RMRh = 0.25*h+0.75*l for humans and RMRp = 0.08*h+0.98*l for other primatesIf we accept that RMRc and RMRp are the same (Klieber), then it's obvious that simply reducing the size of the "other tissues" with lower RMR, actually the RMR per weight would *increase*. Bbut as you told it's constant. If the gut is of the type "low RMR" and you decreased it's size, this would actually result in an *increase* of the summary RMR, because the brain would be relative bigger.
That's a contradiction. It only makes sense if the gut was also a tissue with elevated metabolic resting activity. Like reducing some of the highly active gut while increasing also highly active brain and leaving the rest at the same. "Expensive Tissue" only makes sense if the gut also was highly active, at rest. Maybe this is the case. I think it could be a little more active than other organs at rest because it has to move it's contents.
Personally I think a improved dietary quality alone can explain the gut size reduction. We should think of the purpose of each gut part. What is reduced in humans is the last part where cellulose fermentation is going on in other primates. They need it because they eat very much cellulose material (like leaves) and need to derivate their energy from it.
If a human get's more fruit (untrue) or more starch (true if you think of Wrangham/tubers) cellusole processing is less necessary. The fermenting part gut can be thrown overboard.
Increased dietary quality from
N.B.: However in modern humans the low intake of short chain w-3 fats and/or the intake of synthesis hindering items like tFA, alcohol, excess w-6, excess saturated fats seems to result in some points of differece in IQ to fish oil eaters.
Obviously in most women these organs are rather big, compared to the liver of a child. The amount of milk is not dependant on the size of breasts. But it may be that (I hypothetize this) the DHA synthesis capacity is dependant on the organ size (it creates more cell surfaces and more space for special enzymes to work).
Anyway the size of human mammary glands is very much bigger than the mammary glands in any other mammal I could imagine. Also in primates. As a side effect, the attraction of human (men) to sizy breasts (mammary glands) can be explained with a real evolutionary advantage they provide.
Both, the (relative) big brain and big breasts are unique in humans, because human kids need a longer brain growth time and more substrate.
My conclusion would be that DHA is not a limiting factor to brain growth, except in diseased states like intoxication with tFA, alcohol, excess w-6 acids, high insulin.
If DHA is not a main limiting factor then the best food item in dietary quality are tubers. That supports the theory of Dr.Wrangham. Also the availability of tubers in a savannah seems to be very much better in the aspects of