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

And as new research reveals, even the animal’s dunk tank-size heart jumps between extremes. In a study published Monday in the Proceedings of the National Academy of Sciences, a team of researchers for the first time attached an electrocardiogram tag to a free-diving blue whale to trace its heart rate.

They 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 frequently pushing its own 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.