When we feel tired, we take a nap, have some food or drink a caffeinated drink, but how do animals get their energy? They can’t just grab a coffee when they feel tired, so what do they do?
Herbivores will get their energy from the nutrients of the plants they eat. Carnivores will get some of this energy from the nutrients in the meat of their prey. Ectothermic animals will bask in the sunshine to slow down the loss of energy, allowing some reptiles to go for weeks without food.
Humans are the most evolved animals on the planet, with various ways to produce energy sources. Animals do not have this luxury and must find sources of energy within their habitat. Energy is passed along a food chain or food web from producers up to apex predators.
Food Chains and Webs
In order to understand how animals get energy, we have to understand the food chain. A food chain describes the transfer of energy from one organism to another. In any environment, there may be lots of different food chains.
For instance, within a tree-lined meadow, there would be producers, primary consumers, and secondary consumers.
GRASS (producer) >>
RABBIT (primary consumer)
>> FOX (secondary consumer)
LEAVES (producer) >>
DEER (primary consumer)
>> BEAR (secondary consumer)
Early humans were secondary consumers, but they were often killed and eaten during hunts by other predators. In today’s society, humans are apex predators, with no natural predators of our own.
We are tertiary consumers, eating producers (fruits, veggies, nuts, and seeds), herbivores or primary consumers (chickens, beef, pig), and secondary consumers (mostly fish and seafood that eat other fish). After the last ice age, we became apex predators when there were no other predators that naturally predated us, so our food web was fundamentally altered.
Several food chains within an ecosystem form a food web, and each organism is typically connected to at least two links within a food web. This is because most herbivores feed on several different vegetation types, and predators usually have at least two prey animals within their diet.
A food web describes all the organisms within an ecosystem and which level of the trophic order they fit in. An ecosystem has three trophic levels:
- Autotrophs: Algae and plants. Autotrophs are produced as they produce their food.
- Herbivores: Organisms that eat autotrophs.
- Carnivores: Animals that consume meat.
There are several variations of the carnivorous trophic level.
An obligate carnivore is an animal that relies solely on meat in its diet. Their digestive system does not produce the enzymes required to break down plant material.
Omnivores are animals that consume both meat and vegetation. Bears are a perfect example of this. They hunt fish and small mammals, but they also feed on berries, nuts, and plant bulbs. Some omnivores, like bears, only require a small amount of meat in their diet and are known as hypocarnivores.
Mesocarnivores are animals with a diet made up of approximately 50/50 meat and vegetation. This includes animals such as foxes, raccoons, and bobcats.
Hypercarnivores are those animals that have a larger meat requirement than plant matter. Approximately 70% of their diet comes from prey animals. This includes most cat species.
Within the food web, there are three classes of consumers. Herbivores are primary consumers, as they feed on producers. Carnivores and omnivores are secondary consumers, feeding on herbivores that consume producers. Carnivores that prey on other carnivores are tertiary consumers. The best example of a tertiary consumer would be orcas. They hunt other carnivores, including small whales, dolphins, and seals.
Each food web has at least one keystone species. This species would cause drastic shifts in the ecosystem if that species were no longer present.
In the US, wolf numbers are either falling or remaining steady but low. This means deer are not being predated on, and the population has been increasing each year. There are now so many deer in some towns that they have to venture into residential areas searching for food.
Another issue with the rising deer population is that the forests are being over-browsed, so fruits, berries, and leaves are not available for other animals.
Every ecosystem needs balance to maintain harmony. When one trophic level either declines or increases too much, it affects many other levels of the food web.
Energy Production From Plants
Energy is required for any metabolic reaction to occur within the body. This includes the breakdown of food in the stomach, the absorption of nutrients from the intestinal tract, the production and excretion of waste, and the maintenance of a stable body temperature.
Energy is sourced from food. Plants get their energy directly from the sun, called photosynthesis, which allows plants to synthesize simple sugars using sunlight, water from the ground, and carbon dioxide from the air. Plants also produce nutrients as they grow, such as phosphorus, magnesium, and potassium.
Herbivores eat plants and therefore consume some of the energy and nutrients the plant has produced. The further up the food web you go, the less energy is passed on. Carnivores get some of this energy by consuming the stomach content of their prey and the nutrients held within the meat and organs.
Carbohydrates are converted into glucose molecules during digestion. Glucose is then converted first into pyruvate with water and then into Adenosine triphosphate (ATP), chemical energy. The waste product of this reaction is carbon dioxide, which the body removes via respiration.
ATP production can also be done without water. Cells such as red blood cells do not contain mitochondria, so they must convert glucose into ATP without oxygen. This process creates lactic acid as a waste product.
Muscle cells contain mitochondria and can therefore create energy using oxygen. However, muscle cells are also able to convert energy without oxygen. As we know, this produces lactic acid as a by-product. Too much lactic acid causes the muscles to ache. This is what you would feel after a strenuous workout.
So how do plants and animals get energy when they live in the deep sea? There is no sunlight here for plants to use for photosynthesis, so where does their energy come?
Deep-sea vegetation such as phytoplankton, anemones, and corals can grow because they have evolved to produce energy from inorganic compounds rather than via sunlight. They use hydrogen and carbon dioxide from the seawater and rocks to synthesize sugars.
The plants then use these sugar molecules as an energy source. Deep-sea creatures then feed these plants, absorbing the energy the plans have produced.
Energy From The Sun
In addition to gaining energy via food consumption, some species also gain energy directly from the sun. Endothermic (warm-blooded) animals such as mammals and birds regulate their body temperature using metabolic heat, which requires a lot of energy.
Amphibians and reptiles are ectothermic (cold-blooded). They cannot regulate their body temperature like other animals, so they use their environment instead. This is called thermoregulation.
When an ectothermic animal feels cold, they become lethargic. Their bodily processes slow down to accommodate the drop in energy. To boost their energy, ectothermic animals will bask in the sunlight or rest on a warm surface such as sand or rocks.
This is how animals such as snakes and crocodiles can last for weeks without food. Using the sun to regulate their body temperature means they do not use any of their stored energy. They also do not move around unless they have to. They now only need to eat when they start to feel hungry, which will happen far slower since they use less energy on other bodily processes.
To cool down, ectothermic animals simply need to find shade. This could be under a rock, in a leafy tree, or by digging underground.
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