#started researching at like. six. it's 2:20am. ...at least I stopped to have dinner?

[eyrieofsynapses]

so uh, I am brand-new to Star Trek (as in not even finished with the third TOS season), so I don't have much background here, but I have thoughts? I am also not a biologist, mind you, just researching online and using what I remember from gen-ed classes. I may have lost myself down the rabbit hole for… er. a while. a long while.

For the record, this is me trying to apply Earth biology and logic, which I know is not actually applicable to literal aliens or fiction. The joy of writing is being able to say "fuck it, this is fiction, I make the rules now."

But. Science is fun. So here's me trying to Science it.

First: points of consideration! Vulcan is a desert planet. It's close to its star with a thinner atmosphere, and notably hotter than Earth. A thinner atmosphere makes it harder for a planet to regulate temperature and may block less solar radiation. We also know, thanks to Amok Time, that humans can't breathe as easily there, probably due to lower oxygen. So, this means: 1) The temperature difference between day and night is probably more significant than Earth's (because desert and thinner atmosphere). 2) Less oxygen is available.

(For the record, I don't think humanoids would develop on a purely desert planet, but my tangent on this got so long that I'm cutting it. Also sci-fi hand-wavey suspension of disbelief.)

Second: what we already know! …or what I know from the first and second seasons of TOS, what y'all pointed out in the thread, and what I looked up via wikis. The links on episode titles go to transcripts.

Copper blood: Vulcan hemoglobin (the protein used to carry oxygen around) is copper-based, not iron-based (TOS "Obsession"), hence the green blood.

Blood pressure and heartrate: They have significantly lower blood pressure than humans, but a much, much higher heartrate—assuming Sarek's stats during surgery in "Journey to Babel" are applicable, anyway. (Again, I am very new to Star Trek, so that definitely could contradict something newer.)

Cool blood (and low metabolism): Their blood is cooler than humans', possibly implying a lower body temperature. McCoy also comments on Spock's nigh-unmeasurably low metabolism (TOS "The Paradise Syndrome").

Temperature tolerance: They have a considerably wider range of temperature tolerance than humans. (This is implied by their planet's natural temperature range, and by the fact that Spock is usually okay at human temperatures, but, when he can't regulate his temperature so well, apparently finds 125 degrees (presumably Fahrenheit) "tolerable" in "The Deadly Years").

(…also. According to Memory Alpha, a fan-made medical reference text provides some stats. But it's not considered canon, doesn't reference Earth science, and I spent way too long researching this to end it at that.)

SO. Let's go through those "what we already know" points.

Green blood and copper-based hemoglobin: This is a real thing! Some animals DO use copper instead of iron to carry oxygen around! Except they're invertebrates (ex. octopuses, some spiders), the blood is blue, it's not called hemoglobin, it's in an open circulatory system, and it's not like vertebrate blood. It's called hemocyanin, and it free-floats in the invertebrate equivalent to blood (hemolymph) rather than being shipped around by red blood cells.

(For the record, there are other types of blood that are green, but they don't use copper. But blood color varies for other reasons too. I'm handwaving this one.)

Vulcans clearly have closed circulatory systems (closed = blood pumps through vessels, open = sloshes around organs freeform), so they'd need cells to carry oxygen around, so they probably don't use hemolymph.

If you're wondering "but we evolved from invertebrates, so why don't we use copper?" like I was, it's because COPPER SUCKS AT CARRYING OXYGEN. At least in comparison to iron. (Unless you're in deep sea environments, which the desert is not.) So presumably, Earth vertebrates started using iron somewhere along the line. Vulcans did not.

My conclusion: Vulcan biology uses copper in a way that's analogous to how humans use iron, but somehow finds a way to make it efficient enough to match said iron's oxygen capcity. Also, considering that Vulcan seems to have less oxygen in its atmosphere, I'd guess their systems use oxygen more efficiently overall, or they just don't require as much. So maybe it doesn't even need to be quite as efficient. (Coming back later: to my gratification, Memory Alpha also notes the oxygen issue, although in this case it notes that Vulcans have more efficient respiratory systems to get the oxygen needed.)

Lower blood pressure, higher heart rate: I think beastlyanachronism is right about this one. However, circulatory systems are extremely complex and I know nothing about them (I'm a communications person, not a doctor, Jim!), so I'm not really the person to ask. I did some reading to try to educate myself for the sake of this question, though. Anyway: lower blood pressure implies that the Vulcan heart doesn't beat very hard, but it seems to beat faster to make up for the lower pressure. This clicks with the idea that Vulcans use oxygen more efficiently/need less oxygen than humans, since the heart doesn't have to do so much work.

Finally! Body temperature. I'm combining "cold blood" and "wide temperature tolerance range." And I did Much Research. I'm going to explain some biology stuff that some folks probably already know, but for those like me who don't remember the nitpick bits of biology class, I figure this might be useful.

Generally speaking, there are two types of animal when it comes to regulating body temperature: ectothermic (i.e., primarily relies on environmental heat sources and produces little internal heat; e.g. reptiles, amphibians, fish), and endothermic (i.e., generates internal heat and maintains a more-or-less constant body temperature; e.g. mammals and birds). It's way more nuanced than that, but we're talking broad strokes. (More reading: 1, 2, 3)

(Side note, the ducks kedreeva mentioned are maybe using regional heterothermy.)

Colloquially, you would hear these referred to as "cold-blooded" and "warm-blooded" respectively, but science doesn't use those terms anymore because it's not how they technically work; e.g., lots of active ectotherms keep their body temperature in mammal-range. There are others that conform to their surrounding environments, like fish, but as far as I can tell, they need a fairly consistent environmental temperature range. Endotherms, meanwhile, rely on their internal heat source: the excess heat energy from their internal functions.

Heat's important to animals for… well, multiple reasons, but a big one is chemical processes, because they're fiddly and need specific temperatures. Life itself is basically just a bunch of chemical processes.

The problem with ectotherms is that they're reliant on their environment. Their activity is tied to temperature, and the colder it is, the more they slow down. If it gets too hot, they overheat. Also, they have slower metabolisms, and don't necessarily store energy the way endotherms do. Long, extended periods of high activity are harder—if not impossible—for them to maintain, and they can't support big, energy-hoarding brains like endotherms can. (More on intelligence and endo vs ectotherms here.)

Overall, I find it hard to match this with Vulcans, because they seem to perform consistently across a wide variety of temperatures, their brains are very complex and probably need a huge amount of energy, and they're a lot like mammals (hair/fur, similar physical build, etc.), which are exclusively endothermic. Plus, I'm not sure an ectotherm system would be fully compatible with an endotherm, and humans are endotherms, so… I'm not sure Vulcan/human hybrids would work.

The one exception is metabolism, because McCoy does say specifically that Spock's "Vulcan metabolism" is low. (Unfortunately I overlooked that part until after I'd written most of this post.) And, well, Vulcans are aliens, so they COULD be ectothermic-like and just have other things going on that make hybridization possible. And the boundary between ectotherms and endotherms isn't firm; there's plenty of crossover. Or they might be something totally different! So YMMV.

Meanwhile, endotherms—mammals and birds, basically—maintain a stable internal temperature. This comes with trade-offs: we have a higher metabolism (part of what generates our internal heat) and therefore require more food, and we have to be much more intentional with our temperature regulation. 

But. Because endotherms have a consistent internal temperature, we're actually MORE resilient to temperature changes: we can maintain the same activities at warmer and colder temperatures. We can also support bigger and more energy-intensive brains. (Well, mostly; it's more complex than that, but this post is too long already.) I'd argue that Vulcans kind of have to be endotherms, or at least endotherm-like, in order to function across the vast variety of environments we see them in.

…exceeept there's the matter of the cold blood, and their temperature range is still giant (not to mention the accidentally overlooked metabolism issue). BUT. I have a THEORY.

CAMELS.

Camels let their body temperature range between around 34°C (93 Fahrenheit) to over 40°C (104 Fahrenheit). As desert-dwellers, they'll let their body temperature rise over the course of the day and drop during the night. That also helps them conserve water. They have a multitude of other useful adaptations, of course, but what I'm saying here is: they're desert endotherms that change their body temperature according to their environment. So I figure: why not Vulcans, too?

My pet theory: Vulcans are endotherms (or whatever passes for their evolutionary equivalent), but their internal temperature range varies dependent upon their environment. In human-comfortable room temperature, their body temperature might be several degrees below normal human body temperature, thus sparking Dr. McCoy's "ice water" comment. But once you start getting well over a hundred degrees, their body temperatures may come close to a human's, or even go much higher.

(This still doesn't address the metabolism part, but this is what I get for not reading the transcript line thoroughly until I've written a whole post. Bleh. Maybe I'll figure it out later and add a note.)

I also like to think that, assuming evolution already optimized their systems for less oxygen and somehow got copper to be an actually viable oxygen-carrier for vertebrates, maybe the various temperature-dependent chemical reactions in their bodies are more flexible or have other methods of temperature regulation.

…ANYWAY. there's six hours of my life I'll never get back! biologists, my deepest apologies for anything I got wrong, please feel free to yell at me.

Plus one last fun fact: when we look at climate trends on Earth, animals tend to become bigger when Earth cools (ex. mammoths during the latest Ice Age) and smaller when it warms (ex. early horses became teeeeeeny when our climate rapidly warmed back in the Eocene, and then got bigger when things cooled down). This phenomenon is known as Bergmann's rule. Basically, the bigger an animal is, the better it retains heat, and vice versa.

These are Earth rules, of course, and probably there's large Vulcan fauna that evolved to deal with extreme heat, but. look. I find the idea of Vulcans being giants in a world of miniaturized critters amusing, okay?

Okay, I'm all over the place with my physiological grasp of Vulcans. Are they hotter or colder than humans? Why? Does that make them feel the heat more or less than humans? Is their heartbeat faster or slower?

Also if you have a strong opinion or an explanation or anything to say please tell me! I wanna get a grasp on this one