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The title of the question says it all: Something happens on a planet like Earth. All humans—for whatever reason—simply disappear. Are they dead? Alive? Or something else? No idea. But for all intents and purposes all human life as we know it is wiped away from the planet.

This leaves wild animals and domesticated animals free to roam and do as they wish. If they can. Of course, caged animals or animals locked up in homes would—most likely—die, but for the domesticated animals that can (now) roam the planet freely, what would happen?

I mean even in a case where 80% of the domesticated house pets die, what then happens to the remaining 20%?

And when I talk about domesticated animals, I am talking only about house pets: Dogs, cats, birds, fish, lizards, snakes, etc… Not livestock and farm animals.

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    $\begingroup$ Azimov explored this a bit in Foundation and Earth $\endgroup$ – user535733 Aug 29 '18 at 13:47
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    $\begingroup$ It depends on the animal. You need a different question for each species. Cats and horses would certainly be OK (as species, not as individuals), gerbils and hamsters are doomed, others are somewhere in between. $\endgroup$ – Mike Scott Aug 29 '18 at 13:50
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    $\begingroup$ By comparison: the film maker Jan Lindblad and his spouse Pia Thörn successfully raised two tigers, in the middle of Sweden. The tigers fared well in the Swedish forest, though they did cause a few shivering calls from berry- and mushroom-foragers to the local police. :-D $\endgroup$ – MichaelK Aug 29 '18 at 14:03
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    $\begingroup$ Another comparison: some feral species, where individuals do not grow up with their kin in the wild, may end up untrained/unprepared for such a life, and therefore unable to survive without humans. Sea Otters for instance must grow up with their parents to "learn the trade", otherwise they will not make it. This is why otter pups that are rescued rarely gets let back into the wild, because they missed out on otter survival school. These two cuties Agnes and Mojoe at the Danish national aquarium are an example of that. $\endgroup$ – MichaelK Aug 29 '18 at 14:08
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    $\begingroup$ I suggest you watch the series Life After People. It covers this topic extensively. The short of it is that it is so heavily dependent on the breed that this question might as well be marked as too broad. $\endgroup$ – Trevor Aug 29 '18 at 14:44
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Most, but not all, domestic animal species would survive and breed, though in pretty much every case they'd be much reduced in number. In the long run, as evolution had its way and the ecology adapted, it would not be surprising to see some species die out completely.

In the cases of dogs, pigs, cats, and horses, we already have feral populations, and sometimes have had them for centuries.

The milk cow breeds would probably die out, but there are plenty of other breeds of cattle (including some much-less-overbred "heritage" breeds) which may wind up doing OK. Remember that 99.9% of the individuals in a domestic species can die but still leave a viable wild population. Sheep likewise: Huge die-offs, but some rare breeds might well survive. (Of all the large animals, sheep look the diciest.) Goats would probably make it.

Chickens run wild in some places now. Their numbers would doubtless decline, but they'd quickly re-adapt to lay fewer eggs and have a more practical body type.

Domestic turkeys probably wouldn't make it, but there's still a thriving wild turkey population.

The key point here is that there are two stages: The first stage is whether a domestic species can survive in the wild as it exists today and breed successfully. Most domestic animal species in North America would survive the disappearance of human beings and establish breeding, wild populations.

The second stage is how ecological adjustment works out over the longer run. Will the more marginal species adapt to the wild and thrive long-term or be wiped out before they have a chance? This is basically unpredictable, depending so much on chance. (E.g., a hard winter early in the process during a time when a predatory species is overpopulated could wipe out a marginal population that might otherwise have re-adapted to the wild.)

So, domesticated animals would die in very, very large numbers, but nearly all of them have a decent chance of surviving as species somewhere.

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It depends a lot on the animal, and the extent to which they've been domesticated, but the short answer is : The ones that survive the initial catastrophe will do fine. I'll stick with dogs and cats for simplicity.

Cats are barely on the borderline of domestication and they revert to feral and even fully wild behaviors within a generation or two. I've seen this happen personally with the barn cats on my parents property. Most cats that have been indoor cats their entire lives wouldn't survive, but there are some (like mine) that have the ability to catch their own food even if they don't need to. Within a year or two you wouldn't be able to tell they'd ever been domesticated in the first place.

With dogs it'll depend a lot on the breed. Basically the closer the still is to a wolf, the better it will do. Domestication has had a much larger effect on dog's instincts and body shapes than it has on cats, but there's still plenty of wolf in there. Check out this story from Italy on exactly what it looks like when a whole bunch of domestic dogs go feral.

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