According to biological (生物学的) theories, the larger an animal is, the slower its heart beats, since it takes time to pump blood around its body. Shrews (鼩鼱), the smallest mammals, have a heart rate of up to 1,000 times a minute. Human hearts usually stay between 60 and 100 beats per minute (bpm). Blue whales – the largest animals on Earth – must have even slower heart rates. How slow do they go?