By Taylor Tibbs
When you think of a mouse and a blue whale, the first difference you probably think of is their size. However, another key difference between the two is how long they live. A mouse only lives about 2 years, while a blue whale lives around 85 years. It was this observation that intrigued scientists to determine what factors impact how long an organism lives.
What determines lifespan? At first glance, you might think that an animal’s physical size determines its lifespan, but it turns out that there is a little more to it than that – and it has to do with the heart.
The heart is a muscular organ responsible for pumping blood throughout the entire body, delivering oxygen and essential nutrients to all of an organism’s cells. While all backboned or vertebrate animals (like mice, humans, and whales) have a heart, not all hearts beat at the same rate. The average resting human heart rate is between 60-70 beats per minute (bpm), while the average heart rate of a mouse is around 400 bpm and a whale’s is 8 bpm. That is a pretty large range of resting heart rates. (You can visualize this difference here!) No matter the size of the animal (or the heart), the heart muscle itself has a maximum capacity of around 1 billion heart beats (at least in mammals).
This phenomenon all comes down to math (deep breath, this bit will be quick!). Scientists observed that whether you’re comparing a walrus to a hamster or kangaroo to an elephant, there is a mathematical equation linking heart rate and size to overall lifespan. This principle is called Quarter Power Scaling or Biological Scaling. If you work out the math, this means that an animal’s lifespan is proportional to its mass (bodyweight) taken to the minus one-quarter power (m-1/4). In other words, something that is 100 times larger than itself, like a hamster compared to a cat, lives about 3x as long. In this case, the cat lives roughly 3x longer than the hamster.
Now let’s get back to our original question: why do whales live so much longer than mice? Well, there are two reasons. Reason #1 has to do with their different heart rates: the mouse reaches its maximum capacity of 1 billion heart beats sooner than the whale, whose heart rate is 50x slower than the mouse. Reason #2 comes down to metabolism or how much energy an organism uses. Heart rate can be closely associated with the amount or rate of energy an animal uses. However, mice don’t actually use more energy than whales (per square footage of body size, anyways). Whale cells are just better at using energy than mouse cells. Like the heart, a mouse’s cells use energy very rapidly – and like the heart, a cell has a maximum amount of “metabolic heart beats”. A whale’s cells use this energy much more slowly, extending the lifetime of their cells and ultimately their bodies.
You’ve now learned that all hearts have a maximum of around 1 billion beats; however, there is an exception – us. The maximum number of heart beats for humans is recorded to be around 2.2 billion. Humans have muddled with the natural order of things, likely by discovering hygiene and medicine. We have been able to prolong our “natural lifespan” by taking better care of our bodies, and our hearts, thanks to medicine and science.
Interestingly, human athletes tend to have lower resting heart rates than the general population, around 40-50 bpm, and there is scientific evidence that individuals with lower resting heart rates tend to live longer than those with above average heart rates. Individuals with higher resting heart rates (90+ bpm) are 3x more likely to die prematurely than those with low to average heart rates. Moreover, high resting heart rates have been linked to increased risks of diabetes, Alzheimer’s disease, and even some cancers. Ironically, to lower one’s average heart rate, you actually have to increase it – at least temporarily. Like any other muscle in the body, the heart gets stronger with exercise and exercising regularly can lower your overall resting heart rate.
So, maybe all those posters and TV ads with jacked movie stars have some merit? You probably don’t need to go out and buy a Peleton, but it may be worth it to go on that Saturday morning jog or to take that quick walk around the neighborhood after dinner. Who knows, maybe you’ll be the human who breaks the 2.2 billion heartbeat maximum and tells those mathematicians to go take a hike (literally).
