How Big can Animals Get?

As humans, we're pretty small. Sure we're not like bacteria or insects, but in the animal kingdom, a lot of things are bigger than us. In today's animals, there's elephants, giraffes and rhinos, and as we go further back in time we see even more creatures that were enormous, giant sloth and beavers, and of course dinosaurs. So some animals are pretty big, but how big can an animal actually get?

To answer this, we need to consider what helps, and what limits, animals from growing. Obviously there's no one point after which and animal simply can't grow, but as animals get larger and larger, the problems become more and more evident. 


All animals need energy every day, we get it through food and drink where we eat stuff, then our body breaks it down and releases energy from it. However the bigger the animal, the more it needs. This trend was noticed by Dr Max Kleiber ho came up with this lovely graph that shows basal metabolic rate (the lowest amount of energy an organism uses) against body mass. Known as the mouse - elephant curve, it shows that an organisms basal metabolic rate is proportional to the mass to the power of 0.734. Basically the bigger something is, the more energy it needs.


So why is this such a problem? well we get our energy from our environment, from eating, drinking and in some cases sunbathing, but the more energy we need, the more we need to consume and so the more we have to eat and drink and eventually it would reach a point where groups of these animals would decimate any environment through trying to get enough food to survive.

Next up is movement. Its a little known fact that the more mass something has, the more it weighs! This shocking fact, dictates that the larger the mass of an object, the larger the force of gravity on the object. Why is this important? Because animals with legs spend the whole time that they're standing up fighting against this force. Sure animals like whales could get around it by not standing on land, however if they were in a non buoyant environment (AKA not the ocean), their organs would be crushed by the force of their blubber's weight.


There is a mathematical law called the square cube law, which says that as an object's size increases by a factor, the volume increases by the same factor cubed, so as animals get a bit bigger, their volume increases a lot more. This volume would also cause problems with heat transfer making it likely to be prone to overheating and therefore vulnerable to certain illnesses and potentially cause it to struggle with ordinary enzyme based functions.

So what other problems would an organism of that size face? Well it would need an immense amount of oxygen which would have to be circulated entirely around the body of the animal and so the blood pressure, amount of vessels and the speed at which it goes around the body would have to be enormous.

On top of this it would find it extremely difficult to find shelter from the elements or escape from any predators. 

So in summary, in the unlikely event that its skeleton could handle the immense weight of its own mass, it would be likely to be eaten by predators, starve, cook itself with its own body heat or simply stop working because homoeostasis would be impossible on a massive scale.

So that's it for now, I really enjoyed writing this, it feels like my first actual post in months. Thank you all so much for reading and see you soon :)