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Hearts have become iconic symbols of Valentine’s Day, but when it comes to hearts in the real world, one size doesn’t fit all — particularly in the animal kingdom. At rest, the human heart beats between 60 and 80 times a minute, but in that same time, a hibernating groundhog’s heart beats just five times and a hummingbird’s heart reaches 1,260 beats per minute during powered flight. The human heart weighs about 0.6 pounds (0.3 kilograms), but a giraffe’s weighs about 25 pounds (11 kg), as the organ needs to be powerful enough to pump blood up the animal’s long neck. Here are some other creatures with strange hearts. 

1. Frogs

The glass frog’s organs are visible from the outside. (Image credit: Thorsten Spoerlein via Getty Images)

Mammals and birds have four-chambered hearts, but frogs have just three, with two atria and one ventricle, said Daniel Mulcahy, a research collaborator of vertebrate zoology who specializes in amphibians and reptiles at the Smithsonian Institution, Museum of Natural History in Washington, D.C.

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