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As described in Harry Potter and the Chamber of Secrets, there are a lot of really big spiders in the forest, and they are all carnivorous. I know "what does X eat?" is a kind of obnoxious form of poking holes in fantasy universes, but I can't help wondering if this was considered.

Imagine a "den" of 150 lions. You wouldn't expect there to be many deer left in that forest after too long, would you? What exactly are those spiders eating?

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    wayward students were definitely on the list – NKCampbell Jul 19 '18 at 16:37
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    whatever they want! – djm Jul 19 '18 at 17:14
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    This is the same JK Rowling who created an impossible economy, an impossible sport, an impossible government, and an impossible relationship with the non-magical world. (Read HPMOR sometime and be impressed with how a little thought can destroy Rowling's world.) Why do you think the acromantulas should be more practical and less rule-of-cool than anything else? – Graham Jul 19 '18 at 20:40
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    ... or just much bigger than the OP is imagining. – Harry Johnston Jul 19 '18 at 23:37
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    @Paul Harry Potter and the Methods of Rationality. Very, very recommended. Occasionally the author overdoes it (hey, thia is fan fic, it's not seen a pro editor) but it's good. Be prepared to say goodbye to your free time for a while though! – Graham Jul 20 '18 at 0:14
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The official info from Fantastic Beasts and Where to Find Them mentions that they enjoy 'large prey'

The Acromantula is a monstrous eight-eyed spider capable of human speech. It originated in Borneo, where it inhabits dense jungle. Its distinctive features include the thick black hair that covers its body; its legspan, which may reach up to fifteen feet; its pincers, which produce a distinctive clicking sound when the Acromantula is excited or angry; and a poisonous secretion. The Acromantula is carnivorous and prefers large prey.

We know (based on mentions) that there are a considerable number of larger creatures living in the forest that they could be eating including deer, stoats, birds, foxes and sucking bugbears(?) as well as the odd thestral, hippogriff, centaur and unicorn. It would also, presumably include mundane forest creatures indigenous to Scotland such as badgers, pine martens, hares, foxes and wildcats, all of which would be potential fodder for an Acromantula colony.

Note that Hagrid doesn't think Grawp will have much trouble in the food department

"He can get his own food, no problem. Birds an' deer an' stuff"

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    My point was kind of that the population of "indigenous forest creatures" could not possibly support so many large carnivores. – temporary_user_name Jul 19 '18 at 17:26
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    @Aerovistae - Without knowing how densely populated the forest is, we can't possibly know whether JKRs estimation of population is accurate or not. Also, precisely how nourishing is a thestral? – Valorum Jul 19 '18 at 18:20
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    @Valorum You're asking JKR to make an accurate estimate of something? THAT'S not going to go well. – chif-ii Jul 19 '18 at 20:47
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    @chif-ii - Those creatures (deer, stoats, birds, and foxes) are explicitly mentioned in the text. The others (badgers, pine martens, hares, foxes and wildcats) I've merely supposed are there based on the location of the forest in Scotland. – Valorum Jul 19 '18 at 21:37
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    @jpmc26 the young ones got to eat too – Jungkook Jul 20 '18 at 8:02
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In general, web-building spiders save a lot of energy by not moving, most of the time. Their metabolic rate drops and can stay that way for days, weeks, months even. They tend to only expend energy when building a web to assist in catching food or when moving to actually catch food, or else when disturbed. They also tend to then store the food for later feeding, in handily-spun, juicy webbed packages. This could be consumed some time after capture, giving the spider more than just the stores of its own body to survive on. Think of your average daddy long-legs. How long might it take an acromantula to consume a deer, and for how long might this supply it with energy?

And it may be that the breeding rates of fantastical prey creatures are higher than natural ones.

This is totally up to our speculation. Remember: artistic license applies!

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According to the Pottermore site, the magical creatures that inhabit the forest include:

  • Hippogriffs
  • Unicorns
  • Thestrals
  • Werewolves
  • Acromantulas
  • Centaurs

Non magical creatures include deer and foxes.

While of lesser canon status, the wikia adds the following:

  • Blood sucking bugbears
  • Trolls (rumored)
  • wolfpack (Descended from the above werewolves)

The wikia also mentions possible giants, and blast ended skrewts, but the skrewts wouldn't have been introduced until the events of HP4:GOF at the earliest. (Additionally, the skrewt in the maze I believe is addressed in the book as the last surviving one).

Since acromantulas are carnivorous and prefer large prey, all of the above would qualify.

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    So...what do the Acromantula eat? – The Dark Lord Jul 20 '18 at 12:57
  • @TheDarkLord - I thought it was implied that all of the above could be prey, much as Valorum's answer doesn't explicitly state it beyond "prefer large prey". YMMV. – JohnP Jul 20 '18 at 14:19
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Spiders also tend to be cannibalistic in nature, it is possible that the colony was feeding on itself to sustain the lives of its older and larger members.

  • Welcome to SFF! Here we prefer answers based in canon evidence if you have some can you edit in how you believe this is referenced in the Harry Potter universe? Else is this just pure speculation from what you know of spiders in our universe? – TheLethalCarrot Jul 23 '18 at 15:06
  • Isn't it a question because it isn't addressed by cannon evidence? I was just providing a possibility based on traits of spiders found in nature as that is the closest thing we have to compare it with here. All of this is pure speculation once your past what is laid out in the books... – sneakyemu Jul 23 '18 at 15:51
  • Well canon seems to suggest they hunt "large prey". This is of course a possibility based in evidence from our world but I was wondering if you had anything to back this up from in universe evidence is all. – TheLethalCarrot Jul 23 '18 at 15:52

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