Why are there no mammals or birds with insect-like adult sizes?
On a related note, why do insects only grow as large as they do now, while they used to grow larger in ancient times?
In: Biology
Mammals and birds are hot blooded, which means we use a lot of energy to keep ourselves warm.
It gets harder and harder for an animal to keep it self warm the smaller and smaller it gets. This is because of something called the square cube law. Basically heat is lost through the surface area of an animal. But the amount of heat it can store is based on it’s volume. (For an intuitive example of this think about cooking a turkey and how it can stay hot for hours out of the oven, but a single slice of pizza might go cold within like 10 minutes.)
Eventually a mammal is so small that it takes too much energy to keep warm. The smallest mammal is a shrew (even smaller than mice) and they usually need to eat about about 2-3 times it’s own body weight just to stay alive.
At that point cold blooded insects which need a tiny fraction of the food are able to out complete mammals trying to fill the same niche. So we simply don’t see small mammals.
Edit: Saw your question about why insects don’t grow larger. Interestingly it not related to their body temp. There’s two big reasons they stay small though.
1. they have exoskeletons, which are great for small things but end up being insanely heavy and really difficult to grow for larger animals. So after a certain size animals with an endoskeleton are just better and can out complete again.
2. Insects don’t have the same kind of circulatory system that we do, they basically breath through tiny holes in their exoskeletons, and then they oxygen just kinda…diffuses through them, like of later water through a sponge. Which leads to the same kind of square cube law issues i mentioned earlier. Eventually they are too big they can’t absorb enough oxygen.
Latest Answers