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The presence of one heart is ingrained into our idea of biology. Our own bodies as well as those of many organisms we see around us have a single cardiac organ. But some buck the trend. Here are a handful of animals with multiple hearts.
1. Octopus and Squid
These similar creatures are both cephalopods and have three hearts in total; one systematic organ to go along with two “gill hearts” that force blood to the gills.
2. Earthworms
While it is not technically a heart, the aortic arch of the earthworm performs a similar function and is commonly referred to as one for the sake of simplicity. An earthworm has five arches/hearts that are segmented and pump blood throughout its body.
3. Hagfish
Who knew such a nasty creature could have so many hearts? The hagfish has four of them: one main three-chambered systematic heart and three accessory pumps.
Bonus: Humans
Yes, there are humans with more than one heart: those who have undergone a “piggyback” heart transplant operation that adds an extra heart as opposed to replacing the original. The latter continues to function, but at a much lower level, while the transplant heart takes over most of the heavy lifting. One person who had the procedure as a baby later had it reversed, and when the second heart was removed, her own was able to take back the reins.
A version of this story ran in 2013; it has been updated for 2022.