It's a generally accepted fact that animals have only one heart, at least in vertebrates. Reality shows that duplicates certainly aren't necessary, but is there any reason why they aren't possible? There's plenty of fictional creatures with more than one heart, not always from genetic engineering or artificial additions. A second heart would offer redundancy if one failed, and there's probably some other benefits that could be had.
Edit for clarification:
- I should make it explicit that I'm considering vertebrate animals here, not insects or worms, with a focus on redundancy as per the underlying premise detailed below (one heart failing can cause impairment, but it shouldn't be fatal). Mammalian examples are preferred, since I'm trying to attract answers easily applicable for other world-builders looking to design their own creatures; the majority of fictional creatures, at least the intelligent ones that get all the attention, do tend to be humanoids. Real-world examples of reptiles with multiple hearts are perfectly acceptable as well, however, given that there are plenty of other fictional creatures like dragons that fall into that category.
There is an underlying premise, for the purposes of this question: a self-regenerating creature (a vertebrate, not an earthworm). Having a second heart would allow it to survive the failure of one until it could heal or otherwise rebuild the tissue and restart that heart. Answers that also cover any other singular organs whose sudden failure would kill someone quickly (the liver, for instance) are appreciated but not required; the heart appears to be the biggest sticking point.
I am aware that the standard closed circulatory system seen in vertebrates like us humans would likely have problems with a second heart also pumping blood, so bear that in mind. I've thought of a hypothetical case of having twin (separate) circulatory systems, each with its own little heart, flowing through the whole body, wherein a failure would reduce one's capabilities but not be fatal or actively damaging in itself (duplication and appropriate shrinking of lungs, liver, windpipe, etc., optional). I currently have no idea whatsoever if that's actually feasible, though, so I'll leave it here as inspiration; if anyone wants to expand on this idea and prove it valid for an answer, go ahead!
I'm looking for an answer that can plausibly demonstrate how a creature might have two or more hearts; it doesn't need to be human, although something generally mammalian is preferable to make it more readily applicable for other world-builders. Bonus points for anyone who can come up with a good reason for this to evolve in nature and be favored over a single-heart system for that creature, but genetic engineering is a perfectly acceptable answer.