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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?
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:
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.
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.
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.
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.
