Do you have a small furry friend? A cat perhaps? A hamster? If you pick up a hamster in a pet store, you can feel its heart going putt-putt-putt-putt-putt-putt, much faster than your own pulse, which is probably going putthump...putthump...putthump. Why does a hamster's heart beat faster than ours?

The hamster is probably nervous being picked up by a human which may contribute to its high heart rate (and that in itself is an interesting story), but even after the hamster calms down, its heart rate is much faster than yours. One obvious way we differ from hamsters is how much smaller they are compared to us. There are consequences to being small, and one of them is that all the body organs are small. Obvious isn't it? But size does matter in the way a body operates.

What is the relationship between heart rate and size? Does resting heart rate vary in a consistent way as size changes? Check out the interactive animation below. We've taken four mammals with very different body sizes and show the heartbeat rate. The controls are simple. The ON/OFF button activates the animation and a sliding control at the bottom of the graph changes the animal. The graph shows the relationship of heart rate and body weight in a log-log scale graph. Make sure you have the sound on to hear simulated heartbeat sounds.

It turns out that the size of a mammalian heart is approximately proportional to the body weight. So, a 50 kg dog has a heart that is 10 times as heavy as a 5 kg cat. Generally speaking, something that is small moves or vibrates faster than something that is large. It's not a perfect analogy, but an eagle flaps its wings slower than a sparrow, an elephant steps less quickly than a mouse, and so on. The relationship between body size and heartbeat rate for a mammal is:

Heartbeat rate (beats per minute) = 241 / fourth root of Body weight (kg)

So according to this formula, if the body weight is 81 kg, the heartbeat should be 241 divided by 3, roughly 80 beats per minute. For different classes of animals, like the reptiles, amphibians and such, this relationship will not necessarily hold, because of metabolic differences. Also, remember the heartbeat rate is heavily affected by other factors such as exertion, nervousness, body chemistry, or species specialization.

Interestingly, the blood pressure between different animals is independent of size. Of course, there are exceptions like the giraffe, whose blood pressure is very high so that the blood can be pumped to the brain perched on top of its long neck.

Source: Knut Schmidt-Nielsen (1997) Animal Physiology: Adaptation and environment 5th Ed., Cambridge University Press.