Monster movies are one of the pillars of pop culture. Who has never seen a movie like Jaws, Godzilla, King Kong, Tremors, or Pacific Rim? It’s easy to draw and imagine these creatures, especially when fossilized dinosaurs make them seem plausible, but just how likely is it that these movie monstrosities could exist in real life, dealing with real life limitations and laws?
The topic of size has been a popular one for ages, as far back as Democritus in 5th century BCE Greece (he is one of the founding fathers of atomism). The importance of size in biology, especially in biomechanics, though, is a relatively new idea, spanning only the last century or two. There are two ways to think about size: what effect size has on the biomechanics and life of the organism, and what factors determine size itself. Because the factors that determine size are dependent not just on the biology of the animal (e.g. does it stop growing at a certain point of its life or does it keep growing until death is reached) but also on the environment in which it lives (water vs land vs air), it is nearly impossible to write one or two laws that dictate every animal’s size. It will vary species to species, environment to environment.
This area of research is vast and would take a few book volumes to cover comprehensively. I don’t know about you, but attempting to do even a 7000-word standard literature review would be very boring, for both you and me. So, I’ll take a different tactic. Let’s try to build a monster. Or a few.
Depending on what type of monster we want, where we think it lives, and how we make it, it would have different factors and obstacles that need to be resolved, different principles will need to be examined. Because there are so many options to go through and writing it all as one blog would take forever, I’ve decided to break this blog up into a series of blogs.
In this first installment of How To Build A Monster, we’ll investigate a stereotypical movie monster-making tactic: radioactive sludge.
Because I enjoyed the movie, let’s actually look at the spiders from Eight Legged Freaks (2002). (Because I’m super lazy, I’m just going to write ELF and expect you to know I mean Eight Legged Freaks and not that Will Ferrell movie.) The premise of the movie is that some radioactive waste was accidentally left to leak into a pond and crickets drank the water. The crickets were then caught by an arachnophile who kept an unreasonable amount of spiders in a shed. 1) Crickets drink radioactive water, 2) spiders eat radioactive cricks, 3) Uh-oh! Spiders start growing until they reach unreasonable and terrifying sizes.
There are two main issues with doing this, the first, a general biomechanics principle, and the second, an invertebrate/arthropod-dependent limitation.
If you take a spider and simply scale it up, can it perform the same while looking the same? Would the large spider legs be able to support the new, heavier body mass of the spider and still have the same proportions? I’ll give you a clue: No.
To examine this, we have to delve into some biomechanics. Scaling refers to how structure and function change with size, and allometry is the study of biological scaling.
If you have a square of 1cm on each side (L), then the area of that cube is 6cm^2 and the volume is 1cm^3. What if you double the length of the side? Well, the surface area is going to increase by a factor of 4 and the volume by a factor of 8. Triple it and the surface area increases by 9x and the volume by 27x. These ratios hold true for a sphere and a cylinder as well.
What does this mean for our spiders? Well, we can find out. First, I’m going to pull a “spherical cow” move and model my daddy-long-leg spider as a vertical column with just a block of weight on top of it. The block is the spider’s torso and it’s got one massive, joint-less leg. Here, the column has to support its own weight as well as the weight of the load on top of it. The column would experience buckling at some maximum load weight, or Euler’s Critical Load, which can be simplified to Diameter^4/Height^2. If we just simply doubled the diameter and the height of the column, the critical load necessary to cause buckling would be 4 times larger (2^4/2^2). This is cool. A 2x increase in the 1D measurements results in the 4x increase in the load it can carry.
A/N: Shout-out to Vogel’s Cat’s Paws and Catapults for this twofold-increase analysis.
Now, instead of just doubling the size of our simplified spider, what if we had a ten-fold increase, getting our spider up to B-horror movie size? Our monstrous leg would be able to bear 100x the weight it could at it’s normal size.
But we’re forgetting something.
As we scale the leg up, we’re also scaling the body up. The body is now 10 times larger, has a surface area 100x larger, and a volume 1000x greater! And mass scales with volume, so our spider’s torso now weighs roughly 1000x more.
And we just calculated that the leg could only handle an increase of 100x the original weight. That seems like a problem.
Wait, is that someone yelling that spiders have 8 legs?
Okay then, let’s expand our model to be a spider’s torso resting uniformly on 8 identical pillars. The body mass still increases by 1000x, while the load each individual leg can carry still only increases by 100x. 1000x/8 is 125x. Even spreading the weight out uniformly, scaling the legs up by 10x wouldn’t be enough to support the weight of the rest of the spider. While it’s stationary. If you start adding in joints (built-in buckling points) and the fact that <8 legs will need to bear all weight while the spider is in motion, it just becomes impossible. The legs would need to scale up with a >10x factor in order to support at 10x increase in the spider torso.
At that point, would it even look remotely like a daddy long-leg? My guess is not. You would have a larger and somewhat-normal looking spider body on top of eight extremely large and out-ofr-proportion legs.
You know those body builders that can somehow increase the size of their torso outrageously, but their head is still the normal size and now they look incredibly strange??? It would be spider-equivalent of that.
Jumping back to ELF, even if spiders received massive doses of gamma radiation and grew 10x (if not larger!) than their original size, they wouldn’t even be able to move. They would just sit there, like Jabba the Hut. This would be true if they grew by 10x or 1000x.
This was just one way in which these ELF spiders aren’t really possible. Let’s take a look at another.
I don’t know if you guys know this, most insects don’t have lungs. Lungs developed in fish and as you may or may not know, invertebrates diverged from that lineage way before lungs were even conceived of. They also don’t have blood or hemoglobin, which is how the oxygen that vertebrates breathe in is transferred to the rest of our body. Because of this, inverts have had to find other ways to breath air.
For terrestrial invertebrates, namely insects and arthropods, this is where the tracheal system comes in.
Insects exchange oxygen and carbon dioxide not through alveoli in lungs, but through these tiny tubes called trachea that they have located around their torso. And instead of using a circulatory system to transport the oxygen all over their body, insects have smaller tubes branching off the main trachea called tracheoles, which increases the surface area: volume ratio available for gas exchange to actually happen.
However, there is one, teensy, weensy problem with this system. It mostly relies on passive diffusion of oxygen, in two ways. The first is actually having oxygen enter the tracheal system. Some larger insects are able to use their muscles to pump the air sacs like bellows, increasing the amount of fresh air the system is able to take in. But for a lot of insects, they just have to rely on whatever oxygen is able to find its way into their system. Additionally, diffusion matters at the cellular level. Even when oxygen makes it into the system, it still needs to diffuse across the tracheole walls into the individual cells.
The distance that oxygen is actually able to diffuse is tiny. We’re talking microns here. That’s not so bad when the tracheole walls are tiny, but it might be an issue at larger sizes.
If we take our monstrously large spiders from ELF, the same tiny oxygen molecules in the larger tracheal system would have to diffuse across a thicker tracheal wall than the wall it had to diffuse across in a normal spider tracheal system. Do you think the oxygen would make it to the cells?
Nah, I don’t either.
In addition to that, the trachea are supported by rings of chitin, a sturdy protein that provides structure to the rings and helps to prevent their collapse. We already know that a stationary, vertical column scaled up to 10x wouldn’t be able to support the increase in mass. Do you think the chitin rings would still be able to keep the tracheal system from collapsing under the shear weight of the monster torso?
Nah, I don’t either.
Okay, I have a few more wrenches to throw into this breathing mix. Some spiders do actually have lungs, or “book lungs,” in addition to their tracheal system. They’re very different from vertebrate lungs, though, since they evolved independently in the arthropod lineage. The crux, however, is that the spiders have to move in order to breathe with their lungs. I’m not going to go into the whole biology of that system, but at this point it’s pretty irrelevant. If our massive mutant spiders can’t move, then they can’t use their lungs, either. And for the last wrench: spiders have slower metabolisms than endotherms. This means they need less oxygen than us, and they need to eat less frequently. This is why they can sit still in a web waiting for prey to come along. However, they won’t be able to move to operate their lungs, and their tracheal system would collapse, and after a period of time, they would eventually need to breathe again.
There are a few more reasons I can see why creating super-large spiders would never work, but I’ll stop there for now.
To sum up, not only would our Eight Legged Freak spiders wouldn’t be able to move, but they’d also suffocate slowly after experiencing their radiation-fueled growth spurt.
That kinda sucks, when you think about it.
Now, however, we can extrapolate these results to a whole host of bug-based monster movies.
Hopefully this first part highlights how important basic scaling principles are. You can’t just take something that is tiny and make it bigger. And for other reasons, you can’t take something bigger and make it tiny. Kurzegesagt had a really nice video on this, titled “How to Make an Elephant Explode.” Now that they’ve covered it so amazingly well, I don’t have to.
Lastly, we have one more important topic to cover (very briefly). The fact that mega-large insects really did exist. Just not on “our” earth, as we know it today. They existed on an earth that was very different.
One of the long-lived theories behind these large insects is that the earth had higher oxygen levels ‘back in the day’. Roughly 30%, compared to 21% today. If you rely on diffusion to get air through tiny air tubes in your exoskeleton, then having more of the atmosphere containing the gas that you actually need might help you grow and have a better metabolism. Or so the theory goes.
Another theory published a few years ago is that growing large is a way to cope with oxygen toxicity, which would be a bigger problem with the higher levels. Insect larvae, which absorb oxygen through water and air, would need to become bigger in order to decrease their oxygen intake relative to their body size. And then big baby = big adult.
So, why did insects become small again? According to this study, insect size tracked with increasing oxygen levels in the atmosphere for only the first 200 million years or so. At the beginning of the Cretaceous period, however, oxygen levels spiked while insect size decreased. They theorize that the evolution of birds during the Cretaceous is what forced insects to become smaller again. Works for me.
Whew. That was a lot of information. It took a while to write, too. To sum up, we covered why massive mutant spiders would never be able to happen, both from a biomechanical perspective and a respiratory perspective. And we briefly reviewed how animals were able to overcome that respiration limitation by taking advantage of the oxygen-rich atmosphere of the Carboniferous period.
Next time, we’ll take things even bigger.