Blue whales have a flair for paradox. They live in water but breathe air. They’re the biggest creatures that have ever lived, as far as anyone knows, but subsist largely on tiny krill.

As new research reveals, even the animal’s dunk tank-size heart jumps between extremes. In a study published in the Proceedings of the National Academy of Sciences, researchers found that the rate ranged as low as two beats per minute and as high as 37. Such numbers paint a picture of an animal pushing its limits, and suggest that the whale is not only the largest animal ever, but perhaps as large as an animal with a circulatory system can possibly be.

Jeremy Goldbogen, a marine biologist at Stanford University, led a team that for the first time attached a noninvasive electrocardiogram tag to a blue whale.

In general, sticking to a slow rhythm while diving lets marine mammals conserve oxygen, so they can stay underwater for longer. But this is “just amazingly low,” Goldbogen said.

Goldbogen credits the blue whale’s flexible aortic arch, which is able to hold about 90% of the animals’ blood and slowly release it even when the heart is not actively beating.