In the Alien films the Xenomorphs presented have a very alien physiology. One of these features is acidic blood, much to the dismay of several characters in the film.

Why would a creature benefit from having such acidic blood? Is it purely for defence or does it have another purpose?

  • 8
    Benefit #1: Being completely impervious to heartburn.
    – TLP
    Commented Mar 25, 2012 at 11:24
  • 3
    Benefit #2: They can eat anything and extract nutrients/energy from it. Water is a good solvent, but acid clearly works better. If humans can maintain a mildly acidic stomach with water for blood, imagine how acidic the alien can make its stomach.
    – Adam Davis
    Commented Jun 5, 2012 at 18:31
  • 1
    Because it makes them more AWESOME!
    – user8719
    Commented Nov 22, 2012 at 10:49
  • @AdamDavis Just a little nitpicky correction: throwing stuff in acid usually does not dissolve them but react with them, hence the smoke. Most acids are not solvents but they are actually dissolved in water. Commented Jun 8, 2016 at 13:14

7 Answers 7


A theory states that the acidity is there to provide a charge: or to quote:

Essentially, the Alien is able to operate without otherwise essential functions because it does not possess them, deriving its energy instead from a powerful bio- electrical charge generated within its body by means of acidic reaction. The Alien's "blood" is, in fact, a component of a powerful biological "battery".

from the Alien FUZION RPG conversion



Speculation: The Xenomorphs are a genetically-engineered species designed specifically as weapons. I support this with several simple but powerful ideas:

  1. Their blood is acidic. It would be a very unusual mutation to have blood whose acid content is as powerful as the blood of the Xenomorph. We watch it eat through three decks of metal, easily, in fact effortlessly. Blood with such an acidic level would require a specialized diet to be able to produce such a quantity of acid-laden chemicals (assuming it is an affect of its environment) to create such highly volatile blood.
  2. They are highly armored, very fast predators, capable of running across walls, floors and ceilings with the same ease as crossing a floor. If they have the opportunity to recreate a viable environment using their secretions, their terrain can even cloak them from visual identification, making them even harder to isolate and target.
  3. They do not show up under infrared light. This means their body profile is endothermic (cold-blooded). They are very fast for cold-blooded creatures which at least on Earth, is not the norm. Cold-blooded predators on Earth are generally capable at catching, poisoning, grasping or simply outlasting their prey, but there are few track stars in their ranks. This makes them very difficult for creatures who use the visual spectrum, especially the low end, to detect them. Technology for visualizations toward the high end of the spectrum is more sophisticated and less likely to be easily had. Not impossible, just less likely. The most effective way of tracking them, through their chemical signatures, like the Predators do, is one of the most advanced methods possible and likely the hardest to make portable.
  4. The most damning evidence as a sign of their engineered heritage is their uncanny ability to gestated from a variety of host organisms and the speed of their growth. They do not need to take more than a day or two to reach full adulthood. They are born knowing how to hunt, track and prepare their prey for implantation. We know for certain that they can develop in humans and the Predator species. We have also seen them gestate in dogs, creating functional if not attractive xenomorphs. Since taking human and Predator DNA into account, that is three completely different species who are genetically compatible enough to produce viable offspring. In genetic terms, this would be highly unlikely, on the order of nearly impossible without an advanced technology making it possible.
  5. Since we have also seen the Xenomorph take the characteristics of the creatures it hosts within (taking on the Predator characteristics for example in AVP and AVP: Requium) this implies an intelligence behind the design of the creature to build an organism able to gestate in a variety of organisms of different genetic structures, from different planets and to produce viable offspring of sufficient strength, speed, coordination, and intelligence to work cooperatively, telepathically (assuming a queen is available), or using a very fast other means of interacting that seems like telepathy and be able to quickly take over an area as effectively as these creatures do.

Their aggression, innate intelligence, weaponry, claws, armor and strength is proportional to the creatures they replicate ensuring their superiority in hand-to-hand combat. Their weaponized blood ensures even if an enemy engages them in physical combat and wins, they risk taking damage or even dying. This creature is the very spitting image of a bioweapon waiting to be used.

Considering the Company seemed to want them as weapons, as part of the premise of the series, it seems a logical assumption. Who could have created such a weapon is unknown but it is certainly well beyond the average capabilities of human technologies of the time.

The counter argument says that nature is adaptable and could conceivably create such a creature with a mutable, species ambivalent, genetic structure (though, no creature on Earth comes to mind with a similar ability except viruses and bacteria). Such a creature would become the apex predator of its world, wiping out all life including itself once all the food sources were destroyed. In which case, it is still the most formidable, macroscopic, biological weapon ever seen.

  • 2
    Based on the Prometheus trailer, it looks like the "space jockeys" may be the bioengineers who created them. But I admit this is just speculation/wishful thinking.
    – gnovice
    Commented Mar 25, 2012 at 6:54
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    No. I am saying this creature could not exist. Having the ability to take over the genes of an animal within its planetary biome is acceptable, if difficult to believe. But to control the genes of creatures from completely different biomes is impossible. There is no way for the mutation to develop naturally. Mutations are responses to biological forces. I am approaching it from a biochemical standpoint. Genetics and protein structures they're comprised of are simply not that malleable. Commented Mar 25, 2012 at 14:50
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    @ChrisLutz: The cannon (see commentary on Alien) is that the Xenomorphs are bio-engineered weapons. Commented May 31, 2012 at 8:37
  • @ThaddeusHowze You're wrong. You say something is impossible and then you say it was made artificially - that's a contradiction, if something is impossible it's just not gonna happen, ever. There is no law saying you couldn't find naturally occurring working notebook on some planet, it's just insanely unlikely. Commented Jun 8, 2016 at 13:20
  • This answer doesn't properly address the question. If the question is "For what purpose alien's blood is acidic" it does not matter whether the Xenomorph is artificial or natural or whatever - the purpose remains the same and the purpose was the question, not the origin. Commented Jun 8, 2016 at 13:22

It is a defensive tactic. It teaches its' opponents that killing them is a bad idea. It sets an example. Once one Xenomorph has had it's blood spilled, an enemy would be reluctant to spill the blood of another.

  • 4
    Same idea as any highly toxic terrestrial critter, like the Poison Dart Frog, or the Pufferfish. The aliens just don't have the brightly colored warning stripes.
    – K-H-W
    Commented Mar 24, 2012 at 23:19
  • 1
    Of course a cement monster might not mind so much.
    – Xantec
    Commented Mar 24, 2012 at 23:52
  • I think this tactic would only apply to things that aren't planning to rip out your throat anyway - like butterflies. If you're on a cargo starship, and discover an alien is stalking you, are you going to say - "wait, we can't shoot it - it might bleed on us"...
    – John C
    Commented May 31, 2012 at 10:03
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    @JohnC: If it provides even a momentary hesitation of "Wait... if I shoot it, that blood is going to get everywhere, including the starship walls and my wife next to it", that's an evolutionary advantage.
    – FuzzyBoots
    Commented May 30, 2014 at 12:21

This is half-speculation and half-memory but wasn't there a suggestion that the xenomorphs circulatory system was electrical in nature and the blood is acidic as it makes a better conductor? I seem to recall the Aliens RPG saying something along those lines.

  • I was sure there was some other reason than defense in the acidty of the blood! Can you substantiate this claim?
    – AncientSwordRage
    Commented Mar 25, 2012 at 15:47
  • @Pureferret I don't have my copy of the RPG any more and it's out of print. I just googled it and a couple of torrent sites claim to have it but I didn't get anywhere downloading it. I'm reasonably certain it said something along those lines - there was a whole chapter on xenomorphology - but it's nearly 20 years since I read this (which I'm quite shocked by!) Commented Mar 25, 2012 at 16:50

Besides the defensive angle, the xenomorphs seem to be aware of their blood's acidic nature and use it to their advantage. As seen in Aliens Resurection, where the creatures sacrifice one of their own to melt the floor of their enclosure.

Edit: I also believe in one of the Alien vs. Predator movies one of the aliens has its tail cut, and then it uses it to spray a predator with blood/acid.


First of all, endothermic means warm blooded. Second of all, the idea of an apex predator having such a powerful defense mechanism (if it is in fact, not from their diet) would suggest that either they are a bio weapon, or more likely, there is something larger hunting them. As far as communication is concerned, you cant rule out pheromones. But of course, who is to say that a silicon based lifeform would develop the same way as a carbon based lifeform. The genetic cocktail that is the xenomorph may not seem as specialized when compared to another silicon based lifeform of its size and complexity.


I believe that in Earth species that pH is affected by a number of factors including diet and that the normal, healthy pH for a given species is often one of the mechanisms used for fighting parasites and diseases.

I would suspect that acidity of the Xenomorph's blood is an effect, likely a result of their diet, rather than a deliberate cause. The creatures may use this effect to their advantage, as species are wont to do when useful mutations show up.

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