Summary

• The Xenomorphs in the Alien franchise have acid blood that can burn through multiple floors, but it doesn't dissolve when Ripley shoots a grappling hook through it at the end of the first movie.
• The theory proposed in Aliens: Colonial Marines Technical Manual suggests that the Xenomorphs' acid blood acts as a bio-electric battery, giving them energy without the need for oxygen.
• This theory explains why the strength of the acid blood varies, suggesting that it may depend on how "charged" the Xenomorph is, and why the Xenomorphs still need to consume food to keep themselves going. However, more research is needed to confirm this theory.

Fans became aware of the Xenomorphs’ acid blood in the very first one official theory regarding the Xenomorphs' acid blood might have the answer.

The comic Aliens: Hive teased that Xenomorphs developed acid blood specifically to combat Predators, though that has never been substantiated.

Aliens: Colonial Marines Technical Manual, which was published by HarperPrism in 1995 and then reissued by Titan Books in 2012, is a novel that’s basically a ‘mockumentary’ in book form. It was written as though it was a collection of factual findings from real life Colonial Marines in the Alien universe, despite that world being totally fictitious.

The novel then expanded beyond just being an informational manual, as it also featured entire conversations between Weyland-Yutani employees in the form of official-looking manuscripts. One conversation delved into the Xenomorphs’ acid blood, and while the mysterious nature behind it wasn’t solved, there was one theory within the manuscript that definitely stood out.

Alien: Xenomorphs’ Acid Blood Is Perhaps A Bio-Electric Battery

While Aliens: Colonial Marines Technical Manual is where fans can read this theory and other interesting details regarding the Alien universe today, the book is actually the culmination of a number of stories published in Dark Horse Comics’ Aliens Magazine, for which the author of Aliens: Colonial Marines Technical Manual, Lee Brimmicombe-Wood, was a staff writer in the early ‘90s. In fact, this very theory was published in Aliens Magazine vol. 2 #11.

The theory suggests that a Xenomorphs’ acid blood acts as a bio-electric battery for the alien creature, giving it energy without the need for oxygen. It also suggests that there’s no way to ‘recharge’ this ‘battery’. While it’s suggested that the Xenomorphs can absorb essential elements presumably through the consumption of meat - first from its initial host and then from its victims once the Xenomorph reaches adulthood - there’s no direct correlation between the Xenomorphs’ hypothesized bio-electric acid blood and it's speculated need to consume sustenance.

So, Why Is Xenomorph Acid Blood So Inconsistent In Alien?

Basically, this theory suggests that the strength of a Xenomorphs’ acid blood is directly tied to how ‘charged’ it is if the acid blood is, indeed, a bio-electric battery that effectively ‘powers’ the Xenomorph. If this theory is correct, then the Xenomorphs’ acid blood is why it doesn’t need oxygen to survive, and possibly why it still needs to eat to keep itself going, as food could perhaps be the one way to strengthen the acid. It also explains why its blood is stronger in some instances throughout the franchise than others, as the Xenomorphs’ blood may simply have not been as ‘fully charged’ as it was during an instance when its blood was a stronger acid.

More Research Is Required To Substantiate This Alien Theory

ittedly, there are still a number of holes in this theory. Xenomorphs have been shown to live very long lives without the need of sustenance, so it can be presumed that the ‘charge’ of their acid blood would last pretty close to indefinitely. But, in Alien, the Xenomorph’s acid blood is incredibly weak when Ripley shoots it, indicating that the bio-electric charge was low, despite the fact that it was not only just born, but also that it presumably consumed basically the entire crew of the Nostromo. However, the theorists behind it may have been on to something, as the idea of the Xenomorphs’ acid blood being a bio-electric battery would perfectly explain why its strength is so inconsistent in the Alien franchise, but more research needs to be conducted before anything can be confidently asserted as fact.