Worldbuilding Wednesdays: Terrestrial Adaptations

Welcome to Worldbuilding Wednesdays!  Every Wednesday, we spend what is probably far too much time walking through our worldbuilding process.  In this week's post, we discuss the dangers of running in low gravity.

What We Have So Far

Our world is getting fairly lively.  We have a continent roughly shaped like Australia, combined with the Mediterranean and set on its side.  We have snow-capped mountains more than twice the height of Mount Everest, hyperforests, megasavannas, fog banks that roll hundreds of kilometers, and crazy tides that dance to the movement of the two moons and the sunlike Lantern.  Over these, we now have a variety of creatures leaping and gliding from tree to tree, and a few predators dropping up to a few hundred meters to land on our head.  In the distance, gliding over the fog, we can make out massive soaring "sky whales," which could easily touch both hoops of a basketball court with their wings.

What we don't have yet are any creatures on the ground.  Today, we fix that.

Terrestrial Drift

Just as with the aerial creatures, terrestrial creatures are going to be affected mostly by the low gravity of this new world.  Some of those effects are obvious, but others might surprise you.  Probably the most surprising is that you won't see a whole lot of creatures running in the conventional sense.  This isn't directly due to the gravity, although the motion of running is dictated in part by gravity and will definitely change.

No, the problem is friction.

Friction is one of many forces at play when creatures move.  It isn't too much of a concern to aerial creatures, who are mainly trying to move through air, but for creatures on the ground, a sweet spot must be found.  Too much friction and we move like there are brakes attached to our feet.  Too little friction and we slip.  And, as it happens, the amount of friction we experience has a proportional relationship to our weight.  Drop our weight by a factor of ten, and the friction keeping our feet in place as we move also drops by a factor of ten.  In short, for creatures of the same size and weight as they are on Earth, our world is effectively coated in oil.

There are a couple of methods creatures can employ to keep from slipping.  The easiest is to grow heavier, and so we'll find few creatures of a normal size roaming the megasavannas, but adaptations to increase the friction of their own feet would also be a necessity.  Some of the more unusual adaptations will trade complexity for stability.  In short, our list of animal types is about to get very strange.

All Oiled Up

We're going to work through some conclusions based on the above, just as we did last week.

  Normal Animals Will Be Rare.  While there are certainly some creatures that could move to our world from Earth without any adaptations, they'll be the exception, rather than the rule.  Mostly, these "normal" animals will be those who don't do a lot of running or those who have unusual modes of locomotion- a centipede would work pretty well, and a rabbit might be able to escape predators more easily by leaping through the air and then skidding to a halt.

  Larger Animals Will Be Common.  As mentioned, the easiest way to counter the relative loss in friction is to be relatively larger.  Now, animals could conceivably have a mass of ten times as much as normal and still have all the necessary support of their body, since it will weigh the same, but weight is downward motion.  Our new world doesn't make lateral, side-to-side motion any easier, so larger animals will be more plodding and deliberate than they are on Earth.  Those animals that rely on speed will have to find a middle ground between being large enough not to slide and being light enough to still move quickly.  Those animals who rely on size will almost certainly be close to ten times their normal mass, or a bit more than double their normal size in every direction.  Bears, moose, and elephants, for example, will be, if you'll forgive us for being a little precious, "mammoth."

  Leapers Will Abound.  Since animals will be operating under .1 g, everyone's vertical leap will be much larger than normal.  Those animals that remain close to their normal size will be able to make leaps easily ten times their normal distance; with hang time, it could be considerably further.

We're not saying this will be the norm, but...

  Friction Footing.  A possibly strange aside.  Did you know that the reason our fingers and toes get wrinkly when wet is to make it easier for us to grip things?  True fact.  The exaggerated ridges increase our skin's friction.  And if you can't tell where we're going with this, we'll skip to the end:  animals adapted for our world will likely have wrinkly pads or similar features, such as micro-claws, that will allow them to grip better.  You still won't see them running quickly, since the grip strength of their pads will be working against mass, not weight.

  Tetrapods Won't Be The Only Option.  One of the reasons that tetrapods- four-footed creatures- are so popular on Earth is that it turns out that tetrapods are really good at running.  Four feet on Earth strike the perfect balance between speed and stability.  But that isn't the case on our world.  Here, tetrapods are likely to slip and slide without some assistance.  Conversely, even adding just one extra pair of legs would make an animal 50% more stable.  Granted, having two sets of hips would make their bone structure crazy complicated, but if the trade-off is that they are the fastest runners on our world, you can bet at least a few species will try it.

  The Extra Limbs Affect Other Creatures.  Tetrapods on Earth evolved from fish into the four major animal groups:  birds, mammals, reptiles and amphibians.  If hexapods (six-footed creatures) have a chance to compete, we can expect them to try and fill those ecological niches as well.  Which means that, in addition to the animals we talked about last week, we can expect some varieties of bird or arboreal creatures to have six limbs as well.  In fact, it would be possible for some six-limbed creatures to evolve into creatures with four legs and a pair of wings... which means...

  Mythological Creatures Are Now On the Menu.  We likely won't see horses, or at least not horses with hooves, but something that looks like a griffon- a big feline predator with wings- is definitely possible, thanks to the juxtaposition of creatures evolving six limbs, flight being cheap, gliding being an aid in leaping and climbing, and claws being a useful friction-adaptation.  We can use similar logic to include other winged beasts, including the classics.

  The Fastest Runners Will Be Kind of Nightmarish.  Under these conditions, the creature with the fastest body type will be one with more limbs than you're used to, each ending in a paw, foot, or hand that can grasp the ground, allowing it to effectively "climb" across the world's surface.  It will likely have an insectoid body style to it, considering their success with more-than-four-limb setups.  Up to a certain point, extra limbs and a long, low body will only make it faster.  Add this all up and, instead of horses, this world will have sleek, serpentine monsters that more or less "skitter" instead of "sprint."  They will be quite a bit bigger than your typical human.  And they will be able to move far too fast for you to escape, so hope they're herbivorous.

Conclusion

Good news:  We now have an excuse to put something vaguely like a dragon in our world.

Bad news:  Majestic herds of horse-sized centipedes with hands on the end of their multitude of limbs can now stampede in your general direction.

Worse news:  We still have one group of animals to go, and they like to have too many limbs, too.  Brace yourselves for next week.