and the highest heart rate belongs to the smallest mammal

https://en.wikipedia.org/wiki/Etruscan_shrew

"The Etruscan shrew has a very fast heart beating rate, up to 1511 beats/min (25 beats/s) and a relatively large heart muscle mass, 1.2% of body weight."

(to illustrate - machine guns typically do 600-900 rounds/min)

I wonder whose muscle fiber is stronger per unit mass - the whale's or the shrew's...