A blue whale's heart can beat just twice a minute at the bottom of a dive

In 2019, researchers led by Jeremy Goldbogen suction-cupped the first heart-rate monitor onto a free-swimming blue whale and found its heart rate could fall to around 2 beats per minute at the deepest point of a foraging dive, then spike to an extreme 37 beats per minute at the surface as it rapidly repaid its oxygen debt. Both numbers sit near the outer physiological limits predicted for an animal the size of a blue whale.

— Jeremy A. Goldbogen et al., Extreme bradycardia and tachycardia in the world's largest animal — Proceedings of the National Academy of Sciences, 2019