Why Are Pygmies Short?

Dec 21, 2007 By Lisa Zyga feature
Why Are Pygmies Short?
A Baka pygmy dance group pictured with US Ambassador R. Niels Marquardt in Lobeke National Park, Cameroon, in 2006. Source: US Federal Government.

The question is controversial. Traditional explanations attribute pygmies' small stature to minimizing caloric requirements and walking in dense forests. However, a new study by researchers at the University of Cambridge suggests that there are some problems with this explanation, and offers an alternative hypothesis.

Human pygmy populations are defined by an average male height of less than 5 feet (155 cm). By this definition, a wide range of pygmy societies exist today in parts of Africa, Malaysia, Thailand, Brazil, and Bolivia – different environments that don’t match the traditional hypotheses for small body size.

Besides the differences within pygmy populations, there are also some non-pygmy populations that face some of the same physical challenges as pygmies but haven’t evolved a short stature. For example, many human populations live in dense forests and experience regular food shortages, and yet these populations have larger body sizes.

Now, scientists Andrea Migliano, Lucio Vinicius, and Marta Lahr have performed a study on two pygmy groups from the Philippines, the Aeta and the Batak, and concluded that there may be a better explanation for pygmies’ short stature. Their study is published in a recent issue of the Proceedings of the National Academy of Sciences.

The researchers point out that one characteristic unique to but common among many pygmy populations is their short lifespan compared to other humans. With this in mind, the researchers suggest that pygmies represent the “fast” extreme of life history strategies, with short stature being a side effect.

“We first thought that we would find a relationship between small body size and increased fitness in pygmies – for example, that the shorter pygmies would have more advantages, such as higher fertility, than the taller ones,” Migliano told PhysOrg.com. “However, the smaller pygmies had lower fertility than the taller pygmies. So this gave us the idea that perhaps there was no advantage in being short among pygmies.

“Then, when I went to the field, and started to interview them, I noticed the very high mortality rates – really high compared to any other population,” she said. “So when we checked that different pygmy groups followed the same pattern, we thought that these facts should be linked. Also, life history theory has been used for a long time to understand body size diversity among mammals, and we thought it should also apply to the understanding of human diversity.”

Because of their short life expectancies, the researchers speculate that pygmies have had to shift their reproductive years forward. The average life expectancy at birth for different pygmy populations ranges from just 16 years to 24 years. Very few pygmy women reach the end of their reproductive period, as only a small percentage survive past age 40.

In order to compensate for the lack of older reproductive women, natural selection has shifted the reproductive period forward. The fertility peak of age at first reproduction in the Aeta is around 15 years old, which reduces generation time and compensates for their short lifespan.

In order to make this fertility shift, pygmies must reach full maturity faster than longer-lived human populations. For this reason, many pygmies stop growing at about age 12, several years earlier than other humans. Their childhood growth rate isn’t any more or less rapid than the growth rate of other (traditional) humans; pygmy youth are roughly the same size as non-pygmy youth. (This is the opposite of what is observed in cases of nutritionally induced stunting, where humans delay growth but achieve adult body size later). Instead of experiencing the “teenage growth spurt,” pygmies’ growth is simply truncated.

Migliano also explained why pygmies’ growth rates don’t increase in the early years to compensate for their truncated growth at an early age.

“I think that, besides the high mortality, they have very low calorie intake, so it is a combination of the two factors that lead to the different phenotypes,” she said. “The pygmies grow in the same rates as the Turkana [eastern African Pastoralists], who also suffer from poor nutrition – but because the Turkana have longer life expectancy, they have time to grow for longer and achieve larger body size. I would expect that a population with high mortality and high resources would grow fast and taller.” In other words, human height in general is partially influenced by lifespan.

Still, the life history hypothesis leaves a few unanswered questions. For one, what originally caused the extremely high mortality rates among pygmies? The researchers suspect that the traditional hypotheses of environment, nutrition, thermoregulation, and other challenges may jointly or partially contribute to the high mortality rates observed in a wide variety of pygmy populations. In that case, the traditional explanations may be indirect causes of pygmies’ short stature, although the chain of effects would be much more complex than originally thought.

More information: Migliano, Andrea Bamberg, Vinicius, Lucio, and Lahr, Marta Mirazon. “Life history trade-offs explain the evolution of human pygmies.” Proceedings of the National Academy of Sciences. December 18, 2007. vol. 104, no. 51, 20216-20219.

Copyright 2007 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

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User comments : 11

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earls
2.3 / 5 (6) Dec 21, 2007
Because you're tall.
dsl5000
3.5 / 5 (4) Dec 22, 2007
Nutrition is a huge factor to growth, look at any developing country. Though because of that, the potential to grow tall is masked. It may be true that "short gene" may slip under the radar and propagate. But being tall can still happen. Ah, nutrition forced selective breeding.

Interesting site that tells how certain countries were affected with poor diet:
http://forum.stir...nes.html
Jorient
3 / 5 (2) Dec 22, 2007
I have studied evolution for forty years and non of this makes any sense to me. It seems to indicate, and support, "purpose" in evolution, and says, pygmies are small because they are small.

There are driving forces in evolution, but there doesn't seem to be any purpose, unless you consider survival. For example, insects used to be much larger than they are now because there used to be more oxygen in the air which allowed them to grow larger. They had to downsize to adjust to less oxygen in the air. This was a survival mechanism, not a "purpose" for being.

Am I missing something here?
dsl5000
3 / 5 (2) Dec 22, 2007
Basically this has been seen in multiple animal studies, but somewhat baffling in humans, just because(We are gods amongst lower animals or so some people would believe). I know a test that would prove it :P Take 50-100 of those pygmy infants and raise them in countries with full diet and optimal conditions, if they are still short, then genetics of short stature is prevalent in that population. If they turn out taller and have normal fertility track, then it just means environment has a HUGE impact. (Of course this would never happen)

I think survival is a big purpose, to live, to propagate. Without survival, propagation would not occur/ no fitness.

Um, lower level of oxygen coincides with survival in beetles imo. Their "circulatory system" is based on Oxygen diffusion no? So of course if they are huge, O2 saturation throughout the entire body would be difficult. With that in mind, O2 is used to convert food to energy. O2 is limiting factor, where as in this case food seems to be the limiting factor.

Also consider the difference that the efficiency of utilizing food/effects are different amongst people. E.g. Drugs, some people get certain side effects while others don't.

The comparison between two low diet population means little, what they are eating is important. E.g. History tells a tale of sailors with scurvy but they ate like any other, what was wrong? A specific lack of vitamin C in their diet.

The purpose is fitness, but if you can't survive to reach that, it's pointless imo.
hopefulbl
2 / 5 (6) Dec 23, 2007
pygmies are short because they go in the jungle between 2 and 4 in the afternoon you know...that is the punch line....from beginning
WHY DO ELEPHANTS CLIMB TREES? to get to their nests....WHY IS IT DANGEROUS TO GO INTO THE JUNGLE BETWEEN 2 AND 4 IN THE AFTERNOON?....that is when the elephants are jumping out of their nests...WHY ARE THERE SO MANY PYGMIES IN AFRICA?...they went in the jungle between 2 and 4 in the afternoon...from an old elephant jokebook I read 30 years ago
doctorj
3.3 / 5 (4) Dec 24, 2007
A better question would be to ask: Why do humans have a growth sprit in their teen where the direction their body grows reverses its self (feet before legs, fingers before hands, hands before arms etc.) in the first place? I have what I think is a good idea. Do you have any ideas?
SgntZim
1 / 5 (1) Dec 26, 2007
Hi doctorj. Well, it's 2 days since you posted and nobody has answered, so how about letting us into the secret?
zevkirsh
1 / 5 (1) Dec 26, 2007
i just want to say that i'm a short man at 5'5'' and this article just made me feel that i now understand why i'm short. i'm a closet pygmy who ate well! :)
dsl5000
1 / 5 (1) Jan 01, 2008
Happy New Years!

Zevkirsh, haha, pygmy's average male height of less than 5 feet, it says that in the first sentence. You might as well be called a giant if you compared yourself. I have friends who are 5'3" male. I'm not talking about dwarfism either.

doctorj where is your idea? i'd like to hear it.
Growth spurt huh...that is an interesting question no? why does it happen? Aside from hormonal changes. The purpose of a spurt of rapid growth rather than a continuous constant growth. I'd say it's because our ancestry had quite a tumultuous past. Ever heard of the roman quote? "The small cannot compare with the big, the weak cannot compare with the strong." I would think it would be similar to our savage like nature (We humans are amazingly violent creatures "ya" know). Haha even today when we call it the "modern" world, we still size each other up...it's hilarious. From various body parts to height, etc.

Growing distally is easier, like trees, they grow distally more readily than expanding trunk size. Though of course it needs to be balanced. Going too far in one direction is bad, teenagers are goofy enough as it is (loss of balance and altered self perception). Hence why big kids will generally become big adults(suppose both height and width). Well actually fat kids may have the capacity to grow much taller imo, i wonder if there are any scientific papers sizing up chubby(overly overweight would decrease height gainage) vs. Skinny during pubertal growth.
tyciol
5 / 5 (1) Feb 05, 2008
Very interesting article. Personally, I think I prefer maturing slowly rather than maturing at a normal rate and just stopping.
Meteko
1 / 5 (1) Feb 24, 2008
This human pygmy story is very interesting.

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