You probably don't need me to tell you that abilities such as immunity to heat and cold don't make any sense, biologically speaking, which is a shame, because they're tremendous fun.

For about a year now, I've been running a monthly-ish campaign for one of my gaming groups wherein the characters are agents of a multiversal peacekeeping force. The game is tremendous fun as it lets me drop them into different universes, settings, and genres every session. Last week, the characters found themselves sent on a mission to a world where the base temperature perpetually hovers at about -10 degrees Celsius (that's 14 degrees Fahrenheit for Americans or 263 degrees Kelvin for physicists... well below freezing, but far from instantly lethal to a healthy human). Aside from the humanoids who had made their home there, the world had one indigenous species: a species of troll who had evolved to thrive in the frozen wasteland. When preparing for their assignment, the characters loaded up on cold weather gear and heat-based weapons, on the (entirely reasonable) assumption that cold-based weapons would be ineffective against any creature that lives on an iceworld.  Though reasonable, the assumption was false; the trolls had some cold resistance, enough so that they were immune to being damaged by their natural environment, but any cold damage that exceeded their resistance would hurt them normally. What we see here is that my players assumed that the trolls would have cold *immunity* whereas I gave them cold *resistance* for the simple reason that, from any rational point of view, the concept of cold immunity really doesn't make very much sense.

There are, on Earth, a number of animal species (and non-animal life forms) which are exquisitely adapted to live in cold that would kill an unprotected human. Creatures who thrive in cold environments are called "psychrophiles" or "cryophiles." A Greek-speaker will immediately see the derivation of the word: philo, meaning lover, and cry, meaning cold. In most biology texts, a psychrophilic lifeform is one that is capable of growth and reproduction between about −15°C and +10°C... temperatures that a lot of the world's human population lives in for at least a quarter of the year, albeit with the benefit of walls, clothes, and heating. Fifteen degrees below zero isn't actually that cold; a Canadian mid-winter tends to fluctuate between about -10 and -30, and if you were in Antarctica, -15 is about as warm as you'd ever get. The vast majority of psychrophiles are bacteria and other microbes, not animals. Many of the animals that live in cold environments are actually better classified as "facultative psychrophiles" or "cold-tolerant" organisms, able live and thrive at lower temperatures but needing warmer temperatures to properly grow and reproduce, as opposed to "obligate" psychrophiles which die if their environment approaches what a human would consider comfortable. While the distinction is relatively straightforward in microbes, it can be harder to tease apart in animals. Polar bears, for example, are animals which are superbly adapted to live in the cold, and protect themselves from it by producing a thick layer of blubbery fat and a specialized fur. A polar bear will overheat (in game terms, begin taking damage) somewhere between ten and twenty degrees above zero, which is the range at which most humans will want to wear a sweater at the very least, so you might argue that they're psychrophilic. When a polar bear lives in a zoo, however, and is constantly in warmer conditions, it doesn't expend energy maintaining its fatty layer and tends to grow to be smaller and leaner. An animal otherwise capable of handling the cold far better than any human thus adapts to a warmer climate... so is it facultative or obligate?

Polar bears illustrate one of the ways that an animal becomes resistant to cold. Whales use much the same strategy of a blubber layer, as well as some other adaptations that work better in the water than the air. There are other adaptations, on a microbiological level, that confer cold resistance. For example, the structure of a body's cells is composed of and filled with proteins that have to be in just the right shape, with just the right amount of flexibility to their structure, for them to work. One of the ways that cold temperatures kill things is by freezing those proteins, so that they either become inflexible or else break entirely. Many microorganisms that live in cold environments have adapted to have unusually flexible proteins, so that cold temperatures don't impair them. The flip-side is that when the temperature warms, the proteins become too flexible and lose their function (or "denature"). The organism balances stability with flexibility the same way as any other living thing, just with a different set-point. If you compare humans who have adapted to live in different regions -- an Inuit with someone from the Caribbean, for example -- you don't see such a profound genetic difference, but you do find differences in their metabolic rate, rate of energy consumption versus fat deposition, and so forth, along with the always-important factor of culture and what a person has learned to call "normal" while growing up.

All of this is to say that, to the best of our knowledge, there's no creature in the real world with anything like the complete cold immunity seen in the oozes or white dragons that one can find in the SRDs; a white dragon might certainly be capable of existing at -80 degrees Celsius, although no large animal on Earth is, but the adaptations that would make this possible should, by all rights, mean that the dragon would die at room temperature, and vulnerability to fire doesn't really capture this. Some of the classically cold-immune monsters, such as skeletons, might really be able to function at any degree of freezing just as well as they do in the desert, but if their bones are held together by tendons and ligaments, the way they're drawn in most gaming artwork, then logically they should still freeze solid and become paralyzed at much the same temperatures that a human would. Of course, in the case of demons and devils who are totally cold immune, we can presumably chalk it up to "magic," which doesn't have to make logical (or biological) sense.

More than four years ago, Dr. Eris Lis, M.D., began writing a series of brilliant and informative posts on RPGs through the eyes of a medical professional, and this is the one that appeared here on September 29, 2013. Lis is a physician, gamer, and author of the Skirmisher Publishing LLC OGL sourcebook Insults & Injuries, which is also available for the Pathfinder RPG system.