Human Longevity Compared to Mammals

There are many kinds of laws in nature. A good synopsis of those laws is provided by
Glattfelder (2008):

In biology, so-called allometric laws describe the relationship between two attributes of living organisms as scaling laws:
An important relation to consider is this one:

L ~ M^(1/4)


L = kM^(1/4) = kM¼ = kM0.25


L = Lifespan
k = Constant
M = Mass

The equation

L = kM0.25

can in turn be simplified mathematically as follows:

log (L) = log (k) + 0.25 log (M)

The average weight of a human being in the United States is currently about 80.7 kg (178 lbs), and the average lifespan is 77.6, with a maximum of 119 (Sarah Knauss: 24.09.1880-30.12.1999). Yes, Americans are overweight.

The average weight of a chimpanzee is about 47.6 kg (105 lbs), and the average lifespan in the wild is 35 years, with a maximum of 60 in captivity.

This data shows that the following equations are applicable for United States humans and chimpanzees:

LifespanSpecies = (ConstantSpecies) × (MassSpecies)0.25

LifespanHuman = 25.89067288 × 80.70.25 = 77.6 years

LifespanChimp = 13.32497203 × 47.60.25 = 35.0 years

If human beings followed the same relationship of lifespan to mass as chimps, human beings in the United States would live an average of 39.9 years...

There is in fact an equation that all terrestrial and marine mammals follow which predicts their average lifespan based on their average adult body mass. All placental mammals (Eutheria) follow this equation fairly closely, except one: Homo sapiens sapiens. However, even the estimate above shows that the constant followed by humans is almost 2 times (1.94) larger than that followed by chimps! This shows that humans are not following the same rule all other mammals follow. It is as if someone genetically modified humans. They are not behaving like a mammal as far as longevity goes, just as GM salmon don't behave like wild salmon as far as their growth rates are concerned.

The lifespan (tlife in years) for mammals in captivity has been found to vary in body size (Mb in kg) according to this equation (Sacher, 1959):

tlife = 11.8 × Mb0.20

By inputting the
average weight of a human being in the United States of 80.7 kg into the equation above, we have:

tlife = 11.8 × 80.70.20 = 28.4 years

Since the average lifespan in the United States is 77.6 years, it is obvious that human beings in the US are living 2.73 times longer than they would live if they followed the Sacher equation above closely.

According to these Biology 3295 program lecture notes

"Human lifespan greatly exceeds that expected for mammals of their mass."

The constant of a similar equation Sacher derived for wild birds

tlife = 17.6 × Mb0.20

shows that birds outlive mammals of the same adult mass.

It is also interesting to note that there is an egg-laying mammal, the Australian ant-eater known as the echidna. Monotremes like the echidna are mammals that lay eggs (Prototheria) instead of giving birth to live young like marsupials (Metatheria) and placental mammals (Eutheria). The life expectancy of echidnas is over 50 years. In mammals, only humans have a longer life expectancy than
echidnas (Humphrey, 2001).

If this is not a tell-tale sign of just how exceptional human longevity is, then I don't know what is. The only mammals besides man known to exceed fifty years in lifespan are the Asiatic elephant, and in rare instances the horse (Cobb, 1954). Human lifespan should be considered nearly as extraordinary as human brain size, yet it receives scarsely any attention at all. Whereas the human brain is 3-fold greater than is predicted from body size alone, our lifespan is 2.5-
fold greater than would be predicted from body size alone (Robine, 2007).

Moreover, it is our longevity that matters more, since it even prolongs our mental abilities. Macaques exhibit physiological signs of cognitive impairment, as evidenced by Alzheimer-like neuropathology and cerebral atrophy by ages 22 to 25, and chimpanzees exhibit this by age 30. Contrast these already magnificent mammals with humans, for whom such changes are rare until age 60 (less than 1 percent) and only common (greater than 30 percent) in their 80s (
Kaplan, Lancaster, and Robson, 2003).