The Pump of Youth
Most people, at least in youth, hope to live forever. But we die. More than that, we die in predictable ways, perhaps more predictable than we might imagine. Compare species. A shrew never lives a decade while the average blue whale might live ten. To a great extent the difference between the shrew and the whale appears predictable as a function of how they use their bodies. The shrew’s heart beats a thousand times a minute and, as it does, requires each cell in the shrew’s body to work commensurately fast. The whale’s heart beats slowly, just a half dozen, ponderous beats per minute. Both the whale and the shrew live about a billion heartbeats, as, studies conclude, do nearly all other mammals so far considered. It may be unclear how many licks it takes to get to the center of a tootsie pop, but we know how many licks of the heart it takes to get to the end of a life.
Or at least we know this for some mammals. While dozens of papers consider the relationship between life expectancy and heart rate, these papers consider just twenty or so of the more than 5000 mammal species on Earth. No such comparisons exist for birds, or reptiles, or amphibians or fish. Maybe twenty species is enough to show us the main story, but maybe not. More importantly, much can be learned from the exceptions.
We already know that we are one of the exceptions. Modern, humans in developed countries live out more than 2.5 billion heartbeats if they are as lucky as the average man or woman. This is to say, they/we/you and I live 1.5 extra lives compared to the rest of mammals. We get these extra lives thanks to public health and medicine, both of which allow us to live on even after our cells begin to show their wear, fade and fail. Our ancestors, as recently as two hundred years ago, experienced no such luck. It is a tragic reality of the current dynamics of the world that neither do people, children, born in many countries. Like a shrew or a whale a child born in Malawi, for instance, is likely to get no more than a billion beats.
We know little though about the extent to which some other mammals might also gain extra beats, more than their fair billion. Lacking vaccines, antibiotics and cardiac care centers the extent to which these other species live beyond a billion beats must depend in part upon unique features of their biology. Whatever these features are, if we understood them, we might be able to figure out new ways to extend our own health, push our enfeebled and worn cells and hearts through a few more beats, maybe many.
In order to find these species, we have started a new project in which we are scanning through old books and papers for reports of the longevity of vertebrate species (be they mammal, bird or other) and then also their heart rates. We have already found data on more than 140 species of mammals. You can see the results in the main figure on our project page. Species above the line are those that live longer than their billion beats. In these species we might seek answers as to longevity. Species below the line, they live less than others. In their failings too are stories. But we know that in old books and journals, other scholars have studied other species, and so we are providing here a list of species the heart rates of which we do know. We are soliciting help in tracking down species records missing from this list. Some may be in obvious places. We are still missing some old articles, for instance, with obvious titles such as “the heart rate of the hummingbird squirrel.” Others will be less obvious, buried in old work about unrelated topics. Yet, if we are going to see the big picture, we need those too. We could, of course, just go out and measure heart rates of animals around the world. But such work is less and less easy, the interesting mammals further and further from the places we cast nets, the truth more obscured by the consequences of humanity’s spread. And so for now, we build upon what is already known but not integrated.
And there is more. For the person who, in the next six months, finds the most new records, we will offer a prize, a plush heart. We offer one too for the person who finds the animal that gets the fewest heart beats in its lifetime (right now, the common mole-rat is winning) and the most. As these data come in, we will update our figures and our sense of what means to live a life or, if you are a lucky human, two.