Do animals even drink or need as much water as we do in order to be healthy?? Why do we need so much?
In: Biology
Animals all perform a process called respiration. During this process, we use oxygen and glucose to make a molecule called ATP. The products of making ATP are carbon dioxide and water. This set of chemical reactions almost certainly evolved the way they did because animals started out ***in water***. Water is super convenient for many reasons, it’s a great solvent for most molecules, it has protons it can give up or accept pretty readily so you can do some neat chemistry with it, it takes ***a lot*** of energy to change its state AND it’s liquid in a range where other molecules can interact and chemically change. It’s the perfect liquid to start life in!
About a half a billion years ago, animals started to move onto land (right after plants). The primary difference between water and land is the water. You don’t have a source of water immediately available 24/7. So many of the chemical reactions animals rely ‘assume’ that water is plentiful. Evolution can’t just change these reactions, they’re so fundamental to life that any alteration is probably going to be bad. So, evolution ‘worked’ by creating mechanisms to try to retain water in the water. Animals developed water-proof skin, our urinary systems developed methods for concentrating nitrogen waste products like urea while retaining water, etc. But the fact of the matter is that water is **required** for some things. First, we need water to dissolve oxygen, so we can move oxygen from the lungs into the blood. This is a physical limitation that can’t be overcome. And so our lungs are coated with water and every time we breathe out, we lose water. Our digestive system won’t function if we don’t add water to our food. The water helps dissolve food molecules and allows our enzymes the ability to break down those molecules AND absorb them properly. In humans, we need a certain amount of water in our feces to defecate properly. Too little water and we become constipated and retain our feces (which will have all sorts of bad side effects). Evolution can’t overcome these basic physical constraints, so it “works” to lessen them, but it can’t **eliminate** them. Therefore, we lose water and need to constantly replace it. Some animals are better at water retention than others (compare humans to camels) but all animals will lose water to the atmosphere and urination and need to replace it via ingestion of fluids.
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