Points To Remember When Designing SF Creatures & Species
Just some things to keep in mind when you're creating creatures for your science fiction or fantasy settings, so as to make your settings feel richer and your creatures feel more believable.
Things are the way they are in nature for a reason. When you're designing a fantastic species, it might be tempting to throw out all natural conventions and design something completely different for the sake of being unique. But if you want your species to be plausible, you first need to understand why real-life species are the way they are lest you alter something so drastically that your species would realistically have little to no chance of survival in a real ecosystem. So take some time to research and learn why real-life animals are the way they are or were so you don't end up accidentally making a lifeform that by all rights should have gone extinct - or should have never even evolved in the first place.
Conversely, researching real-life animals (both living and extinct) is a great way to get an idea of what species that do the things yours are supposed to do might plausibly be like. For example, if you're trying to create an ambush predator, look into how various ambush predators look and how their bodies are structured. Also take note of what you do not see in real ambush predators, because there's probably a reason for that, too. So whatever creature you're creating, look into similar ones to get an idea of how they work.
The purpose of DNA is to make more DNA. So whatever cool feature or nifty ability you might be considering for a given species, ask yourself: "How does it make this species more likely to survive long enough to reproduce, given the conditions it lives in?" Go this route, and you'll be more likely to end up with a plausible species than otherwise.
(And while we're here, this is why the claim that humans only use ten percent of their brains is so patently absurd. Why would humanity have developed all of this brain power in the first place when it can obviously get on just fine without using it? Why waste so much energy and nutrients supporting so much body tissue that isn't even going to be used?)
Also, even if your species doesn't use DNA to reproduce, the same principle applies as long as the rules of natural selection apply to your setting or species - and the rules of natural selection always apply if your species reproduces in any way (EG, vampires turning humans into new vampires counts as reproduction), and if there is any chance of the offspring coming out with slightly different features from the parent or parents.
There is absolutely such a thing as being too successful. For example, a species that has no difficulty finding and acquiring its food, reproduces prolifically, and has an extremely low death rate compared to anything else is very likely going to eventually proliferate to the point where it destroys the very resources it needs to survive soon after die out from starvation - unless it's cognizant enough to realize that it has to carefully manage what it does so as to avoid this kind of disaster.
Beware of the "fish in the forest" fallacy. This is a trap occasionally fallen into by designers of speculative future animals - someone might take, for example, an aquatic animal and plop it into a dry environment with few changes to its body plan and/or with no regard to how similar aquatic creatures adapted to life on land. Fish did take to a completely terrestrial lifestyle once - by becoming reptiles, birds, mammals, etc. Thus it's likely that any other fish that took to the land would end up just as different from the fish still living in the water as these creatures are. If it was so easy to live as a "fish in the forest," the fishy ancestors of these creatures would not have needed to change so drastically, and thus would already be the proverbial fish in the forests. Meanwhile, actual animals that didn't change very much from their fully-aquatic forms when adapting to land - EG, snails and terrestrial isopods - must still live in fairly moist environments.
The same principles go for any animals whose ancestors have adapted to any vastly different environment. Look at the ones who have done it (preferably as many species as you can) and take into account the changes it took to make it possible.
Life stages do not universally follow the same proportionate lengths across species. It's commonly believed that if a species lives for a very long time, then it must also take a very long time to mature. But this is false. Macaws can live just as long as humans do, but take only three to four years to reach sexual maturity. The seventeen-year cicada, as its name implies, takes just that long to reach adulthood - and then dies after a few months. A cat reaches sexual maturity in about a year, but can live for an average of fifteen - if the average human lifespan was proportionate to this, then they would be living well around two hundred years. For more information on what does affect lifespan, this is a good page on it.
If you're building up an entire ecosystem, don't forget the microfauna! It's sometimes easy to get focused on creating the large, majestic-looking animals that one forgets the little things, too - the creatures that fill the same ecological niches as small rodents, songbirds, arthropods, etc.
And don't forget the aquatic animals! It happens sometimes that people end up richly developing everything on land, but overlook what's going on down in the water. Realistically, aquatic animals should be just as varied and diverse as anything on land.
Don't forget your species' close relatives! If your species evolved via natural selection, then it should realistically have - or have had - close relatives out there. For example, consider bats. There isn't just one bat and one bat alone, but there are many different kinds of bats of all shapes and sizes that live in different climates and eat different foods. Same goes for just about any type of animal you can think of - so when you're designing animals, keep this in mind.
Run some thought experiments. Did you come up with a predator that hunts using an unusual method? Imagine how an encounter between your predator and its prey might go down, considering that the prey very much does not want to be eaten and will be doing everything it can to get away. Did you come up with a creature that has an unusual defense mechanism? Imagine that creature using it against a predator that wants very much to eat it. If your creature ends up losing out, what you had in mind might not be such a great idea.
The tail of land vertebrates is simply an extension of the spine. You can see for yourself how this works by looking up pictures of cat skeletons, dog skeletons, bird skeletons, etc. This also means that if you want to put a tail on a humanoid, the correct thing to do is to visualize what this humanoid would look like if its spine didn't stop right above the buttocks, but went on past it instead.
Realistically, human/animal hybrids should not have two sets of pinnae (external ears). Although some mammals may have pinnae that are set higher up on the head than those of humans', the ear canals themselves are always in more or less the same location as humans' are - below the level of the eye sockets and just behind the jawbone. You can see this for yourself in the skulls of various animals here. (For information on the hearing of aquatic mammals like whales and dolphins, go here.) And while we're at it, a second set of ears would imply a second jaw, as the bones of the inner ear are modified jaw bones.
If you liked this, you might also be interested in:
Evolution: 24 Myths And Misconceptions
Understanding Evolution @ Berkeley University
How To Create A Scientifically Plausible Alien Lifeform
The Desert Adaptations of Birds & Mammals
Antarctic Animal Adaptations
Winter Adaptations Of Animals
Rainforest Animal Adaptations