This poster displays the different sections of mammals that exist in our world today. The first section depicts the monotremes, or egg-laying mammals, which include echidnas, playtupuses, and the spiny anteaters. To the right of that are marsupials, or mammals that bear offspring at an early stage of development and allow the offspring to grow outside the parent's body. The rearing of the offspring often takes place in the characteristic pouch of the marsupial, displayed most prominently in the kangaroos. The last category, which contains most mammals, are the placental mammals, or mammals whose offspring develop inside the parent. (2 AS)

Mammalia contains a vast diversity of forms. The smallest mammals are shrews and bats, which can weigh as little as 3 grams. The largest mammal (and largest animal on Earth) is the blue whale, which can weigh 160 metric tons (160,000 kg). (AW 7)

Diagnostic Characteristics: The class mammalia is defined as having mammary glands, which all mammalian mothers use to nourish their babies and nourish babies with the milk they make. Mammals are also distinguished by hair, warm-blooded (endothermic) bodies, active metabolisms, four-chambered hearts, and efficient circulatory systems. Most mammals are born, rather than hatched, and have considerably larger brains than other vertebrates of similar sizes, making them fast learners. Furthermore, mammals have teeth in a variety of sizes and shapes, unlike their reptilian ancestors, for eating different kinds of foods: incisors and canine teeth for shearing, and premolars and molars for grinding. Mammals' lower jaws consist of only one bone whereas all other vertebrates have more than one bone in their lower jaw bone. (EK) Lastly, mammals are distinguished by inner ear bones which, in ancestors, had made up the jaw joint. Mammals have three middle ear bones (CH). Mammals are further divided into two sub divisions: Prototheria and Theria. Prototheria includes monotremes (platypus and some species of echidna) and Theria includes all other mammal that give birth to live young. (EK22)

The video below provides basic information regarding the characteristics of mammals (5 MB).

Acquiring and Digesting Food: Mammals can be carnivores, herbivores, or omnivores. Herbivores graze, and meat eaters either hunt animals alone or in packs. In digesting food, mammals break up the food physically and chemically into its elements, which diffuse into the bloodstream, and feed the cells of the body. It starts in the mouth, where food is consumed. The food is chewed, and physically broken down. At the same time, salivary amylase, an enzyme, starts to break down carbohydrates in the food. After that, the food is swallowed, and passes down the esophagus, which uses rhythmic muscle movements (called peristalsis) to send the food to the stomach, which uses acid to break the food down further. After the stomach, the food passes into the small and large intestines, where nutrients and water, respectively, are absorbed into the bloodstream. Finally, the solid waste is excreted via the anus.

Sensing the Environment: Animals have five basic senses: taste, touch, sight, hearing, and smell. Some work better than others for each particular mammal. Special senses include sensing the magnetic field of the Earth, as some animals use for migration. Also, mammals like bats and whales use echolocation, a system involving emitting high pitched sound, and using the amount of time the sound takes to echo to establish location in relation to physical objects. This is the same idea that the military uses for sonar. Other mammals, such as cats and dogs, have vibrissae, or whiskers, which are specialized hairs that make the animal aware of it's contact with it's environment (EG 10).

Locomotion: Mammals use limbs, in the form of arms, legs, fins, or wings, to move from place to place. These limbs are easily rotated, and can be used by mammals to walk, jump, swim or fly. There are many different types of ways that mammals use their legs to move (9HL):

  • Diagonal walk- using diagonally opposing legs, front left with right back and front right with left back (9 HL)
  • Giraffe walk (and a few other animals, such as camels)- use both legs on one side and then both legs on other side (9 HL)
  • Leaping- sometimes with two legs and sometimes with four (9 HL)
  • Gallop- succession of leaps (9 HL)
  • Crawling- most sea mammals crawl, such as seals (9 HL)
  • Walking on two legs (9 HL)
  • Swinging- some arboreal mammals, especialy primates, commute through the forest canopy by swinging from branch to branch. (8RM)
  • Flying- the only mammals who truely fly are bats, members of the order chiroptera. As their name sugests (chiro-(greek)hand, ptera-(greek)wing), chiropterans' wings are formed from the bones which compose the hands of other mammals. (8RM)
  • Swimming- members of the Sirenian (sea cows such as manatees) and Cetacean (whales, dolphins, and porpoises) orders evolved specialized flippers and body shapes because they spend all their time in the water. (8RM)
  • Gliding- a few forest dwelling mammals such as the flying squirrel, sugar glider, and some lemurs evolved the ability to increase the range they can jump from one tree to another by extending flaps of skin between their fore and hind limbs. (8RM)

Skeletal System (KA):
All mammals are vertebrates, meaning they have an internal bony support to which muscles and ligaments attach and use the bones as levers for movement. An important structure of the skeletal system is the spinal column, a series of small bones that have one or more holes through the center, which houses and protects the spinal chord, a long sheath of nerves and ganglia that extend down the body plan. The vertebrae of most mammals can be divided into five regions:
1. Cervical Region:
The neck region of the animal. For digging and/or swimming mammals, these vertebrae are often cemented together for increased strength and support.
2. Thoracic:
The bones from which the ribs extends. These vertebrae usually have dorsal spines that help support muscles that are responsible for lifting and controlling the neck and head.
3. Lumbar:
Where the back legs connect to the spinal column. These vertebrae usually have numerous spines.
4. Sacral:
Supports the pelvic girdle.
5. Caudal:
These are the bones of the tail that are generally smaller and less complicated than the other vertebrae. This region does not contain the spinal chord, but does have some nerves and blood vessels.
*In humans and chimpanzees, the caudal vertebrae have been reduced to four and are fused to form the coccyx.

Integumentary System: (15 AN)
the integumentary system, or the organ system that protects the body, is very specific to mammals. It is comprised of the epidermis (the surface layer), the dermis (the middle layer), and the hypodermis (the lowest layer). The epidermis is the thinnest layer, and is constantly losing its uppermost cells. Its main purpose is to "waterproof" the body. The Dermis makes up bony structures and blood vessels, as well as hair, which is a defining characteristic of mammals. The hypodermis is made of adipose tissue and it stores lipids and provides cushioning. (15 AN)

Respiration: Mammals use lungs for respiration, which are made up of branching ducts and have a spongy texture, and have a two cycle breathing system: intake, and respiration. In intake, air flows in through the mouth or nose, and down into the trachea, or windpipe. The trachea branches into two bronchi, leading into each lung. From there, the bronchi branches into finer tubes called bronchioles, and then eventually into tiny little air sacs called alveoli. It is on the membranes of the alveoli that gas exchange occurs. This is because, in the alveoli, there are extremely thin blood vessels, called capilaries, where gas exchange occurs (9T2). Around the alveoli capillary, or small blood vessel. When air is breathed in, oxygen from the air diffuses into the blood across the alveoli membranes, and carbon dioxide dissolves out of the blood into the alveoli, to be exhaled as waste. Breathing itself is accomplished when the diaphragm and inter-coastal muscles contract. This increases the amount of space in the thoracic cavity (the part of the chest where the lungs reside), decreasing the pressure in the chest, and causing a pressure gradient. Air from outside the body rushes in, filling the lungs. When the diaphragm and intercoastal muscles do the opposite, mammals exhale.
Human respiratory system

Metabolic Waste Removal: Cellular Respiration creates many waste products, which diffuse into the blood, including carbon dioxide and nitrogenous wastes. The CO2 diffuses out of the bloodstream during respiration. In mammals, nitrogenous waste takes the form of urea (SW 6). It is filtered out of the blood by the kidneys, and excreted in the form of urine. Structures in mammals that remove this waste are the kidney, ureters, urinary bladder, and urethra (SW 6). The ureters are muscular ducts that transfer urine from the kidneys to the urinary bladder; the urinary bladder is a hollow, muscular, and elastic organ that collects urine; and the urethra is a tube that connects the bladder to the outside of the body (SW 6).

Circulation: Because mammals are vertebrates, they have closed circulatory systems of, also called cardiovascular systems. Blood, which carries all the nutrients the body needs, flows in a circuit, and is pumped by a four-chambered heart. Blood vessels called arteries take blood from the heart to capillaries, while veins do the opposite. Circulation begins when the heart pumps blood to the capillaries next to the alveoli of the lungs. The blood is oxygenated and rid of carbon dioxide through diffusion, and veins bring the blood back through the heart. Then the blood is sent by artery to capillaries throughout the body. Oxygen diffuses into the cells of organs and CO2 out of the cells, and the blood is sent back to the heart, to start the cycle over again.

Self Protection: Mammals have many different means of self-protection. These include claws, antlers, poisons and horns to defend themselves. Many, like the skunk, emit a foul odor to keep animals away. Often, animals will herd together, as herds make it hard for predators to corner and kill an individual animal. Mammals also display cryptic coloration in their fur to either help avoid detection or predation. Some species, especially that live in arctic regions, change from brown to white as a result of either the change in temperature or photocycle affecting the protein production for the hair (DPOD 16).

Osmotic Balance: If a mammal has excess water, it will get rid of some by urination. The kidneys of an animal regulate the amount of water that is in an animal's urine. (14 AL) Depending on whether an animal is dehydrated or over hydrated, the kidneys will either conserve or excrete water. (14 AL) If a mammal is dehydrated, it will get thirsty and seek hydration. Salt water will not help with dehydration, so marine mammals must maintain osmotic balance by consuming food and subsequent metabolism, since they also do not have a specialized gland for secreting salt. (J. Sun 21)

Temperature Balance: Mammals generally have a body temperature of 36-38ºC, which is usually hotter than the ambient temperature. Therefore, mammals tend to lose heat. They counteract this by producing heat by variable amounts of cellular respiration, based on how much heat is being lost. Also, moving or shivering can increase the amount of heat produced by the body. The Mammalian body temperature can vary between the core and the periphery. (2 VK) Mammals have temperature sensitive neurons and nerve endings in the brain, skin, spinal cord, and body core. (2 VK). Some animals have special hormones, which, in extremely cold climates, cause mitochondria to increase activity and produce heat instead of ATP. This is called Non-shivering Thermo-genesis. Another adaptive mechanism mammals have is insulation, in the form of blubber, thick skin, and fur. For example, land mammals react to cold weather by raising their fur, to keep in the air close to the skin, so that when the body gives off heat, and the air absorbs the heat, the heat is not lost. So me animals have evolved adaptations for cooling, such as panting, sweating, and bathing in water or mud.

Small mammals such as ground squirrels, rodents, and bats hibernate during the winter to avoid food shortages and the cold temperature. Hibernation is a way of conserving energy. As the animal's metabolism slows down significantly, the animal's body temperature decreases. Despite contrary beliefs, bears do not go into "true-hibernation" since their degree of metabolic depression is much less than that observed in small animals (CH 4).

Reproduction: All mammals reproduce sexually, and invest lots of time and resources into reproduction. Yet, their methods of reproduction are extremely diverse. Generally, mammals can be divided into three categories, based on their reproductive strategies:
1. Monotremes- platypus and echidnas. These animals are the only animals who lay eggs. The eggs are reptilian in structure and development, and contain enough yolk to nourish the embryos inside. After birth, the baby sucks on the mammalian glands, which unlike other mammals, have no nipples.
2. Marsupials- opossums, kangaroos, koalas, bandicoots. Babies are born while still embryos, underdeveloped and helpless. In most marsupials, the mother has a maternal pouch called a marsupium (from whence they get their name), which contains the still embryonic baby. For example, baby red kangaroos are born 33 days after fertilization, and are about the size of a honeybee, barely developed. The baby fully develops and is nursed inside the marsupium.
3. Eutherian Mammals- every other kind of mammal, including humans. These mammals have the longest period of pregnancy. Embryos completely develop inside the uterus and are connected to the mother by a placenta.

Mammals evolved from an ancestor that reproduced by making amniotic eggs, which have shells that retain water and nutrients. These eggs enabled the ancestor of reptiles, birds and mammals to reproduce on land. Though most mammals do not produce eggs now, the placenta inside the mother essentially performs the same function. (AR 5)

Review Questions:
1)Mammals are known to reproduce in different ways.List and explain the three categories in which mammals are grouped based on reproduction.(3DO).

2) What distinguishing features do mammals have that no other class of animals have? (8-SC)
3) What is the integumentary system and how does it protect a mammal's body? (J. Stein)
4. What five general regions can the vertebrate of mammals be divided into? (CS 10)


First image obtained from:
1. Goli, Roberta. "Thermoregularity Adaptions to Heat in Mammals." Anatomy and Physiology 22 May 2009. 25 Oct. 2009 <>. (2 VK)
2. "The Mammalian Skeleton." Web. 24 Oct. 2009. < mammals/skeleton.html>.
3. "ADW: Mammalia: Information." Animal Diversity Web//. Web. 25 Oct. 2009. <>.
4. Wikipedia. "Hibernation." 20 Oct. 2009. Wikipedia. 31 Oct. 2009 <>
5. "What is a mammal?" 31 Oct. 2009 <>
6. "Lecture 2 Outline: Ionic and Osmotic Balance." 2 Nov. 2009 <>
7. Wund, M. and P. Meyers. "Class Mammalia." Animal Diversity Web. 2005. 2 Nov. 2009 <>.
9. Harris, Tom. “How Animal Camouflage Works.” HowStuffWorks. 2009. 5 Nov. 2009 <>.