There exists a planet with a similar biological history of Earth that experienced a sudden inundation of the undead during it's equivalent of the middle Pleistocene. These undead are capable arising from the death of any animal, with the relative violence of their demise determining the speed at which they return. Violent death results in a resurrection event occurring within minutes, whereas death by age or starvation would take hours, up to a day at most.

Once the fresh undead is reanimated, it may be assumed to be capable of moving without energy input, but is still reliant on the musculature of the original animal, restricting its power output and mobility. Undead are also capable of thermoregulation, running at temperatures similar to most endotherms, but are incapable of healing themselves and providing an immune response to infection. Thus, the undead population is controlled by the rate at which musculature rots and bones are worn and broken with use.

Unfortunately for the traditionally alive species of this planet, becoming undead also imbues animals with a predatory instinct, causing them to act similar to other predatory species in their respective environment might. Thus, a deer dying would later act like a wolf, and hunt prey as a wolf might. The exception to this is when an undead is freshly risen, or in the presence of a heavily wounded or freshly dead animal, at which point the undead goes into a "bloodfury", where the animal becomes highly aggressive and demonstrates exaggerated control over its form. This includes the ability to move and relocate bones and muscle tissue within its body, perform limited repair to any existing wounds, and merge bodies with any other animals nearby, living or undead.

It may be assumed that all undead possess an animal-like intelligence, with problem-solving skills being limited to that of predators in their respective environments. Undead in a bloodfury may experience greater problem-solving skills more similar to those of a chimpanzee or an octopus.

Lastly, the undead possess a supernatural ability to sense the direction of nearby animals, living and undead. This ability degrades with distance and relative health of the target, meaning that the undead most capable of sensing a wounded, nearby animal.

So, my question is what would the resultant biosphere look like given almost a million years? Things I assume would be true:

  • Predatory species and most obligate carnivores would be out-competed by the undead
  • Scavengers would feed exclusively on animals dying of causes other than predation
  • Breaking bones would become an even more favorable strategy for fighting
  • Animals would be hesitant to kill rivals for mating or territory privileges
  • $\begingroup$ @user535733 In case I was unclear, this is not an expansion scenario. Any death results in reanimation. I would disagree that the inclusion of one magical aspect in a story means that everything about the real world can be tweaked. In fact, the most interesting fantastical stories to me come from the exploration of small changes to the world $\endgroup$ – Zer0ah May 15 '20 at 18:30
  • $\begingroup$ I'm not clear about how it's not an expansion scenario. One zombie elk attacks two friends during bloodfury. Now there are three zombie elk. Then seven. Then twenty...in perhaps one day. Your question has only described zombie strengths, no weaknesses. Do the undead bodies keep killing until their teeth wear down? Do they putrefy into goo in about five weeks? Will insects feeding on zombies become zombies too? Also, it's okay to introduce magic into a scenario, but you must place clear limits upon it. Just trying to help you improve the question. $\endgroup$ – user535733 May 15 '20 at 18:41
  • $\begingroup$ @user535733 Ah I see the misunderstanding, my mistake. You are right in that case, which is something I was concerned about with herd animals, but should probably make a note of. I think ProjectApex does a good job of addressing the issue in their line about elephants. The core limitations here would be decomposition and inability to repair themselves unless under bloodfury, and never repairing bone. So insect feeding would not result in infection. I suspect that means that insects win big time in this scenario... $\endgroup$ – Zer0ah May 15 '20 at 18:56

Evolution or extinction

Just to clarify here and simplify things for myself, I'll assume throughout this answer that all of your undead will simply become highly predatorial (meaning that changes in the food chain won't affect their behavior too much), and that the bloodfury variant will be naturally more competent overall, but with its ability to assimilate organic matter being limited to the first 6-12 hours after its creation, keeping its other enhancements until its "death". Additionally, I'll assume all undead have, at best, very limited pack hunting capabilities,usually acting more as lone hunters.

Your other species would need to adapt to the undead plague. As far as you said, every dead animal is bound to become an undead. That means that scavengers as we know them will be enhanced, as dead meat will be only safe to eat during a very short time spam, and will require the scavengers to learn which are safe to eat (will take longer to convert) and which will arise soon and can't be fed on until they reach a decomposition stage in which they can move no longer. Undead hunting shall be only done by the greatest carnivores at the peak of their lives, as for weaker creatures, especially when dealing with their own undead counterparts, an older or weaker animal will likely die if they try to do so.

Regarding the selection among animals, a good sense of sight, hearing and especially smell will likely be selected as good ways to avoid the undead, and pack hunters will have a severe advantage over lone grazers and hunters, with social species being selected due to their higher capabilities to fend off the undead. The food webs will be partially variable, with undead creatures being located all throughout it. I have no doubt that the undead will be placed relatively close to their living counterparts, and will compete with the living carnivores for food.

The relationship between carnivores and the undead (assuming the former don't go extinct altogether with the undead serving as a means of population control instead, which is unlikely as bodies exposed to the environment don't last long and will be progressively weaker after being made, even if we have more being made all the time) will likely be the fiercest, with many species adopting hunting strategies like wolves and lions (group hunting). Additionally, carnivores will adapt to keep its prey alive for as long as possible, restrain prey either by breaking limbs or other methods and evolve large Jaws and feeding methods which involve ingesting as much meat as fast as possible, so that by the time it comes back they'll have eaten enough and already left. Snakes and the like will go extinct. As the last thing you'll want is a whole undead animal inside of you. Additionally, more than ever the sick and weak will be favored, as their death will mean weaker undead.

Now for the top of the food chain. It'll obviously be the bloodfury variants. With their ability to assimilate organic matter and higher intelligence and lacking both food and sleep to survive, they'll easily outpower and outlast the living carnivores. There's also a chance that they might instinctively look for dryer, colder environments, in order to slow down their own decomposition. Any normal animal will have a hard time facing it, and we can't rule out that they might understand how to make more, as well as engage in pack behavior as well. Additionally, as they can reshape their muscles and bones, even boloodfury variants which originate from herbivores can modify themselves to achieve claws, carnivorous dentition and other killing tools, as well as repurposing internal structures to favor movement speed and overall power. That means they'll usually have more than just a few advantages against other creatures.

Lastly: intelligence. In your case, I'm not sure if intelligence will make much of a difference, considering that smarter animals will mean smarter undead. Evolution will likely favor overall strength and resistance instead, in order to ensure that those who die , meaning less dangerous undead roaming around.

Animals which can understand death, like elephants, instead of mourning their dead by rubbing their trunks against it, will do so by stomping/smashing, burying it so that once it returns, it doesn't represent a threat.

Plants will also adapt to this. Some might use the stench of rotting meat as a defense mechanism against herbivores. The use of seeds that cling to fur and skin might become more successful, as even after death animals will be able to spread these seeds.

Lastly, decomposition: the one reason I see of why the ecosystem is still going is because of decomposition. If decompositors didn't exist or took too long to break down dead tissues, the undead would most likely cause the extinctions of most animal life, as well as the death of some plant species which heavily rely on animal pollination.

Additionally, it's great that the undead plague as you described doesn't seem to occur on a celular level so to say, or there's a good chance that all life on the planet would quickly cease to exist, giving place briefly to an ecosystem solely composed of undead animals, plants and unicelular agents, which would then erode away given enough time, resulting in the end of the biosphere.

  • $\begingroup$ Interesting, so decomposition rate defines the primary population control for the undead, makes sense. I wonder how tweaking the set body temperature of the undead would affect things, or even if decomposition wasn't a major concern $\endgroup$ – Zer0ah May 15 '20 at 18:45
  • $\begingroup$ @Zer0ah remember that decomposition's best friends are humidity and heat, so it won't be uniform (which is why I suggested That the bloodfury variant might avoid these to stay mobile for longer). The high body temperature will surely help out the decompositors to clean up their insides. I ended up forgetting to put it in the answer, but if decomposition didn't happen by some means or took too long to happen, you'd likely end with a world without animals, as they'd all go extinct hunted by their undead counterparts. Only plants would remain. $\endgroup$ – ProjectApex May 15 '20 at 18:49

If decomposition works as it does on our world, the undead would become unviable as predators quickly, within days-to-a-couple-of-weeks at most. Skeletonized zombies are just inanimate, are they not?

However, if violent deaths cause immediate reanimation, then there can be no carnivores. They hunt with violence, and if your food gets up and walks away before you've gotten a chance to eat, you starve... especially when it's now capable of defending itself when just minutes before it could not do that. So there are no predators except the undead, except in some very limited circumstances (predators who can consume their prey entirely without minutes or who swallow them whole). Fish and sharks maybe. Some insects.

Honestly though, as a plot point this doesn't much work for me. Zombies are only compelling plot devices because they occur in stories with humans, they're allegorical for various fears that we have (and not the ones most people assume). On an alien world this becomes little more than a novel biology competing with the original, and not in a very interesting way.

  • $\begingroup$ Half the fun of an alien world is the novel biology though! Have no fear, humans are involved, I just didn't include them in the question since they were not immediately relevant to the ecology aspect I'm looking at right now. Same goes for thematic utility $\endgroup$ – Zer0ah May 15 '20 at 18:49
  • $\begingroup$ @Zer0ah Keep in mind that if this started happening in the Pleistocene, you're probably looking at ecological collapse of the highest magnitude. Maybe all megafauna are gone... with the simple paradigm, predators only become plentiful after the prey has become plentiful, and they die back when the prey become scarce. This inverts that... they become more plentiful even while prey are diminishing, because every single death converts the latter to the former. This isn't just a world without lions, it's probably one without gazelles. And zombie nematodes are nightmare fuel. $\endgroup$ – John O May 15 '20 at 19:49
  • $\begingroup$ "If your food gets up and walks away"... clearly you've never seen a predator eating an animal it's hamstrung... while its meal is still alive. You're welcome for the nightmares 😉. Also, "within minutes" is still long enough to dismember something. Predators will adapt. $\endgroup$ – Matthew May 20 '20 at 1:54
  • $\begingroup$ @Matthew Given the depictions of zombies in the movies, it's very doubtful that dismemberment will be a complete impediment. The predators are still based on biology, they can't win win the rules are stacked against them. $\endgroup$ – John O May 20 '20 at 4:18
  • $\begingroup$ Complete impediment? No... but I'm having a very hard time imagining a predator having any serious difficulty chowing down on a dismembered arm, even if it is still trying to move. Again, predators are capable of eating still-living animals that have been suitably immobilized. "Blood-rage" or not, an animal that can't stand up is not a serious threat. Hamstring it, kill it (or in the opposite order), tear up some more critical tendons, wait for the blood rage to wear off if you even need to do that... and presto, you've got your meal. $\endgroup$ – Matthew May 20 '20 at 15:47

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