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.
>Why are there no mammals or birds with insect-like adult sizes?
There actually is substantial overlap between the two. For example, the largest beetles are both longer and wider and much heavier than the smallest mammals or birds. There are beetle larvae that weigh up to 200 g / 7 oz, which is about the same weight as a Syrian hamster. And a Syrian hamster is by no means the smallest mammal. The largest adult beetles are smaller at about 50 to 75 g, but that is still 20 – 30 times as heavy as the smallest mammal.
https://www.nhm.ac.uk/discover/what-is-the-biggest-beetle-in-the-world.html#:~:text=The%20heaviest%20Actaeon%20beetle%20larvae,weight%20as%20a%20Syrian%20hamster.&text=Goliath%20beetles%20(Goliathus)%20from%20Africa,adult%20only%20about%20half%20that.
But of course you are correct that mammals are, on average, larger than insects, and so are birds.
Why? Well, it’s worth remembering that everything competes with everything else from an evolution perspective. Tiny mammals would compete with tiny insects and it shouldn’t be that surprising that one or the other would mostly win out. The biggest reason, though, is that there are fundamental differences in anatomy between insects and mammals or birds. Insects are invertebrates and have very distributed nervous systems such that a lot of them can do relatively sophisticated behavior like grooming themselves even if you cut their heads off. They have much different systems for distributing oxygen to their body, which are more efficient than mammalian or bird systems that include lungs at small size. Exoskeletons are also much better than endoskeletons if you are small. From a structural perspective, it’s much better to have the stiff / hard components on the outside of the body rather than on the inside like mammals or birds.
>On a related note, why do insects only grow as large as they do now, while they used to grow larger in ancient times?
The most widely accepted explanation for this is that the oxygen content of the atmosphere is substantially lower than during the Carboniferous period, which is when there were dragonflies the size of medium to large birds. Because of the much lower oxygen content now, the ways that insects breathe and distribute oxygen are incapable of supporting such large bodies. More recent research has also pointed to the evolution of birds, as both predators and competitors to insects, as a cause for the decrease in the average size of insects.
A lot of stuff in evolution happens just by random chance. And when you’re talking about things that happened hundreds of millions of years ago, you’re not going to get story that is both very detailed and true.
https://news.ucsc.edu/2012/06/giant-insects.html
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