system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do smaller animals have faster heart rates than larger ones?

Are there any exceptions to this trend?
gpt
This is for mammals only, and it's because of metabolism. Small things cool down faster than big things, so small things need to burn more energy to keep warm, so small things have faster heart rates. It also helps that it's easier to move a small thing than a large one.