by Martin L. Shoemaker
Martin L. Shoemaker discusses the conception of the fictional slug species featured in “Sluggish,” his eleventh Analog story, now available in our [January/February issue, on sale now!]
Like any good scientific investigation, the natural history of Quijote Orragi (the Orrago 5 slugs from “Sluggish”) has many roots. While you don’t have to know this history to enjoy the story, I thought you might appreciate a look into how it came to be.
The first and most obvious root is the incident that inspired me, first as humor and then more seriously, to write this story. A few years back, some friends of mine went on a writing retreat. (Names withheld because they want nothing to do with this story. It’s too gross for them. My apologies if it’s too gross for you as well, but biology is messy.) They packed off to the northern California woods, and had a great time . . . except for the slugs. Ariolimax californicus californicus, gig green banana slugs, common to Northern California woods. They posted pictures of these disgusting little green turds on Facebook, thoroughly grossing out their social media friends.
But not me! I thought the whole incident was hilarious. (I fully concede that I was three time zones away from the slugs. Had I been closer, I might have been grossed out, too.) And so I joked about writing a story of the writers in the slugs.
The surest way for a writer to be stuck writing a story is to joke about writing the story, not taking it seriously. That’s how the story ideas sneak past your defenses and take hold of your brain.
Soon I was researching slugs, slug biology, and slug behavior. The evolutionary logic of animal behavior can be utterly fascinating, even if it’s gross. There are lots of different slugs in the world, with lots of feeding and reproduction habits. I took a few, juggled them together, and eventually ended up with the Orrago 5 slug. Only for story purposes, it needed to be a lot bigger: a 2-meter slug.
And this brought me squarely up against the second historical root of the Orrago 5 slug: the giant lobster story.
No, there are no giant lobsters in “Sluggish”; but there’s one in my head anytime I design a creature, especially a large creature. When I was around 11, I read in Boys Life that discussed designing monsters for books and movies using realistic science. Being a kid who loved writing stories, especially science fiction, the article fascinated me for 50 years. It introduced me to something mind expanding for me, even though I couldn’t understand the math behind it at age 11: the Square-cube Law. The article stuck with me because of its basic scientific truth: If you tried to design a giant lobster the size of a small office building, and you tried to make it scientifically realistic, you end up with something that doesn’t look anything at all like a lobster. In fact, the author argued that a lobster the size of a Tyrannosaurus rex would end up with a body design that was pretty similar to T. rex or other dinosaurs. And the primary reason was the Square-cube law, which tells us that you can’t radically enlarge creatures without changing their body plan if you want them to survive. A horse the size of an elephant would have to look an awful lot like an elephant, or its legs would break under its own weight. I’m sorry to say that the Square-cube law forever ruined Godzilla for me, and even giant robot stories. When the body gets bigger, the body plan has to change, or the body will fail.
Note that I’m not saying you can’t enjoy your Godzilla movies or Transformer movies. It science fiction, after all. If that’s the type you enjoy, I’m happy for you. But I like my science fiction with a heavy dose of realism. Hard science-fiction in the old Analog tradition.
The surest way for a writer to be stuck writing a story is to joke about writing the story, not taking it seriously. That’s how the story ideas sneak past your defenses and take hold of your brain.
Without getting lost in math swamp (heheheh, swamp . . .), what the square cube law tells us is that if the body is twice as long but otherwise proportional, it has four times as much skin surface —as well as stomach surface, lung surface, and brain surface—and eight times as much mass. And so on. A person ten times normal human size would have a hundred times as much skin and lung tissue, but a thousand times the weight.
And that’s a problem because your lung surface is how you take in air. Your stomach surface determines how you take in food. You have a thousand times as much muscle and or other organs to feed, but only a hundred times as much stomach surface. That ten times human shape is going to starve and suffocate because it can’t get food or air in fast enough. The reason why an elephant-size horse would break its legs is because the strength of the leg for a given leg shape is dependent upon the cross-sectional square area of that leg, but the weight it’s holding depends on the cubic. The only way that an elephant-sized horse can walk around without breaking its legs is to have legs that looked more like an elephant: much thicker, with bigger, flatter feet.
And that is the secret to designing larger creatures: You have to change their body plan to make it possible for their body to support their weight and for their skin to hold in their guts. I love the tremors movie series, but a Graboid very likely would collapse because there’s too much body inside that Graboid shape. It should collapse like a pancake. I’ll confess, though, I’ve never seen an anatomical drawing of a Graboid. That tough skin may be thick enough to support their mass.
But a mere two meter slug? It’s going to be a real problem holding all of that body mass inside that shape. As a D&D player, I love a gelatinous cube; but in reality, a cube of Jell-O one inch by one inch by one inch will just sit there, while a cube ten feet by ten feet by ten feet is going to collapse under its own weight. The sides of the cube can’t hold up what’s inside.
One answer to this problem of how an Orrago 5 slug supports its body weight is that it lives on entirely aquatic lifecycle. That’s true of some real-world slugs as well, but on Orrago 5 it’s essential. The water can do a lot to support the weight and to distribute that support all way around. Instead of a slug sitting on the ground with only that portion touching the floor holding up the weight, in the water, nearly all the slug’s surface helps provide buoyancy. This spreads the load around.
And the other major adaptation—as you know if you’ve read the story—is that compared to a normal slug, an Orrago 5 slug is practically hollow. It may look like a slug shape from the outside; but if you dissected it and dissected a banana slug, the overall body plan would be very different. A major factor constraining the size of many organisms is how much oxygen-bearing and moisture-bearing air they can get to all the tissue in their bodies to keep them alive. In insects, for example, they don’t have lungs, they have spiracles: tiny little pores all over their chitinous exoskeletons to let air in to contact the tissues within and provide them with oxygen also with water. If you traveled from the icy north of, say, Michigan to the sunny tropics, you may notice that the insects in the South are often bigger. Sometimes much bigger. At a zoo one time, I saw an exhibit of some sort of insect where the thorax was as big as my two fists put together. That size insect is only possible in a tropical environment were more humid air can get through the spiracles. If you want to get bigger than that, or if you want that organism to survive in less humid areas, that organism is going to have to evolve lung. Lungs are more efficient than spiracles because their big sacks honeycombed with countless little alveoli, micro sacs like a sponge to let the air flow through the spongy material and contact the tissue in vastly more places than if you could only breathe through your skin. The lungs are like a huge volume of skin crammed into a tiny space in order to take up oxygen.
So with the Orrago 5 slug, I let them evolve lungs, and also to use them as swim bladders, such as we see in many bony fish. Air in the bladder helps them to maintain neutral buoyancy.
And because the Orrago 5 slug has such large lung pockets, the Square-cube law is bypassed to a degree: where an ordinary slug is mostly heavy muscle which must be fed and oxygenated, the Orrago 5 slug is mostly empty space. So it might be 25 times the length of an ordinary slug, and its skin surface might be 625 times the surface area, its mass is nowhere close to 15,625 times the mass of an ordinary slug. I haven’t worked it out precisely—I don’t have an Orrago 5 slug to weigh – but my guess is the mass ratio is probably very close to the skin ratio, or 625 times the mass of an ordinary slug. So suddenly slugs which can leap out of the water for short periods become more believable. The muscular outer skin of the slug is just about proportional to the mass of the slug, so the slug can actually move with surprising agility.
There is more that goes into the design of the Orrago 5 slug. The sexual metamorphosis is common in certain species of slug. The egg laying and fertilization mechanisms are common as well. I didn’t take many liberties there, though I don’t honestly know if the reproductive mucus of slugs hardens in the air. That was invented strictly for the story.
But I tried my best to make a believable giant slug. No doubt a real malacologist could poke holes in my design, but it’s plausible enough. And what I learned in writing this is a newfound respect for some of my fellow Analog authors who create plausible creatures on a regular basis. I’m mostly known for my engineering hard science-fiction: spacecraft and space stations and exploration gear and rendezvous orbits. I know my way around those pretty well for an educated layman.
But biological hard science-fiction has a whole different set of constraints from engineering. It’s hard. And it’s gross.
There’s one final thing I’d like to note about the natural history of Quijote Orragi. After I told this story to a friend who has excellent Internet search skills, that friend eventually hunted down the issue of Boys Life that taught me that a plausible lobster would look like a Tyrannosaurus rex. It was the May 1974 issue, in the article “How to Make a Monster,” written by . . . Ben Bova, the second editor of Analog. It’s like Analog has been guiding me to write for them since before I knew I wanted to be a writer. And I thank them for that.
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