Picture new parents cooing at their baby on the couch. The baby's quiet and calm, and everyone's at rest. So their heart rates are low, maybe 60 or 70 beats a minute for the adults. What about the baby? How do you think the baby's heart rate compares to the parents'?
Newborns have resting heart rates around 100 to 150 beats per minute, way higher than that of adults. In fact, a newborn's resting heart rate is in the ballpark of the target heart rate for an adult doing a heavy workout. What gives? Why do newborns' hearts beat so much faster than adults'?
Part of the answer lies in the fact that babies grow so dramatically. They burn food in every cell of their bodies to fuel that growth. The process requires not only an abundance of milk but also oxygen, and it produces large amounts of carbon dioxide as waste. Babies' hearts work overtime to keep supplying oxygen and removing carbon dioxide to fuel their amazing growth.
But growth isn't the only factor, because it's not just baby animals we're talking about. Adult Chihuahuas have much higher heart rates than adult elephants. Why? Chihuahuas have a greater surface-to-volume ratio. In other words, the Chihuahua has a greater proportion of skin than internal tissue, which means it loses more heat to the environment, relatively speaking, than a larger animal.
Babies' hearts beat much faster than their parents', both to fuel their growth and make up for greater heat loss to the environment.
So little hearts maintain a higher metabolic rate, pound for pound, than big hearts.