The rate-of-living theory is popular on the Internet. The intuition is that all animals are born with some “budget” that they burn out over time according to their rate of energy expenditure. A proxy for how much energy you spend is your heart rate.
Anyhow. You are a candle. The harder you live, the shorter you will live. You have a fixed budget, spend it wisely. Live slow, live long; live fast, live short.
The famous science-fiction writer David Brin penned an essay entitled Want to Live Forever? to support the theory :
Elephants live much longer than mice, but their hearts also beat far slower, so the total allotment stays remarkably similar.
Brin’s essay basically says that evolution tried to maximize our lifespan, but that we are hitting the limits set by the rate-of-living theory.
This sounds good and intuitive.
But is it true? Notice how Brin refers only to mammals. What about birds? Well, parrots live to be 75 yet their hearts can beat up to 600 times per minute. And some animals (like the jellyfish and the hydra) are considered â€œimmortalâ€.
So maybe there is something special about mammals that makes us burn like candles with each heart beat.
Maybe. But we know that large dogs live much shorter lives than small dogs. The difference is large. A Yorkshire Terrier lives up to 17 to 20 years old. A Great Dane is expected to live only 6 to 8 years. Yet, there is no correlation between body size and heart rate in dogs. So big dogs get far fewer heart beats than small dogs.
So maybe it is not heart rates that matter but your energy expenditure.
Well. Athletes certainly do “live fast”, building up a large muscle mass and burning lots of energy even while at rest. Do athletes live shorter? We have no evidence of this effect.
What about child rearing? For a woman, giving birth and raising kids is certainly a major drain. Except that women who have children live longer.
The question was investigated thoroughly 25 years ago by Austad and Fischer and they demonstrated that the rate-of-living theory is wrong:
Our results fail to support a rate of living theory of aging in either a weak or strong form. The strong form suggests that mass-specific energy expenditure per life span would be approximately equivalent across species. For 164 mammalian species, energy expenditure per lifetime ranges from 39 to 1102 kcal/g/life span. Within bats, there is nearly a fourfold range of lifetime energy expenditure while among marsupials the range is nearly tenfold. Thus even within mammalian orders, this generalization seems not to hold.
A weaker form of the same idea holds that lifespan should be generally inversely related to metabolic rate. This notion predicts that: (a) marsupials should be longer-lived than similar sized eutherians; (b) heterothermic species should be longer-lived than similar-sized homeotherms, and (c) homeothermic bats should have life spans approximately equal to similar-sized nonflying eutherians. Our data fail to support any of these predictions
These results suggest that at least between species, there is an uncertain relationship between basal metabolic rate and aging. Therefore the finding that a proven life-extending treatment such as dietary restriction in laboratory rats and mice does not routinely reduce metabolic rate is perhaps less
surprising than it otherwise might be.
It is clear from the preceding that there is no simple relationship between mammalian longevity and metabolic
rate. Species with low or high metabolic rates may have evolved short or long life spans, depending upon their ecology.
At this point, one might be tempted to try to find interventions that “slow your life” and also extend your lifespan. But these examples would not rescue the theory. Indeed, you might have a theory that all animals living in the ocean are fishes. If I point out that your theory is wrong because there are whales, you can’t argue back by giving me examples of fishes that live in the ocean.
The lesson is that animals are not candles. You can work and play hard, and yet live long. You have no excuse to be boring and slow.