The largest mammal, the blue whale, has a heart the size of a sofa, and their heartbeats have been recorded as low as two per minute. That's the slowest heartbeat of any warm-blooded mammal. Small animals have a faster breathing and heart rate because they lose heat through their skin faster than big animals do. They need to produce energy and heat at a faster pace and then redistribute it repeatedly through their body as it disperses.

Source: NHPR