Re: Do smaller organisms have a faster heart rate than larger ones if so why?

Hello Deshan,

Your question is very interesting.  It turns out that the simple answer is 
"yes," but the whole story is a bit more complicated than that.  Let me 
explain.

It is valid to make a broad generalization and say that small animals have 
faster heart rates than large animals.  Thus, your cat's heart rate is 
faster than yours, which is in turn much slower than an elephant's.  These 
kinds of comparisons work very nicely when the animals being compared are  
related; in the example I gave you, all the animals are mammals, so it 
works.  

However, when you're comparing animals that are not closely related, the 
comparison might fall apart.  For example, a frog that is smaller than a 
rabbit may have a slower heart rate, because the frog is an amphibian and 
the rabbit is a mammal.  Mammals and birds are endotherms, and thus have a 
high metabolism manifested by increased heart rate, respiratory rate, and 
body temperature.  Frogs, reptiles, and fish are generally ectotherms and 
are typically characterized by having a lower metabolic rate than mammals.

Now, why does heart rate vary with size?  It turns out that within the major 
groups of vertebrates, mass-specific metabolic rate increases with 
decreasing body size.  This means that for a given amount of body mass, a 
small animal has a higher metabolic rate than a large animal.  Since heart 
rate is a major factor of metabolic rate, the higher metabolic rate of small 
animals is manifested in a higher heart rate.

Please note that although I've answered your question as it pertains to 
vertebrates, the same principles apply to invertebrates as well.  

I hope this helps!

Allison J. Gong
Mad Scientist