Re: Brain body size evolution

On 7/16/2024 9:49 AM, Athel Cornish-Bowden wrote:

On 2024-07-16 12:40:13 +0000, RonO said:
 

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,

 Does that require speculation? Surely we *know* that the brain uses a lot of glucose.

The speculation is for the second half of the sentence.  There may be some point where the brain to body weight ratio can't keep up with the standard ratio of correlated increase, and smaller relative brain size would be selected for to reduce the energy load.  I do not know what they think now, but when I took biology and comparative anatomy the notion was that you needed larger brains to control the larger body mass, and that is why brain size showed the correlation with body weight.  The high energy demands of the brain may mean that at higher body weights you may have to select for smaller relative brain size to reduce the energy demands of the brain.  For humans we selected for bigger brains that allowed us to collect more calories from the environment.  When we subjected ourselves to a poorer diet by taking up agriculture we selected for smaller brains.
Animals can obviously select for more efficient brains.  Bird brains are amazingly efficient in terms of what they can do for their size, and octopus brains are likely equivalent.  Avians and molluscs can do a lot with small brains.  Avians needed smaller brains because they needed to reduce the energy load and weight to fly, and octopus are cold blooded and needed their brains to work under those conditions.
Ron Okimoto

 

 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.
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Ron Okimoto