Imagine we take mice, a cat and an elephant and throw them from the top of a skyscraper on a stack of mattresses. The mice will fall, feel stunned and then shake the body and walk away. The cat will break all his bones and die. The elephant will explode and spray out all its guts and blood on the unlucky few pedestrians.
So how did the mice live while it killed the other two? Size matters. Big is not always better.
Size determines a lot for us, how we are built, how we live and experience the world and also how we die. This is because the physical laws apply differently for different sized animals. Life as we see around and also the ones we don’t see comes in different sizes. It ranges from the invisible bacteria to ant to mice to dog to humans, tigers, elephants, sharks and the blue whale. Each size has its own unique place, its upside and downside.
Now let’s see how the mice survived. That is because how scaling size affects everything. Very small size is practically immune to falling from heights because you won’t have to care much about the gravity. Imagine an imaginary animal which is spherical and is of the size of a marble. It has three features – its surface area which is the skin, the length and the volume with all the organs inside it.
Now imagine if we make it the size of a basketball, that doesn’t mean all its features grew 10 times. The skin will grow a hundred times and its volume will grow a thousand times. This volume is actually the mass of the animal. The more the mass the higher will be the kinetic energy while falling and stronger will be the impact. The more surface area you have in proportion to your volume, the better your chances of survival as your impact gets distributed and softened. The air resistance will also slow you down more.
The elephant has less surface area compared to its volume therefore a lot of kinetic energy is concentrated a small space resulting in such an explosive death. Now comparing it with insect’s, they have a high surface area and very less volume.
But there are other forces which can be harmful to the tiny world but normal for us. I am talking about surface tension. It is a deadly threat to insects. Water tends to stick to itself as its molecules attract each other – it’s called cohesion. It creates tension on the surface just as an invisible skin. For humans this invisible skin is weak and we hardly notice it. When we are wet about 1 percent of our body weight in the form of water sticks to us, but for a wet mouse about 10 percent of its body weight sticks to it. So imagine as if 10 full water bottles are sticking to you when you are coming out of swimming pool. For insect’s, the surface tension is so great that it becomes a question of life and death.
If you are shrunk to the size of an insect then the water will seem to you as glue and will quickly engulf you and you will drown. So nature made them water repellent, some insects have developed a thin layer on wax on the surface to make it partially water repellent. Many insects have developed hairs which increase their surface area. This prevents the water droplets from touching there exoskeleton and therefore makes it easier to get rid of water. Some insects have developed very short and dense layer of water repelling hair/fur, which can be as dense as a million hairs per square millimeter. When they get into water, the water cannot cross the hair barrier. Something amazing happens next, as the oxygen between the hairs run out, the carbon dioxide defuses out and new oxygen defuses in. The insect carries an outside lung so it can swim underwater.
The smaller you get, at some point even air feels like solid. Let’s get down to the level of the fairy fly which is only 0.15 millimeter long. It’s a weird world at that scale. The air seems like a thin jelly or like syrup like soup always surrounding them. Flying is not easy for them, they have to grab onto the air with their wings and literally swim in the air. Evolution had to work through billions of years to come to the optimal solution.
So why don’t we find ants of the size of horses or elephants of the size of amoeba.
Let us imagine we shrink an elephant to the size of a mouse and enlarge a mouse to the size of an elephant. The tiny elephant will be dead in minutes. The giant mouse will also die. Our body is optimized to function at a particular size range otherwise it is sure death. But why?
Life if dependent on cells and are actually pretty similar across all life forms. The blue whale and the crow have the same cell size, the only difference being the number of cells. Cells have to work hard and therefore need a lot of energy. This energy is sourced from the food we eat and oxygen to make it to usable chemical energy. This process happens in the mitochondria which is the powerhouse of the cell. They are like little power stations spitting out small batteries that can be used by the cell to do everything it needs to do. The mitochondria get hot while working and the skin cells of humans can get up to fifty degree Celsius. Some cells have up to 2000 mitochondria so you can imagine how much heat it produces. So all life forms produce a lot of heat to stay alive and with more number of cells, more will be the heat. The body has to look for ways to let out this heat or it will be cooked from inside. This is the problem of bigger animals.
In this context animals have three properties – the length, the skin and the internal organs and what is inside. When things grow, the insides grow faster than the outside. This is the problem of bigger animals because the heat can only be released through the surface. The giant mouse will have 3600 times more surface to lose heat but has 216,000 times more volume. This means a lot more on the inside and less on the outside so the giant mouse will die quickly( more heat produced inside and less surface to get rid of it). Then how do big animals lose heat. The elephant has big ears to increase the surface area for heat to escape. But it is not sufficient, and nature found a way out. Elephant cells work much slower than the mouse cells. In bigger animals the cells work just enough to function properly so the metabolism is slow.
The small animals work just the opposite. The Etruscan shrew is the smallest mammal. It has a length of 4 cms and weighs just 108 grams so it would cool off very fast so its cells run in hyper mode to stay warm. It has a heartbeat of 1200 times per minute and breathes 800 times a minute. So it has to eat a lot to keep the cells working and therefore has a high metabolic rate. So if an elephant works on the principle of shrew then so much heat will be produced that the fluids inside will start boiling and the elephant will explode.
The baby in the womb is more a part of the body’s organ than an individual and has the similar speed of life that of the mother. But when it comes out a switch is flipped by nature and all its internal processes speed up to function independently. 36 hours after birth a baby’s cells work in the same speed as that of any mammal of its size.
But in one thing the big and the small are similar, that is heart beat. All mammals have almost similar number of heart beats that is about one billion. So though these two mammals function completely differently but they are also similar in one way.
you have written great brother