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A creature in my story was largely inspired by the xenomorphs of the Alien franchise. In wondering how long it would take one of these creatures to grow from conception to maturity, I decided to look at my original inspiration which only posed more problems. Xenomorphs develop insanely fast! Growing from an egg to a nymph in a matter of hours and from a nymph to an adult drone in maybe days. So my question is as stated: How fast could a large creature feasibly grow and how much would it need to eat?

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  • $\begingroup$ Blue whales can grow to be about 100 feet (30.5 meters) in length and may weigh around 160 tons. Newborn blue whales are about 23 feet (7 meters) long and roughly 30 tons and can add 200 pounds a day! Life can grow very fast. For an adult blue whale it needs around 1.5 million kilo calories per day to stay healthy. $\endgroup$ – Jeremy Barrett Feb 3 at 6:39
  • $\begingroup$ Are you just going from egg to juvenile & bypassing the face hugger stage or including it, because if included that's two fetal stages both starting from scratch as a microscopic ball of cells that grows to bunny size. $\endgroup$ – Pelinore Feb 3 at 18:33
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  • How fast could a large creature feasibly grow?

Growth & more specifically Cell growth is what your asking about here.

Growing from an egg to a nymph in a matter of hours

That given the size of the alien you reference that pops out of the egg is perhaps the most implausible part of the the thing as growth of embryos in animals is not pounds or ounces per minute or hour but rather cell divisions per "x" period of time, & you start with a single cell.

and from a nymph to an adult drone in maybe days

While this given the starting size of stage in it's life cycle when it bursts from someones stomach is perhaps more plausible, if we assume a level of cell division comparable to the early stages of cell division in a fetus rather than in juvenile animals after birth.

Duration of the Cell Cycle "Certain fly embryos sport cell cycles that last only 8 minutes per cycle!"

If we assume this is about as fast as it gets you could perhaps be looking at exponential growth (a doubling in size) every eight minutes.

  • and how much would it need to eat?

A bit more than it's own body weight every eight minutes.

Which may be a problem for a large complex multi cell organism as you'll need

  1. An extremely efficient stomach & gut to absorb all those nutrients & calories fast enough.
  2. An extremely efficient cardiovascular system to get it all to cells that need it quickly enough.
  3. Extremely efficient lungs for oxygen absorption to power the cell processes you're feeding.

Which may all need to be implausible efficient to sustain anything like that speed of cell division in large organisms, there's always hand waving though & you've not included a reality check tag.


The organism is going to need a certain number of calories just to sustain itself, I've had difficulty finding details on this so I'm going to have to extrapolate from Google searches on human statistics.

The average calories needed by an adult male is 1,800 calories a day.

The average adult male weight is 197.6 pounds

Extrapolating from those search results (which is of course wildly unsafe for all sorts of reasons, but all I have) gives a daily figure 9.1 calories per pound of body weight just to sustain itself.


You should consider the above more along the lines of my showing my workings for a math question rather than my answer, gotta go for now so that will follow later, possibly a lot later, sorry about that :)

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    $\begingroup$ keep in mind for humans about 1/3 of those calories are just going to the brain, assuming your creatures brain is significantly smaller it won't need that much.Humans are also endotherms which presumably xenomorphs are not. ectotherms need around 10-15% the calories ans endotherm needs. $\endgroup$ – John Feb 3 at 18:22
  • $\begingroup$ @John ^ If pound for pound a proportionate brain to body weight ratio is assumed (which I had thought was self-evidently the case) then that 1/3 is built in isn't it, so null point, not an issue :) $\endgroup$ – Pelinore Feb 3 at 20:20
  • $\begingroup$ @John : "Humans are also endotherms which presumably xenomorphs are not" I always presumed they were, mostly because if they weren't defeating them in Alien would have been insanely easy, just turn the heating off, then go find the comatose (or near-comatose) aliens & slaughter them. $\endgroup$ – Pelinore Feb 3 at 20:26
  • $\begingroup$ Why would you assume they have the same proportional brains, humans have absolutely huge brains, Xenomorphs don't show human level intelligence. As for defeating an ectotherm just by turning off the heat, that really doesn't work, many ectotherms can still be quite active in cold temperatures if forced to, You'd have to get it cold enough to freeze, which would work on endotherms as well. The fact they have no form of insulation on their bodies is a good indicator they are ectotherms. $\endgroup$ – John Feb 3 at 22:24
  • $\begingroup$ @John "many ectotherms can still be quite active in cold temperatures if forced to" only for a very limited time & over extended periods they'll die, unless they can enter some form of suspension of life processes which is seemingly less & less common the larger the animal species is, & if they can you can safely kill them, & then we're back to my reason for assuming they weren't, because that didn't happen in the film. $\endgroup$ – Pelinore Feb 3 at 22:27

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