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In the movie Alien (1979), the first alien we see is the facehugger. It is only the size of a baseball mitt.

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A few scenes later, the offspring bursts out of a crewmember's chest. (Thanks to Johnny Bones for reminding me of that in his answer.) It is the size of a adult human's arm.

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When we see the alien next, a few hours later, it has grown larger than the average human.

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Is there an in-universe explanation how the alien gained so much mass? Did it eat food that was aboard the ship?

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First of all, the Facehugger is a different alien. It's job is to find a host and lay an egg (much like a parasite), and the egg then hatches from inside its host. The newly hatched alien then kills several people and grows in size, although they never specifically show any interim iteration as I recall. Presumably it just grows at a faster rate than humans, which isn't unusual (think "elephants"...).

  • You're right about Facehugger being a different alien. Can't believe I forgot that step! I'll edit my question. However, the humanoid alien is full-size when it kills Brett, and that's its first kill. – BrettFromLA Jul 28 '17 at 18:30
  • If you regard the Xenomorph as a parasitic organism similar to Toxoplasma gondii with different stages of its lifecycle in different forms then the FaceHugger and the Chestburster are different stage of the same organisms lifecycle. – Sarriesfan Jul 28 '17 at 23:22
  • The problem with that is that the Chestburster and the Xenomorph resemble each other, while the Facehugger does not resemble either. So it would have to go from Chestburster to Facehugger to Xenomorph, which doesn't seem likely. – Johnny Bones Jul 29 '17 at 15:41
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I've always suspected the xenomorphs have some ability to draw carbon out of the air, similar to what plants do with photosynthesis, because they often seem to grow without eating sufficient mass to account for their size.

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I don't think it gets explained. But dramatically, it does make it scarier in first movie when they go looking for the chest burster they don't realize its morphed into this huge monster. I was so busy being scared (and I saw the movie the first time in the theater when it came out in the 80s) I didn't think about it.

  • I was busy being scared in the theater too! I was 11 when I saw it, and that was way too young. Didn't even think of the question until 38 years later. – BrettFromLA Jul 29 '17 at 19:42

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