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Land Vertebrates: Reptiles, Birds & Mammals

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Things changed a lot when vertebrates left the water and moved to land. This lesson will compare reptiles, birds, and mammals to one another and explain why they look the way they do.

From Water to Land

You want to make the transition from water to land. Seems simple, right? Well, if you are anything like the early vertebrates, or animals with a backbone, you're going to face some challenges. During the Paleozoic Era (542 to 251 million years ago), the first vertebrates slowly transitioned to life on land. So, what challenges did these vertebrates face? Here's the short list:

  • Challenge 1: Being able to get oxygen from the air instead of from the water.
  • Challenge 2: Having a different diet on land versus in water.
  • Challenge 3: Critters that live in water feel lighter because of the buoyant force. Living on land makes it harder to move around, which takes more energy!
  • Challenge 4: Having senses that deal with life on land versus life in the water.

Let's take a look at reptiles, birds, and mammals to see how they overcame this list of challenges!

Lungs

In order to deal with Challenge 1, or being able to get oxygen from the air, land vertebrates needed to trade in gills for lungs. Many critters living in the water use gills to extract the dissolved oxygen in water. Some aquatic animals, like amphibians, can actually exchange oxygen through their skin.

But land vertebrates cannot extract oxygen from water so they need lungs. Reptiles are ectotherms, meaning their body temperature is dependent upon the environment. As a result, they do not require as much energy as birds or mammals, both of which are endotherms, which regulate their body temperature internally. As a result, reptiles have simple lungs where oxygen-rich air mixes with oxygen-poor air. Although this isn't extraordinary efficient, it works fine for a reptile's energy needs.

Birds, on the other hand, need to deal with the extreme demands of flight so they need an efficient method of obtaining oxygen. Birds have air sacs, which prevent oxygen-rich air from mixing with oxygen-poor air. This ensures that every breath a bird takes maximizes the amount of oxygen delivered to the body's cells.

Finally, mammals have a diaphragm, which is a muscle that helps lungs inflate. Although mammals are not as efficient as birds when it comes to their respiratory system, they are more efficient than the ectothermic reptiles.

Reptiles, birds, and mammals have lungs. Mammals also have a diaphragm
Lung

Digestive System

Challenge 2, or having different food sources on land, has created a wide variety of diets, each suited to those different food sources. And although reptiles, birds, and mammals share many similar digestive structures like the esophagus, stomach, and intestines, there are some variations.

Mammals, reptiles and birds have an esophagus, a stomach, and intestines
Digestive system

For example, the digestive systems of mammals are extremely diverse. Herbivores, or animals that eat plants, have additional structures like the rumen, which is filled with protozoa, bacteria, and fungi that aide in digestion of plant material. Carnivores, or meat eaters, usually have a shorter digestive tract because nutrients are more easily obtained from meat.

Birds have a digestive system built for flight. It is quick and efficient so that birds do not have to carry the extra weight of food when they're up in the air. For example, a type of bird named a thrush can digest berries within 30 minutes. Birds also don't have teeth. Instead, they have a gizzard that helps break down food as well as a beak that can crush and tear.

Ectotherms, such as reptiles, have a lower metabolic rate, so this means their digestive system is much slower than that of birds or mammals. Since their energy requirements are less, reptiles also do not need to eat as much or as often.

Circulatory System

Land vertebrates expend a lot of energy moving around, which brings us to Challenge 3. These vertebrates deal with the extra energy requirements of land by having an efficient circulatory system, or the cycling of blood through the body.

Birds and mammals have a four-chambered heart, which keeps the oxygen-rich blood separate from the oxygen-poor blood, thus reducing mixing. This four-chambered heart makes for an efficient circulatory system.

Since birds and mammals are endotherms they require more energy to maintain their body temperature. Although reptiles have a more complex circulatory system compared to fish and amphibians, they are less efficient than endothermic mammals and birds. As a result, reptiles have a simpler three-chambered heart.

Nervous System

And finally, Challenge 4. Light, smell, and sound travel differently in water than on land, and moving on land requires different motor skills compared to traveling in water.

Although reptiles are considered less intelligent than birds or mammals, they have larger cerebella and cerebra compared to amphibians. The increase in these brain structures helps improve motor function and the processing of senses such as smell, vision, and hearing.

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