregulatory Adaptations to Heat in Mammals: How Desert Animals Maintain Body Temperature in Harsh Climates |" Roberta Goli | Web. 30 Oct. 2011. (AP)

7. Breed, Michael D. "CHAPTER NINE." Animal Behavior Online. Web. 30 Oct. 2011. <>. (SP)
8. (AM)

9. (JS)
10. "Thermoregulation." School Work Helper. St. Rosemary Educational Institution. Web. 06 Nov. 2011. <>. (SP)

11. "Mammals."UXL Encyclopedia of Science. 6, 2011) .


13. (ZJ)

14. (RK)

"Reproduction in Mammals." The Earth Life Web. Web. 13 Nov. 2011. <>.

17. "Mammal." Biology. Ed. Richard Robinson. New York: Macmillan Reference USA, 2009. Gale Science In Context. Web. 27 Nov. 2011.