Fantasy creatures come in all shapes. Some are basically existing animals but with special traits or abilities. Others are chimeric, with two or more species smashed together wholesale. And still others are completely bizarre monstrosities. They can be fishlike, insectoid, reptilian, mammalian, you name it. Many are very much like humans. But some fantastical traits tend to stick out, and one of the more pervasive concepts in fantasy and folklore is to make truly massive monsters.
Off the top of my head, only two common “classic” fantasy creatures come to mind that aren’t at least human-sized: the phoenix and the “small humanoid” variety of elves and similar fae. The former is traditionally eagle-sized, which is still pretty large for a flying bird. The latter comes in all kinds of forms, from insect-sized to humanlike or even taller (hence the “small humanoid” qualifier we started with).
Compare that to the likes of krakens, thunderbirds, dragons, and of course literal giants, and you can see what I’m getting at. There’s a good reason for this, of course: something about the idea of largeness just inspires awe. And if you’re going to invent or exaggerate or theorize about a strange creature, why not make it truly awesome?
But like any living thing, megafauna – that’s the official science term for “really friggin’ big animals” – face their own set of challenges. We’ve already discussed how large sizes make flying tricky, so I won’t repeat all that here. But even when they’re firmly on the ground, there are a few things you’ll need to keep in mind if you want your massive monsters to actually survive in their environment.
The square-cube law strikes again!
As mentioned in the above links, the square-cube law is a geometry fundamental that has a lot of impact on living things. If you need a refresher, this law basically states that as the size of an object increases, factors influenced by its volume increase at a higher (cubed) rate than factors influenced by its cross-section or surface area (squared). Naturally, this is going to have an impact on real-life massive “monsters.”
One interesting way this impacts animals is by limiting the size of insects and similar arthropods. See, animals like insects and spiders don’t have the complex lungs we have. Our lungs are full of pockets called alveoli which are used in the exchange of oxygen and CO2 into and out of the blood. The shape of the alveoli increase the lung’s surface area for gas exchange. This is important because the need for gas exchange increases with volume. Simpler respiratory systems, like the “tracheal” systems insects have, just can’t keep up beyond a certain size.
So, why did we have bigger insects in dinosaur times? While part of it had to do with the rise of insect-eating birds in the Cretaceous period, there was also more oxygen in the atmosphere back then. This gave larger insects more to work with, so to speak, even with their simple respiratory systems.
Okay, so what about vertebrates?
Of course, oxygen isn’t our only concern here. In the article on giant fliers, we talked about how weight increases based on volume while strength increases by the area of the muscle’s cross-section. And while that discussion was focused on flight muscles, you need muscles for more than flying. You also need it to, well, just plain hold up your own weight.
Let’s take a look at the heaviest land animals out there. Elephants, rhinos, and hippopotami are all stocky. They have relatively squat, thick legs and muscular bodies to support their heavy heads and frames. Large cattle like the gaur aren’t as stocky in the leg, but they aren’t quite as big either and they still have a lot of muscle mass. Perhaps the biggest outlier is the famously tall giraffe, with its long and relatively slender legs. But giraffes have some very specialized anatomy to help support themselves with less muscle, with distinctive suspensory ligaments working with the long leg bones themselves to keep them upright and a compression sock-like design in the tight leg skin needed just to keep their blood flowing correctly.
All this, of course, becomes much less of an issue when you live in the water. Fully aquatic animals are largely supported by the water itself, and whether they can move easily to the surface is a question of density, not weight. This is the main reason why whales can get so much bigger than land animals. In fact, most whales and dolphins are so well built to be supported by the water that they can’t breathe when they’re out of it for too long at a time.
The hunter and the hunted
So those are some traits and limitations to massive animals, but what about massive monsters? One of the big draws of fantastical or folkloric giants is that they invoke a sense of danger. And another obvious way to inspire danger is to give your monsters a reason to attack your probably-human protagonists unprovoked.
This, of course, is where the fear of large predators comes in. And consequently, a lot of massive monsters are meat eaters.
But let’s step back for a moment and take another look at the heaviest land mammals. Notice a pattern? Every single one of them is an herbivore, or a plant eater.
That’s because bigger animals need to eat more to sustain themselves. This deceptively simple fact is important for two reasons. First and foremost, plant life is more abundant than the animals that eat it. This is because the way energy is transferred from producers (such as plants) to consumers causes some of that energy to be lost in the process, especially since consumers will have to eat more than once in their lifetime.
Thus, a given amount of plant life sustains a smaller amount of the animal life that eats it. The animals that eat those animals, then, have to be even smaller in number. And if anything eats those animals, their numbers must be smaller still.
Because that amount, or biomass, is actually a combination of the size and number of organisms, predators can sustain larger populations by not being as large individually. It basically gives them more food to go around.
The second reason predator species are limited in size is because they need to actually catch their food. Think about it: trees and grasses can’t run. Mice and deer can. And even ambush predators need to move quickly to strike once they’ve snuck up on their prey.
And while predators need to be relatively large compared to their prey if they want to take it down, that’s comparative. Once you’ve reached a certain size, all that bulk is going to make speed and especially maneuverability tricky. Conversely, truly large prey animals can use their size itself as a defense, becoming too thick-skinned and strong for predators to take down even if they aren’t maneuverable enough to escape them.
Incidentally, you may have noticed that sea animals don’t follow the “biggest animals are herbivores” rule. Food webs in the ocean are a bit odd because of the role plankton play in the ecosystem. It means that tiny fish and krill are abundant enough to support even massive predators. And it’s no coincidence that the largest ocean predators are baleen whales. The krill they eat, though mobile animals, are slow enough to easily catch.
So, what about my monsters?
All these factors paint a picture of what it means to be a truly large creature. Massive monsters are still entirely possible, but you can fit them a bit better into your ecology if you remember the following guidelines:
- If you want giant bugs, get creative. True insects and arachnids are limited in maximum size by their body plans, but maybe that’s the first clue that your monster isn’t what it seems. Your horrifying “giant spider” could actually be a member of an entirely new order of animals. This has the bonus effect of letting you tweak your critters in other “non-spidery” ways for a surprise twist. Conversely…
- Consider basing your ecology around a high-oxygen world. Insects weren’t the only serious megafauna in high-oxygen times. And the existence of megafauna doesn’t stop smaller creatures from being around either. Maybe there are all kinds of huge critters roaming in your world.
- Consider how your monster’s size affects its mobility. Most megafauna are stocky and muscular, more focused on strength than speed. Of course, the giraffe shows us that every rule has its exceptions.
- Consider making your massive monsters herbivores. Even if you want your giant beasts to be dangerous, predation isn’t the only way to do it. Herbivores can be extremely territorial, defensive, and unpredictable. And revealing that your supposed “man-eating” monster is a simple grazer could make for an interesting twist. Especially if that twist lulls your audience into a false sense of security until your hero discovers just why that monster got such a nasty reputation…
- If your monsters are predators, they’re probably territorial. Most large land carnivores are solitary, protecting a swath of land for themselves. And the exceptions work together to bring down larger prey and still defend territories from rival groups. This ensures that they don’t run out of food, and your monster predators should behave the same way.
- Save your biggest beasties for the water. This is the best place to find megafauna, even among hardcore predators. And if you’re going for the “scare” factor, you can’t do wrong by combining the fear of dangerous beasts with the fear of the deep.
However you choose to use them, fantastical megafauna are sure to leave a truly massive impact on your setting.
Sources: UCSC on giant Mesozoic insects, Giraffe leg support by LiveScience, Forbes on herbivorous megafauna, Wikipedia article on ecological pyramids