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.
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!
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.
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.
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.
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.
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.
A bird's nervous system is well adapted for flight. For example, the cerebellum is very large and this is attributed to the complex movements required for flight. The increased size in other parts of the brain is attributed to improved vision, hearing, and more complex motor skills.
Mammals have larger brain portions than their vertebrate counterparts. For example, a mammal has a large cerebellum so it can complete difficult movements. Mammals are unique in that they are the only animals with a neocortex, which allows for spacial reasoning and conscious thought. These adaptations have allowed mammals to inhabit nearly all of the land habitats found on earth.
Life on land came with several new challenges and these challenges shaped birds, mammals, and reptiles into what they are today.
Although birds, mammals and reptiles all have lungs, the efficiency varies. Reptiles are ectotherms, so they do not have the energy requirements of birds and mammals (both of which are endotherms), and as a result their lungs are not as efficient. Birds, who have extraordinary energy requirements for flight, have air sacs that improve lung efficiency. Mammals are somewhere in-between birds and reptiles, with lungs that are efficient enough to sustain an endotherm.
On land there is a new array of food. This lends to a new variety of diets, each suited for consuming different materials. Some herbivores have adaptations like a rumen, which helps them digest plant material. Birds need to be lightweight, so they do not have teeth. Compared to birds and mammals, ectothermic reptiles have a slower digestive system.
Life on land requires more energy. Birds and mammals have an efficient four-chambered heart that minimizes the mixing of oxygen-rich and oxygen-poor blood. Reptiles require less energy, hence their three-chambered heart.
Finally, life on land requires the use of different senses and complex motor skills. This leads to a larger brain with more portions (especially in mammals and birds). A bird has a large cerebellum to deal with flight, and a mammal has an extra portion called the neocortex, which allows for complex thought and movement.
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