Re: Brain body size evolution

On 7/16/2024 1:05 PM, Ernest Major wrote:

On 16/07/2024 13:40, RonO wrote:

https://www.sciencedaily.com/releases/2024/07/240708101004.htm
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https://www.nature.com/articles/s41559-024-02451-3
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The Nature article is open access.
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The authors claim that they have developed a model for the relationship between brain and body weight for mammals and the evolutionary trajectory for different lineages.  With this model they can identify lineages that do not conform to the usual brain size evolution relationship to body weight.  As pretty much every other study has indicated humans have evolved bigger brains for their body weight and primates have a higher rate of brain size increase.  Some lineages have lower brain size to body weight than expected.  As you might expect these are the largest mammals.  They speculate that brains take a lot of energy to maintain, and that there is likely selection against larger brains at some point in body size increase.  Population sizes for large mammals have to be smaller because it takes more food to maintain individuals.  The estimate that I have seen is that it takes 80% of our energy production to run our brains.  If you have smaller brains you could maintain larger populations.  Hunter gatherer populations were probably restricted by our brain's energy needs.  With the poorer agricultural diet our brains actually decreased in size as our population increased.  We could maintain much larger populations on the same amount of territory, but it wasn't a diet amenable to supporting large brains.

 The number floating around the web is that the brain (2% of body mass) consumes 20% of resting energy usage (and presumably a lower proportion when performing hard physical labour).
 Elsewhere I find it said that the heart and kidneys use more energy per gram than the brain, and the liver and spleen combined use more energy that the brain does.

That is what it seems to be.  For some reason I recalled it to be 80%. Maybe I just transposed the percentages.
The web claims that kidneys use twice as much energy per gram as the brain (>400 kcals/kg per day, supposed to be the same as for the heart).   One estimate for the heart was 440 kcals/kg.  The brain uses around 200 cal/g per day (200 kcals/kg).  The the two kidneys only weigh around 400 g total, and the heart weighs around 300 g, while the brain weighs 1,300 g, so the brain uses more energy than your kidneys or heart that use the most energy per gram.
A paper I found had resting energy needs.
https://www.sciencedirect.com/science/article/pii/S0959438822001623
Brain 21%, Kidney 8%, Heart 8.9%, Skeletal muscle (36% of total mass, while the brain is 1.9%) 21.2%.
Ron Okimoto

 

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Ron Okimoto
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