Like all dogs, Gus is getting slower as he ages. His main pump is failing; slowly but inexorably. If you accept that there is there is a relationship between the mammalian heart rate and overall life expectancy, you will already to know to expect late life heart issues for otherwise healthy pets. This may say nothing about their enthusiasm. For example, Gus loves the snow and today, he was out and about, running on snow and slipping on ice. He was segmenting and ranging on familiar paths, back and forth, excited by the snow while I was taking photographs. Today’s ambition, for me, was to take a series of high dynamic range photos of relatively mundane subjects as I walked quickly around our local hill.
The limit to longevity is indicated by studies that have concluded that all mammalian hearts have an endurance of about a billion heartbeats. That there are variations for body volume will come as no surprise to those who notice that whales are quite different to gerbils. You may know there are allometric rules that matter, assuming that you know allometry is what’s left when isometric relationships fail.
You’ll recognise allometric growth when I remind you that your head takes up a much smaller proportion of your body than it did when you were born. We’re not sure why but it seems likely that we need a big brain right from the start; there is a lot to learn. And of course, you know that your skeletal structure got disproportionally stronger and stiffer as your body size increased. That happened because your total volume was increasing faster than your height. What happens in middle age is for different reasons.
In summary, the human heart grows in proportion to body size, that is to say, isometrically, increasing linearly with the body. Your brain grew allometrically, it having been disproportionally larger, relative to your body, when you were a child than when reading this journal as an adult. You can see allometry everyday in birds and planes. Compare the wing sizes in the wren and the condor, the 737 and the A380 to the volume of the bodies they carry.
Except that the use of, the sense of, allometry has changed. These days it tends to be used to refer to biological scaling relationships in general. But this doesn’t matter to Gus as he approaches his billionth heartbeat. He’s recently developed gingivitis which in humans, increases the likelihood of heart attacks. But we treat him for this as we treat him for his arthritis, his heart and his general senescence.
Humans, as in many things, are anomalous in that we get about 2 or 2.5 billion beats of the pump. We’ve been very successful in extending our life expectancy so I suppose it’s a relief to know that our hearts have the durability to capitalise on the medical and nutritional advances. I’m reminded of something Bill Bryson wrote in The Body.
‘In 2011, an interesting milestone in human history was passed. For the first time, more people globally died from non-communicable diseases like heart failure, stroke and diabetes than from all infectious diseases combined. We live in an age in which we are killed, more often than not, by lifestyle. We are in effect choosing how we shall die, albeit without much reflection or insight.’