September 01, 2009

How humans differ from animals in height and mass variation (McKellar & Hendry 2009)

The authors found that humans within populations have more variation in mass than most animals do. In other words, there are many "thin" and "fat" people in human populations. This isn't very surprising to me, because in developing countries, socioeconomic differences may account for these differences (e.g., some people starve), while in developed countries, most people are employed in jobs and perform activities where having an optimal body mass is not that important for your survival and reproduction.

When it comes to height, humans show a very low within-population differentiation. In other words, most humans are around the "average" height, and really short and really tall ones are not that common. My guess is that this has something to do with the extreme socialization of humans; really short and really tall individuals (although the patterns are gender-specific) do have trouble in human society, if we judge from marriage ads where desired height is often specified, or from various pieces of technology (shields, spears, doors, steps, clothes, etc) which are designed for people of a particular height.

However, between-population differentiation in height is substantial. While we are in the 8th/4th percentiles in our within-population differentiation (very uniform), we are in the 47th/51st percentiles in our between-population differentiation. Thus, while in absolute terms between-population differences are average, these contrast greatly with our very low within-population differences: Human populations appear to be very different from each other in terms of their height.

From the paper:
One interesting result was that humans, in comparison to other animals, show a high level of within-population variation in mass considering their within-population variation in height (Figure 1). Specifically, when considering residuals from a regression of within-population CVs for mass on within-population CVs for length, human males and females fell into the 71st and 91st percentiles, respectively, for the entire distribution of animal species.


Another interesting result was that humans show low within-population variation in body height in comparison to body length in non-human animals (Figure 2), but the same was not true for human mass relative to animal mass (Figure S1). These differences can be quantified through several different comparisons. First, the mean within-population CVs for male and female human height correspond to the 8th and 4th percentiles, respectively, of the mean within-population CVs for animal length. In contrast, the mean within-population CVs for male and female human mass correspond to the 56th and 60th percentiles, respectively, of the within-population CVs for animal mass.
Specifically, the mean among-population CVs for male and female human height correspond to the 47th and 51st percentiles, respectively, of mean among-population CVs for animal length. Illustrated another way, humans show relatively low levels of within-population variation in height given their among-population variation in height (Figure 3).

PLoS ONE 4(9): e6876. doi:10.1371/journal.pone.0006876

How Humans Differ from Other Animals in Their Levels of Morphological Variation

Ann E. McKellar, Andrew P. Hendry


Animal species come in many shapes and sizes, as do the individuals and populations that make up each species. To us, humans might seem to show particularly high levels of morphological variation, but perhaps this perception is simply based on enhanced recognition of individual conspecifics relative to individual heterospecifics. We here more objectively ask how humans compare to other animals in terms of body size variation. We quantitatively compare levels of variation in body length (height) and mass within and among 99 human populations and 848 animal populations (210 species). We find that humans show low levels of within-population body height variation in comparison to body length variation in other animals. Humans do not, however, show distinctive levels of within-population body mass variation, nor of among-population body height or mass variation. These results are consistent with the idea that natural and sexual selection have reduced human height variation within populations, while maintaining it among populations. We therefore hypothesize that humans have evolved on a rugged adaptive landscape with strong selection for body height optima that differ among locations.


1 comment:

Ponto said...

What the results show is that human beings have domesticated themselves more thoroughly than dogs, sheep, goats, pigs and bovines. Think blue eyes. A totally useless iris "color", the color being an illusion, variety. Or the ability to drink raw milk post weaning when milk can be processed into butter, cheese, yoghurt and other more palatable forms of dairy products.

Or put it another way, humanity is the result of a supernatural being or beings interfering with human dna as in 2001 A Space Odyssey.

You can accept whichever explanation you like or reject both.