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Homeostasis is the epitome of regulation. Is there anything more that needs to be said? Keeping trillions of cells in an animal working at the same time is an amazing feat. Regulation may be difficult, and it requires lots of energy, but animal life could not exist without it. Sensors throughout the body are like a natural twitter feed. Their entire job is providing perpetual status updates on the levels of things throughout the body, from iron levels to water levels. All of the tweets are processed by the brain, which decides if anything must be adjusted to maintain the perfect balance. If even the smallest solute level is disrupted it can wreak serious havoc on the cells. In addition to cellular level regulation, animals have entire systems that they must coordinate, and the only way to bring them all together is through extensive coordination and regulation. Simply put, animals must regulate everything if they are going to thrive as large multi-system organisms.
Two words: sinoatrial node. It is the pacer for the entire body. If there was ever a master regulator of the entire body, this is it. If the heart beats too fast or too slowly it can be problematic. Most of all, it is problematic if it doesn't beat at all.
The regulation of gases in the blood falls under the category of homeostasis, and, like circulation, it is a chief point of regulation. It is so important, in fact, that an animal's brain doesn't even trust them to do it. It is taken care of subconsciously by the medulla and the pons. If you have ever tried to take over your own breathing, it can be a daunting task. Try for one minute to breathe regularly but consciously. It can be difficult to keep as even a rhythm as your brain can. By the end of the minute, your brain is probably dying to take over again like someone who has let someone else drive his or her car. Regulation of respiration is just too important to be handled by anyone but the brain.
All living things on this planet are in a constant battle for survival. They are competing against each other and their environment. This dangerous environment allows animals with certain characteristics to survive and reproduce better than others. This causes animals with certain characteristics to become more prevalent and animals with other characteristics to disappear from the population. This is called natural selection and is how animals' characteristics can change over time and how new species can be created. The differences between the animals on the planet are direct evidence of evolution.
Animal movement is one such characteristic that can help or hinder animal survival and reproduction. Think about the pronk gait. Very few animals use the pronk gait, but it must have provided an advantage over other forms of gait to be naturally selected for.
Imagine a population of gazelles many years ago grazing. Suddenly, a cheetah appears. Most gazelles have the type of legs that make it easy for them to rotary gallop, while others have legs that allow them to pronk. When the group of gazelles tries to escape, the gazelles that use a rotary gallop often get caught and eaten by the cheetah. The gazelles which pronk high in the air are better at surviving cheetah attacks. These gazelles are then more likely to reproduce. Over time, more pronking gazelles are born. Eventually, all gazelles are pronking gazelles. Think about other animal movements, all the unique movements of animals were probably selected for in a similar way.
Temperature regulation is a key part of homeostasis. Since temperature regulation is so important for animal survival, it is often a reason why certain characteristics are selected for. For example, many animals have different types of fur or hair. These differences were all selected for at some point.
Perhaps at one time there was a group of bears. One of them had a warmer coat than the other bears. This allowed that bear to survive the cold weather better. Over time, many of these heavy-coat-wearing bears appeared. Eventually, the shaggy bears began to live in a colder environment than the other bears and the two populations began to separate. Today, they might be completely different species of bears.