I’ve managed to understand the square-cube law, which explains why scaling up small creatures like ants wouldn’t work. But that just makes me wonder, how did enormous dinosaurs like titanosaurs manage to support their own massive weight and move around?
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hollow bird bones. (source: watched jurassic park in theaters when it originally released)
They were not built like ants. Their bodies devoted way more space to holding up their own weight. Bigger bones, sturdier muscle and cartilage, slower movements.
They were also often relatively lightweight, to help reduce the impact of their size.
The ratio of oxygen to other gasses in the atmosphere was much higher and could support much larger creatures.
The air pressure and ratio of oxygen to carbon based gases in the atmosphere change how circulatory systems worked. During the biggest period of dinosaurs, you also have larger insects too, and there’s a strong connection between the amount of atmospheric oxygen.
This difference in atmosphere pressure and levels of gases in the atmosphere lead to different biological conditions to form in the dinos. Bone structure of most dinosaurs were hollow or semi-hollow with an internal structure to help fill in that hollow space and reinforce the bone strength without making them heavier. On top of this, many are believed to also have more bird like respiratory systems using air sacs.
On top of this, the ecosystems provided an abundance in food supply for many dinosaur species, so combined with a slower metabolism, and the ecosystem conditions, allowed many dinosaurs to grow so large and be able to sustain itself.
Presumably, if you raised a chicken (most probable descendent of the t-rex) to the size of a t-rex, it would collapse on itself because it’s food source can’t sustain itself, and its respiratory system couldn’t handle the current atmospheric pressure and balance of oxygen to carbon.
Bigger animals are built differently and move differenty then small animals. Generally, they have thicker bones and larger muscles proportionate to their size, allowing them to move their bigger bodies, but also move more slowly and carefully.
The problem with giant insects is twofold:
Insect respiratory systems are relatively inefficient. Beyond a certain level the insect can’t get oxygen to its cells. Earlier in Earths history when oxygen levels were higher, insects were larger.
Exoskeletons are heavy compared to endoskeletons. A dinosaur could be larger than an insect because it was relatively lighter
I’ll add that ‘giant insects’ have two important differences from other “megafauna”.
Their skeletons are on their outside. Dinos and other giants can have big, strong bones. Insections are basically lobsters, they have a strong shell they use as a skeleton. The shell can’t scale up with the meat so eventually they crush themselves.
They breathe through their skin. Insects breather through holes in their skin without having what you’d call ‘lungs’ to pump the air. (They have something more like gills). A giant insect just can’t breathe fast enough to get oxygen to all their parts.
When giant insects did exist (dragonflies the size of cars, centipedes the side of buses) there was a lot more oxygen in the air so it balanced.
Dinosaur bodies were designed from the ground up to support such massive sizes. Ants were not.
The big Sauropods were designed to carry weight as efficiently as possible. Their legs were built like columns, with ligaments and joints in the right places to offer stretch and support. Their bones were hollow, but denser and stronger than mammal bones. And thanks to the avian respiratory system made up of specialized air sacs, not only could Sauropods breathe efficiently, their bodies were comprised of 80% air!
An ant by contrast has such spindly legs, that if scaled up to Sauropod size, would collapse under its own weight. Its exoskeleton would be far too heavy at that size thanks to the square cube law. Breathing is another issue, insects breathe through holes in their exoskeleton, this is nowhere near as efficient as a mammal, or especially a dinosaur. An ant the size of even a dog wouldn’t be able to get enough oxygen to breathe.
First, dinosaurs had bones, while ants have exoskeletons. Each of those has advantages, but exoskeletons don’t scale very well with size (especially outside of the water).
Second, ants also suffered a square-cube problem with their breathing. Ants don’t have lungs to circulate air, they have tubes that carry air from the outside directly to their tissue. That only works when you’re very small.
Everyone is talking about why, so here’s an example that everyone is excluding. Elephants are some of the largest land animals and they are able to walk despite being giant “against the square-cube law.” Animals are just “built for their environment.”
Isn’t it the fundamental movement structure, like we have bones and muscles and fibers, while smaller creatures really have an exoskeleton, and a bunch of fluids. At a small level the fluids require less work to create a larger pressure. You can see this in action with a penny and water, the water molecules themselves can create a bubble of surface tension. As the ants get larger so does the required amount of fluid to create the pressure. They get some of this pressure from the air itself, so there is a limit to the amount it really gains an advantage based on that, if we had a thicker atmosphere (by having more air, or just a heavier mix of elements) this calculation would change.