Heading underground

I like to write about worldbuilding, and about the biology of organisms that are unfamiliar to us. Terrestrial ecology and biology and geography are our only guide to developing alien worlds, alien biologies. Yet.

Since yesterday was Halloween, I wanted to write a worldbuilding/biology exercise on a scary topic: caves! Creepy, dark, and full of bats. What could be better?

Hall of the Mountain Kings cave

Let’s treat a cave as an alien habitat and work from there. What are its important characteristics? How would ordinary terrestrial animals have to adapt to live there? Are there things they don’t need any more?

Caves are remarkably difficult places to live: dark, often wet and very humid. Not everything about caves is hard to live with. The temperature is usually constant, with no highs or lows with the season. Aquatic creatures, amphibians, and other soft-bodied animals prone to drying out might do well in caves if they can adapt to the darkness.

The hardest thing about living in a cave is one I haven’t mentioned yet: the lack of food. Photosynthetic plants form the base of nearly all food chains. In the darkness of a cave that whole level is taken away. All of the food in a cave has to come in from outside, whether falling in through an opening, or washed in by underground streams, or carried in. Animals that wander in and die are an important but irregular contribution.

Some creatures like moist dark places, and can live in caves, under logs, or in leaf litter (troglophiles). These species look the same whether they live in caves or under your porch. They can spend their entire lives in a cave, or no time at all. Human cavers probably fall in this category.

Other animals use caves occasionally, often as shelter (trogloxenes). Bears do this, and bats. These species may be somewhat adapted to caves, but they can live other places as well. They can’t spend their entire lives in cave. The animals that go back and forth are key because they bring in calories and nutrients. Entire systems live on bat guano, a very rich resource.

Townsends bats

The creatures that live only in caves are the most interesting. These troglobites are adapted to their surroundings, and no longer look much like their ancestors. They can’t survive very long outside their caves. Cave species have often lost all pigment and even their eyes. Eyes and coloring are both wastes of energy if they will never be used. Instead, cave dwellers often have longer antennae or bristles, and an improved ability to navigate by touch or sound.

Cave fish

Reproductive isolation is key to the formation of new species, so each separate cave system can have its own cave fishes, and cave crickets, and cave salamanders. They may have all descended from the same ancestor, but after many generations of isolation have become new species.

Cave dwellers have adapted to the low-energy environment. Most of them have slow metabolisms, and can go for long periods without food. As a consequence, these species can also be very long-lived.

Cave salamander

My favorite cave creature is the olm (Proteus anguinus). These foot-long (30cm) aquatic amphibians are found only in southern Europe. They illustrate all of the cave-dweller traits I’ve mentioned: the juveniles have eyes, but the adults lose them. They have no pigment, although olms can produce melanin if exposed to light. The olm does have the ability to sense light, presumably so they can find their way back underground if they get washed out of their caves.


Olms have a wide array of sensory equipment. They can detect organic compounds in the water in very low concentrations, and can even sense electrical fields.

They have the slow metabolism characteristic of cave-dwellers. It takes an olm fourteen years to reach sexual maturity, and they can live to at least seventy, and maybe one hundred years. An olm can go at least ten years without food.

My very favorite thing about olms, though, has nothing to do with their ecology. In Slovenia, olms would be washed out of their caves after heavy rains. Baby dragons living underground? Their parents must be down there somewhere!

Things to consider for worldbuilders when thinking about the local scale: what are the key physical characteristics of your habitat? What traits do your creatures need to live there? Or not need? Where do needed resources come from (energy/nutrients/water)?

Animals on our planet can be incredibly plastic and adapt to all sorts of apparently-inhospitable environments in remarkable ways. Studying our own biology and ecology, especially at the fringes, offers the science fiction writer all sorts of wild ideas. (And next month I’ll be writing about something even weirder!)

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