I need to add danger to my world since its a post apocalyptic sort-of scenario where animals were left to their own devices after humans went extinct because of war. Humans resurface after 1000 years.

Could animals suffer a change in their body and/or behaviour permanently to make them more aggressive or dangerous because of chemicals and stuff like that through the many generations they would've gone in the span of 1000 years? If so, would they need to be in a concentrated area where said chemical affected them constantly? What other sorts of handicaps would it bring to have a behavioural change?

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    $\begingroup$ Brain damaging drugs could do it, for behaviour at least. Or maybe testosterone or other hormone imitating chemicals. $\endgroup$
    – DKNguyen
    May 21 at 23:49
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    $\begingroup$ Although not a perfect duplicate, the problems covered in that question apply to your question as well. $\endgroup$
    – jdunlop
    May 22 at 0:28
  • $\begingroup$ Please, one question at a time. $\endgroup$ May 22 at 1:48
  • $\begingroup$ Something like Them!? $\endgroup$
    – Fred
    May 22 at 9:54

Frame Challenge: You don't want chemicals, you want engineered viruses.

Bioweapons Run Rampant

The post-apocalypse tropes of "giant animals" was made popular in the 60s, but isn't remotely realistic. Given current global conditions, bigger and fiercer isn't a particularly good survival strategy; smaller and more fecund is. So how do you make your animals more threatening?

As part of the apocalyptic war that largely wiped out humans, one particularly hackneyed plan modified rabies and made it airborne, further modifying the pathogen to delay onset and increase aggression towards larger animals (or, via some magical science, humans in particular). The resultant swarms of animals of all sizes attacking humans spread quickly beyond the borders of the country upon whom it was deployed, but after humans vanished from the planet, became largely latent aside from driving all large primates to extinction.

As humans reemerge, a pathogen that occasionally drove animals into suicidal rage but mostly remained in faunal reservoirs goes active again in a big way. Animals routinely start frothing at the mouth and seeking out humans to attack, and a bite is almost inevitably fatal unless treated immediately.

The animal populations on the whole appear largely unchanged over the thousand intervening years - they'd all be recognizable to a contemporary human - but this nasty little virus makes every mammal bigger than a squirrel into a potential death sentence.


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