Designing Aliens

Destiny's Forge introduces a new alien species to the Known Space universe, the Whrloo. They are small and quasi-insectoid, and they can fly, awkwardly, in Kzinhome's heavy gravity. They speak with a heavy buzzing inflection, and their eyes are on stalks - and we don't learn too much more. The Whrloo are background rather than major players in the book. Most of their story remains untold, perhaps for future writers to develop. Even so I had to consider a lot of factors in creating them and there's a lot of background there which simply won't make it into the book.

I started by deciding I wanted to make a flying sentient species, which is already a design challenge. Weight is at a premium in any flying creature, and brains are heavy. The largest flying creature we know about is the pteranodon, which sported a wingspan of up to thirty feet, but even a large one would have weighed under a hundred pounds. Also because of the inexorable laws of physics the power requirements for a winged creature scales up faster than its available muscle power does. The inevitable result of this is that big flyers like eagles and condors (and pteranodons) are inevitably soarers who get their energy from riding updrafts. Even a loon needs a long running start to get airborne. If you want something that can hover (and I did), you need something the size of a hummingbird or a bumblebee, which most certainly rules out a large brained sentient. Of course it isn't just raw brain size that counts, but the ratio of brain size to body size, otherwise elephants would be getting people to move heavy logs for them, but there must still be a certain minimum brain size necessary for complex thinking. I made the Whrloo about a metre tall, which seems to be as small as you can get before a big brain becomes problematic. Homo Floresiensis, the recently discovered "hobbit" subspecies of humanity, was about this tall, and evidently had a reasonably advanced civilization. If we cross a flyer as big as pteranodon and a thinker as small as Homo Floresiensis we can just barely get a flying sentient, but it's asking too much of evolution to make it hover.

Fortunately physics gives as well as takes away, at least in science fiction where the writer has broad liberty to adjust the background assumptions. The Whrloo fly with gravbelt assistance on Kzinhome, with its high gravity and not-too-dense atmosphere. Their homeworld is something else again, and we can adjust it as necessary to create large and intelligent flying creatures. Normally a world shapes its inhabitants. Here the inhabitants will shape the world.

Everything grew big in the age of the dinosaurs, and one possible reason is that the atmospheric oxygen level ranged from 30-35%. Oxygen availability is very directly linked to size for insects, which breathe rather inefficiently through spiracles in their exoskeleton and so need to maintain a certain ratio of surface area to volume at a given partial pressure of oxygen. Since surface area goes up as a square function while volume goes up as a cube function, this effectively keeps insects on the small side today, when oxygen levels are sitting at 21%. However in the Cretaceous dragonflies got to be a couple of feet across. It is speculation to extend that logic to say that dinosaurs got big because they were better oxygenated and thus could do more with less muscle than we can, but it isn't unreasonable speculation and that's what science fiction is all about. The Whrloo homeworld will have 35% oxygen content, as much as Earth has ever had, and as much as we can reasonably put into a biosphere. Any higher than this and dry vegetable matter will spontaneously combust, a process which soaks up oxygen and serves as an effective limit on how high oxygen levels can get. The pre-civilized Whrloo wouldn't have had any trouble discovering fire. Their problem would have been preventing fire from discovering them.

This much oxygen isn't enough to make something that can hover, so lets do some magic with that 35% oxygen content. What counts is the partial pressure of oxygen and not it's total percentage in the atmosphere. So we'll say that we'll have .35 atmospheres of oxygen partial pressure, which puts up against the spontaneous combustion limit, but we can pile on all the other gases we want. This will reduce the total percentage of oxygen in the atmosphere, but it won't change the .35 atmospheres of partial pressure. This way we can have more than one atmosphere of total pressure. That's a good thing, because the denser the air is, the easier it is to fly. How dense can we make it? The total pressure on the surface of Venus is ninety atmospheres, enough to make the air there a thick, sluggish fluid. Flying would be easy there, but an atmosphere this thick comes with its own problems. Venus's largest problem is the greenhouse effect, which makes its surface temperature hot enough to melt lead. We can solve part of that problem by making all the extra gas nitrogen instead of carbon dioxide (our two primary choices for an earthlike world). A living biosphere will actively scrub CO2 out of the air, so this assumption is plausible. However even a thick atmosphere will hold more heat than ours because it will hold more water vapour, and we need lots of water to make a biosphere work. Theirs will be a cloudy world.

This argument tells us that the Whrloo homeworld has to be farther from its sun than Earth is from Sol, or that its sun has to be less bright, in order to balance the planetary heat budget in the face of the greenhouse effect. Either choice works, but since Sol is larger than most stars we'll say theirs is smaller, which means cooler, which means both dimmer and redder. That combined with the clouds means the Whrloo will need big eyes to see as well as we do. Their evolution may favour visual acuity over discrimination, which will make them colour blind. To make up for it, we'll give them the ability to determine the polarization of light. They'll have excellent night vision, but it will be attuned to a redder peak frequency than ours is. A Whrloo on Earth or Kzinhome will need sunglasses in the day, but they won't want ones with polarizing lenses. A smaller star is probably an older star, because small stars live longer, and it also means less available energy flux at the surface for photosynthesis, a problem exacerbated by the clouds. Less energy means it will take more plants to support the food chain, which limits both the size and the density of large herbivores and the predators that will eat them. The Whrloo have another good reason to be small. This also convieniently explains why they have only about .4% oxygen in their atmosphere (though the oxygen partial pressure is still .35 atm, remember). Less photosynthesis releases less oxygen to the world.)

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