Body Size Vs Lifespan: and the debate continues.

A few months ago, I read in one of the news media that the average lifespan or life expectancy of a typical Nigerian is 40 years (you read that right, four and zero). It sounded amusing to me (just like it would be to every research minded Nigerian youth) simply because I could not fathom where and when the research was conducted before arriving at such conclusion, even to the extent of the latter making the news. Contrary to the news and according to the World Health Organization, Nigerians have an average life expectancy of 54.5 years - not like it is more comforting in any form.

^{By Rcragun - CC BY 3.0, Link}

On a more general note, virtually everyone knows that the lifespan or how long a person would live is dependent on several factors, especially those relating to the lifestyles of an individual. We all know that smokers are liable to die young, don't we? :). Beyond all the factors however, there are natural processes within the body system that causes aging/senescence of all biologically living organisms. These processes are referred to as metabolism. The faster the metabolisms of a living organism, the shorter its lifespan and vice versa. This assertion which is generally referred to as rate of living theory does not hold true in all cases as we will find out later in this article.

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Body size and lifespan

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How would you feel as human if you were told that your body size is a limiting factor that determines the number of years you are going to spend alive? Would you try to become smaller or bigger if you learn that either would grant you a longer lifespan respectively?

Well, the idea that the size of an organism has a say in its lifespan stemmed from the observation of Max Rubner in the year 1908. Through series of investigation, he was able to come into conclusion that larger animals lives longer than their smaller counterparts. According to his theory, larger animals tend to have slower metabolism compared to the smaller ones, hence they live longer.

^{By Giladleh - CC BY-SA 4.0, Link}

If indeed larger animals have slower metabolism when compared to smaller ones, then the theory propounded by Rubner makes perfect sense. If the body of organisms is likened to a system in which chemical reactions is continously taking place, a faster reaction means products would be reached quickly and reactions would come to an end. Such is what happens with organisms with faster metabolism, which is the smaller organisms.

The theory that larger organisms have slower metabolisms compared to their smaller counterparts can also be explained using a simple analogy. Considering an important process of circulation of materials in higher animals. Oxygen needs to be transported into various cells of the body tissues from the lungs while carbon dioxide needs to move in opposite direction. For animals with large bodies, the blood would need to travel more distance in order to reach the various cells compared to animals with smaller bodies. Hence, the metabolic transportation of oxygen and carbon dioxide from and to the lung is completed faster in the animal with smaller body compared to its larger counterparts.

However, it is interesting to know that while the positive correlation of body size and lifespan theory appears to be true for some organisms, the reverse seems to be the case for a lot of others. Depending on whether the phenomenon is investigated within or across species, conflicting results can be obtained.

It has been established that most big mammals have higher lifespan than small mammals, though this can be argued as far as Homo sapiens is concerned. Agreed that elephants do have higher life spans than mice which generally conforms to the body-size/lifespan theory, this does not holds true if man and elephants are to be juxtaposed. The lifespan of elephants ranges from 48 to a maximum of 70 years while human beings that got as old as 125 years have been recorded (My grandma actually lived till 130 years). This consequently means there are exceptions to the body-size/lifespan theory and a host of other factors might actually be in play.

In a research conducted in 2005, Aziz and his co-researchers found that larger male strains of Drosophilia melanogaster had higher life span when compared to smaller sizes of the same strains of the organism. Drosophilia belongs to the insecta class of the Phylum Arthropoda, the same class that the much bigger houseflies belong, yet the houseflies do not live beyond 28 days while Drosophilia lives for 40 to 50 days.

The report on the Drosophilia above thus suggest that correlation between body-size and lifespan may best be considered on species by species or even strain by strain basis. A research conducted by Colin Selman and his team to assess the relationship between body-size and lifespan of different breeds of dogs revealed that a strong inverse correlation exist between the body-size and lifespan. This simply means that the bigger dogs lived shorter when compared to the smaller ones, a suggestion that is true to reality. I do not own dogs, if you own dogs and have a contrary experience, please feel free to express your observation in the comment section of this article.

^{pxhere CC0 Public domain}

Similar to what is obtainable in dogs, research by Thomas Samaras and Harold Elrick has also suggested that humans with big body size could live a shorter life compared to those with small body size. The outcome from their research completely negates the earlier notion that big body-size humans live longer than their small body-size counterparts due to slower body metabolism. It thus means that a whole lot of other factors (including genetic factors) come in play when the lifespan of man is in question. The discussion on the factors is for another day, maybe in my future posts.

Contrary to what is obtainable in dogs, It is generally believed that lifespan in birds depends on and directly varies with the size of the bird. That is, the larger the bird, the more the lifespan. Although the correlation found in this case has a lot to do with survival within the environment by small birds. Small birds generally are prone to predation and other environmental factors that tend to limit their lifespans. Hence, the larger birds’ sizes help them to stay atop the food chain as well as conferring survival advantages in times of diseases, accidents, famine and so on.

^{By Pslawinski -CC BY-SA 3.0, Link}

However, small birds have been reported to live a considerable lifespan with every other factors being constant. For example, the king vulture is far bigger than the parrot but has a life expectancy of 30 years, whereas a parrot in captivity has been reported to have lived for more than 80 years.

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How about in plants?

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The concept of body-size having correlation with lifespan is also applicable in the world of plants. Tree species most often outlive shrub species while shrub species always seem to outlive herbs and grasses. Hence, it can be said that body-size and lifespan in plants directly varies with each other, which is positive correlation.

The above assertion was corroborated by the report of Núria Marbà, Carlos Duarte and Susana Agustí of the Mediterranan Institute for Advanced Studies in Spain in one of their researches during which about one thousand natality and mortality rates in different species of plants were examined. More information on this research and why it is a testimony to the fact that lifespan of plants is dependent on their body size can be found here

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Summary

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There is no doubt that the issue of body-size and lifespan in living organisms is an interesting one that has attracted a lot of research. While both phenomena happens to be positively correlated in some animals, it is negatively correlated in some while some animal species do not show any specific trend. There have been various arguments and hypothesis to explain the different trends but like every law, there are always exceptions.

In plants however, the observation that body-size is directly correlated to life span has been established and actually holds ground to an extreme precision as shown by the research of Núria Marbà and his colleagues.

Do feel free to express your opinion in the comment section below.

Thank you!

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References

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science direct

ncbi

world life expectancy

wikipedia

wild birds

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