8.27.3 Lesson: Endotherms

  • Due No due date
  • Points 100
  • Questions 4
  • Time Limit None
  • Allowed Attempts Unlimited




H.1 – Identify the characteristics of Kingdom Animalia and its various phyla.


Air Sac

Air-filled space in the body of a bird that aids in breathing and in temperature regulation


Describes when young birds are born in an underdeveloped state and require extensive care from the parents


Thin branches or filaments that contribute to the composition of a bird feather


Behavior used by an organism to select a partner


Enlarged part of the esophagus that is used to temporarily store food


Muscular portion of a bird’s stomach that uses grit to grind food


Achieved when an organism maintains an internal environment at a stable equilibrium


Mammal that gives birth to young that are undeveloped and are typically carried in a pouch


Movement of animals from one region to another based on seasonal temperatures


Mammal that lays eggs


Describes when young birds are born in a relatively advanced state


Part of the stomach that partially digests food or cud

Temperature Regulation

Animals must maintain their internal body temperatures at specific levels to prevent under or over-heating. By doing so, their cells and organs continue to function properly. Homeostasis is achieved when internal conditions are relatively stable (in this case, body temperature). Consider the complexity of each bodily system and then account for the fact that each process requires just the right conditions to operate! For instance, enzymes work best at particular temperatures to speed up the rate at which metabolic processes and reactions occur within the cells of living organisms. For humans, this temperature is 98.6 degrees Fahrenheit. If enzymes are unable to catalyze reactions, the result is an uncompleted cycle or pathway without the needed product. Consequences include the potential for build-up of toxic chemicals and/or the depletion of ATP.

Recall that endotherms are animals that can regulate their own body temperatures; they do not rely on the environment to do so. Mechanisms within their bodies allow for the internal generation of heat, a process the requires much energy. Birds and mammals are examples of endotherms and are discussed in more detail below.

Class Aves

Class Aves is a group within phylum Chordata that includes birds. These organisms are all endothermic and maintain their body temperature independent of the outside environment. Compared to humans, birds have higher natural body temperatures, which enables them to live in a wide range of habitats: temperate, tropical, and polar.


The design of a bird’s body is remarkable, especially when considering how various structures serve several different purposes. Feathers, for instance, are a defining characteristic of birds. They function as structures for flight, attraction for mates, and as warmth and protection against outside elements. They also contribute to waterproofing for birds that swim, cushioning birds in nesting periods, and camouflaging birds for protection from predators.

There are several types of feathers:

  • Flight Feathers
    • Form the wings and tail of a bird
    • They are long and have strong barbules, or filaments, that give them strength for flight.
  • Down Feathers
    • Insulate birds by trapping air
    • Soft and fluffy
  • Contour Feathers
    • Give shape and color to the bird
    • Found everywhere except beak, legs, and feet

Others include semiplume feathers (also used for insulation), bristle feathers (stiff in structure and can be found around the eyes to function as eyelashes), and filoplume feathers (small in size and attached to nerve endings).

feet and legs

The lifestyle of the bird determines the structure of its feet and legs. Many birds have scaled, clawed feet on their hind legs with 3-4 toes; this structure is useful for birds that perch in trees or for birds such as chickens that scratch at the ground for food. There are various birds that spend much of their time in water, such as ducks, and instead have webbed feet. On the other hand, the ostrich is an example of a bird that cannot fly; therefore, a different means of escape from predators is needed. Thus, they were created with highly muscular legs that enable them to run swiftly.


Below is a diagram of a bird skeleton. Features include:

  • Beak: A modified upper and lower jaw
  • Keel or Breastbone: A flat, large structure used for the attachment of flight muscles
  • Clavicle (wishbone): Provides strength to withstand rigors of flight
  • Cervical (neck) Vertebrae: Create a highly flexible neck for flight and grooming



Beak and Digestive System

Birds have large energy needs and require a great deal of nutrients. However, birds that fly cannot be weighed down by the effects of heavy consumption; rather, the digestive system needs to be efficient and capable of quickly processing food to provide immediate energy to the bird while keeping it as light as possible.

A bird’s beak is shaped according to what the bird eats. It is used to acquire food that will be swallowed whole since bird’s do not have teeth. The food will then travel down the esophagus and into the crop, where it is temporarily stored before going into the stomach. Upon entering the stomach, it travels through two sections. The first section is called the proventriculus, where digestive enzymes begin breaking down the food. Next, the food enters the ventriculus, also known as the gizzard, which grinds the food with previously digested sand and stones. The food, now ground and coated with digestive enzymes, proceeds to the intestine for further digestion. Any undigested material leaves the body through the cloaca.


Respiratory System

The bird’s need for food and energy causes a complimentary need of oxygen. Since a bird’s lungs are not large, they need an alternative method of gaining oxygen molecules. This comes in the form of air sacs, which serve a similar purpose to that of lungs but are much lighter in weight.

Circulatory System

Birds have a four-chambered heart and a closed circulatory system. Blood that has been oxygenated is kept separate from deoxygenated blood. The heart rate of birds is relatively rapid in order to process acquired oxygen.

Reproductive System

The reproduction method of birds involves more than just fertilization. The process initiates with courtship when a male bird attempts to attract a female bird. He achieves this through use of songs or calls, or by displaying his feathers. In fact, most male species of birds are more vibrantly colored for the purpose of attracting female birds. The female bird’s dull colors aid in camouflage technique as she hides her nest while sitting on her eggs. Two cardinals are pictured below. The female is on the left and the bright red male is on the right. To impress the female, the male cardinal has just given the female a bit of food.


Sexual reproduction involves the production of sperm within the male testes and the production of ova (egg cells) in the female ovary. Sperm are transferred to the cloaca of the female and oviparous methods of birthing are used to produce offspring. The female lays shelled eggs that hatch outside of her body. The young can be classified into one of two categories: altricial or prococial stages. Altricial birds are the least developed and hatch after about 14 days. They are blind, featherless and helpless at birth. Typically, if the young are altricial, the mother will not lay a large number of eggs. This allows the parent birds to meet the responsibilities of caring for such dependent young. Precocial birds are more developed and hatch around 30 days after being laid. These young birds have a down coat of feathers, and though they still rely on their parents for survival, it is at a lesser level. Eggs of this type typically come in groups of larger numbers.

Interesting Behaviors

Seasons change, and with the changing temperatures, supply of food often fluctuates. Some birds overcome the limitation of food availability by migrating. Migration is the travel to a different geographic area on a seasonal schedule to reach warmer temperatures and gain food supply. God has given birds an instinctive sense of direction.

Watch the video below on bird migration.

Class Mammalia

Mammals comprise another distinct class of endotherms. Common characteristics of these organisms include a four-chambered heart, the coverage of the body with hair, and the female production of milk for her young.

Species Specific Traits

The type of hair that a mammal has depends on the environment in which the organism lives. Polar bears, for instance, have hair that is designed to keep them warm and to help them hunt by camouflaging them from their prey. Marine mammals, however, have very little hair. Dolphins are an example of mammals that have hair at birth but not in adulthood. In the oceanic environment, having hair at a mature stage of development would actually be considered a disadvantage, as it would create friction against the water while the organism swims.

Other structures that depend on the mammal’s environment include teeth and limbs. Types of teeth, similar to the beaks of birds, are connected to diet. The example below is of the human jaw, displaying the different kinds of teeth and their shapes. Since humans eat different types of foods, varied structure is required.


  • Incisors: Flat, front teeth used for biting
  • Canines: Pointed teeth at the front corners of the mouth used for ripping and tearing
  • Molars: Thick, back teeth used for grinding and chewing

Appendages, or limbs, are another component of mammals based on environment specificity. Animals that burrow and live underground, such as chipmunks, have short, muscular forelimbs. Animals that need to run quickly or jump to evade predators, such as kangaroos, have thicker hind legs.

Digestive System

Digestive systems have variability across mammal species as well. One of the most interesting adaptations is seen among herbivores, animals that eat plant materials which are full of cellulose. Though cellulose is indigestible to most animals, God has given herbivores a unique design to obtain nutrition.

Using a cow as an example, let’s follow the path that food takes through the digestive system:

  1. The cow chews and ingests the plant material.
  2. The plant material is swallowed and travels down the esophagus into the rumen, a section of the stomach that is filled with bacteria to digest cellulose.
  3. The cow regurgitates the cud (the partially digested plant material from the rumen) back into its mouth to continue chewing. Rechewing the material further mixes the plant material with digestive enzymes.
  4. The food is then swallowed again and passes through the cow’s long digestive tract.

Respiratory System

The respiratory system of mammals is composed of two lungs and a diaphragm. As the diaphragm contacts, air is drawn into the lungs. This process of inhalation allows for the supply of oxygen to the body. When the mammal exhales, carbon dioxide is expelled from the body.

Circulatory System

Hearts of mammals typically have four chambers: two ventricles and two atria. The system is closed, with oxygenated blood and deoxygenated blood kept separate at all times.


The process of fertilization is internal for mammals. In males, the testes produces sperm, and in females, the ovaries produce ova.

Most animals are placental mammals, meaning that young are developed inside of the mother and are nourished by the placenta. A placenta is a cord or a vessel within the mother that attaches to the offspring and serves in the exchange of gases and nutrients. Once sperm is inserted into the female and reaches the egg, the egg is fertilized in the oviduct and is now known as a zygote. The zygote becomes implanted in the uterine wall and a placenta soon develops. After gestation and when the baby is born, hormones are released to signal the mother’s mammary glands to produce milk for her young.

Marsupials compose another style of mammal reproduction. In these animals, reproduction starts the same, but the zygote does not implant in the uterine wall. Instead, it has a yolk sac to feed from. When the yolk sac is depleted, the small, underdeveloped organism crawls out of the uterus towards the mother’s milk supply. Examples of organisms in this group include koalas, opossums, and kangaroos.

The most unusual method of reproduction is used by monotremes. These mammals lay eggs! The eggs are kept warm until the young hatch from them and need milk from the mother. Examples include the duck-billed platypus and the echidna.

Further Classification

As we travel down the taxonomic hierarchy, the next level of classification is “order.” Organisms are grouped together based on more specific characteristics. Below are examples of how mammals can be identified based on a closer examination of structure and behavior.

  • Order Rodentia
    • Mammals with a pair of incisors in both the upper and lower jaws
    • Examples: Mice and rats
  • Order Carnivora
    • Includes all meat-eating mammals
    • Examples: Cats, bears, dogs, and foxes
  • Order Pinnipedia
    • “Fin-footed” or aquatic carnivores
    • Examples: Seals, sea lions, and walruses
  • Order Cetecea
    • Aquatic mammals that breathe air by coming to the surface of the water
    • Examples: Dolphins, porpoises, and whales
  • Order Primates
    • Mammals that have opposable thumbs, can walk upright and have a tendency toward social behavior (living in groups)
    • Examples: Apes, chimps, and lemurs
  • Order Perissodactyla
    • “Odd-toed ungulates;” members have either one hoof or three toes
    • Examples: Rhinos, tapirs, and zebras
  • Order Artiodactyla
    • “Even-toed ungulates;” members have either two or four toes
    • Examples: Cattle, deer, and camels
  • Order Edentata
    • Toothless mammals; if teeth are present, they usually lack enamel
    • Examples: Sloths, anteaters, and armadillos
  • Order Chiroptera
    • Flying mammals
    • Examples: Bats and flying foxes
  • Order Insectivora
    • Mammals that eat insects
    • Examples: Hedgehogs, moles, and shrews
  • Order Sirenia
    • Large, herbivorous marine animals with a tail instead of hind limbs
    • Example: Manatees
  • Order Proboscidea
    • Animals with trunks; the trunk is a modified nose and upper lip that is used to breathe and to reach/grasp
    • Example: Elephants


Only registered, enrolled users can take graded quizzes