Reproduction, Development, Respiration, Transport, Nutrition, Excretion, and Regulation.

The Class Mammalia is home to around 5,000 species of mammals, all of which are placed into 26 seperate orders.
Orders are determined by reproduction, development, and evolutionary history.
House Mouse
This tiny mouse and this enormous whale are both mammals.
Humpback Whale

But what defines a mammal? The answer, is very simple.
Mammals are members of the Kingdom Animalia and the Phylum Chordata. Mammals are dictinct from other classes of chordates because of their reproduction. All mammals secrete milk from specialized sweat glands used to feed their young.
A spinal column showing a spinal cord

Chordates are all animals that have a nerve cord sometime in their life.


Most common of mammals, are the Placental mammals, which include nearly 4,000 species of known mammals.
Placental mammals all bear live young, which are nourished before birth in the mother's uterus through a specialized embryonic organ attached to the uterus wall, called the placenta. The embryo develops over a long gestation period, and emerges from the womb fully formed. Monotremes and Marsupials have placentae as well, but they have little function in these different mammals.
Human fetus inside Placenta
African Lion (Placental Mammal)

Marsupials are the group of mammals commonly thought of as pouched mammals, but the pouch is more complex than most think. This sub class is home to 260 species of known marsupials. Like most other mammals, marsupials give live birth, but have much shorter gestation periods than placental mammals. Instead, they give birth much earlier and the young animal, pretty much a helpless embryo, climbs from the mother's birth canal to the mother's pouch. Inside of the pouch are the mother's nipples, to which the embryo attaches to and remains until it is a mature juvenile.
Sugar Glider (Marsupial)

The rarest of all mammals, are the Monotremes. Home to only three species, monotremes are the only known mammals to give birth to their young in eggs. Like reptiles, monotremes lay their young in eggs, but unlike reptiles the eggs are retained for some time within the mother, who actively provides the egg with nutrients.The Duck-Billed Platypus and two species of Echidna are found only in the continent of Australia.
Short Beaked Echidna (Monotreme)


Human Embryo at six weeks
As like all chordates, mammals begin their lives as embryos. Mammals take this one step further, by developing their offspring in a placenta. When sperm cells from a male mammal enter the ovary of a female mammal of the same species, the sperm cell fertilizes the egg cell and forms a zygote. The zygote then begins to go into mitosis, rapidly growing until the zygote becomes an embryo. In humans, after eight weeks of gestation, the embryo is called a fetus. The unborn offspring is called a fetus until its birth. All mammals in embryo form have a set of gills, a tail, and pharyngeal pouches. However, all mammals lose their gills as they grow, and the pharyngeal pouches are replaced by lunds and other organs as the embryo matures.
Human Embryo
The fetus of most mammals develops similarly to the Human fetus. After the first stages of development, the human embryo reaches a stage very similar to all other chordates.
Note how these animal embryos all resemble each other, even the mammal embryos share similarity.


The lungs of mammals have a spongy texture and are honeycombed with cells, having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung.
Breathing is largely driven by the muscular diaphragm at the bottom of the thorax. Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward. Air enters through the oral and nasal cavities; it flows through the larynx and into the trachea, which branches out into bronchi. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the diaphragm contracts, forcing the air out more quickly and forcefully. The rib cage itself also is able to expand and contract to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient.
The lungs of mammals are the most efficient lungs evolved on planet earth because the can extract much more oxygen from much less space than any other animal.
The Human (Mammal) Respitory System

TRANSPORT (Circulation)

Another advantage that mammals have over other animals is their highly advanced circulatory systym. A mammalian heart has four walled chambers, which efficiently pump blood to the lungs, to the body, and back to the heart.
A Human Heart, showing the Lungs
How does blood flow through the body of mammals?

Well, the heart pumps the blood through vessels called veins or arteries. The heart pumps blood through arteries, such as the Aorta, to the lungs. Other veins bring the newly oxygenated blood back to the heart. When the oxygenated blood gets to the heart, the heart pumps the blood through arteries flowing throughout the body. Veins then bring the unoxygenated blood back to the heart, and the cycle starts again. This has happened in just one heartbeat.
A Human Heart, showing all four chambers


Mammals are one of only two classes to be considered Endotherms. Being an endotherm means that you do not need to waste precious time basking or collecting heat energy because your body heat is produced internally. This is not all of a blessing to mammals. Thanks to our internal "furnace," mammals need to consume much more "fuel" than other animals to keep their temperature regulated.

The mammalian diet is a vast variety of food, containing everything from grass to other animals.

Some mammals are herbivores, which means that their diet consists only of plants and plant material.
The herbivorous diet is the most common diet found on Planet Earth.

This African Elephant is an Herbivore

Other mammals have evolved to a wider range of dietary needs. Omnivores also consume plant material, but also other animals as well.
We humans are omnivoires, and our specialized teeth are examples of our needs.
This American Badeger, is an Omnivore

Some mammals have evolved to the point that they no longer eat plants, but only other animals. These mammals are called Carnivores.
All cats and dogs are carnivores. Carnivores have a much faster metabolic rate than other mammals.
This Bengal Tiger is a Carnivore

The size of mammals also has an affect of their diets. The larger the mammal, the more they need to eat to keep going. An example is the Blue Whale, who has to eat more than one ton of Krill each day to survive.
Blue Whale


When a mammal eats, it must deficate to release all of the undigested food, or it will die.
Most mammals have similar digestive tracts to humans.
The Human Excretion System

A mammal's diet also contributes to how the mammal digests.
This Cow has four stomachs due to the rough grass it eats.


The mammal needs not only to remove wastes my means of excretion, bust also by means of its Lymphatic System. Over three liters of fluid leaks from the circulatory system each day, and it's the lymphatic system's job to collect and replace these fluids whil still filtering out bacteria and other waste products.

The Lymphatic sysetm is network of vessels, nodes, and organs that collect fluid that is lost by the blood and returns it back to the circulatory system (see above for circulatory system). The fluid found in the lymphatic system is called Lymph. Lymph collects into small lymph capillaries, which flow into much larger lymph veins. The collected lymph the travels through the veins and then goes to two openings in the Superior Vena Cava, thus returning the fluid back to the blood stream.

Along the lymph veins are small bean shaped objects called Lymph Nodes. When the lymph flows through the nodes, bacteria and microorgansims get trapped in tiny "filters" contained in the nodes. This cleans the lymph for re-entry into the blood stream.

Lymph vessels are also helpful in nutrient movement. Fatty Acids and Glycerol are pulled into the lymph vessels and transported throughout the body.

The Spleen is an important part of the lymphatic system. It stores Phagocytes that engulf and destroy harmful bacteria.
The Human Spleen (Lymphatic System)

The endocrine system is an integrated system of small organs which involve the release of extracellular signaling molecules known as hormones. The endocrine system is instrumental in regulating metabolism, growth and development, tissue function, and plays a part also in mood. The Pancreas is a major part of the endocrine system because it releases hormones into the circulatory system. Blood Plasma also plays a large role in moving hormones throughout the body.
The Pancreas (Endocrine System)

The Nervous System
All mammalian brains possess a neocortex, a brain region that is unique to mammals.
The Central command center of the mammalian nervous system is the Brain. The brain controls all of the functions of the body by sending electrical impulses throughout the nervous system.
The Human Nervous System

The Brain

The mammalian brain is the most evolved brain in the Animal Kingdom.
There are many more lobes of the brain in mammals than in other animals. Mammalian brains also contain a region called the "Reptilian Brain." This small portion of brain is located near the back of the brain. It controls all of our instincts.
Human Brain
Advanced Mammalian (Human) Brain

Hopefully this website will prepare you for the upcoming EOC in Biology.
Good Luck!