Paul:

It's good to be back. 

Geoff:

Much appreciated!  I've gone to some effort to keep the archives organized…stop by anytime.

Kate:

"Why Are We Hungry" isn't done yet…I'm just taking time to explore other topics.  Frankly, I could write an entire book about hunger, and perhaps I should.

Mich:

Pleistocene climate was indeed a wild ride…and there's no reason to think it's over.  Food for thought.

Anand:

Aiello and Wheeler are clear, in the original paper, that the ETH only applies as stated to the great apes.  They don't have solid data for other apes, let alone the rest of the animal kingdom, and they don't attempt to apply the ETH to the rest of the animal kingdom.

Why not?  Because it's clear that, as you mention, there are many more possible energetic tradeoffs between different body tissues.  Brain vs. gut was the biggest tradeoff for great apes, but muscle mass and bone mass can also be traded.  Then there is general activity level: you can have lots of muscle that spends most of its time at rest, or less muscle that spends more of its time in motion.

"Why do carnivores have a small brain?"  Answer: it's as big as it needs to be.  Larger-brained lions (for instance) have no survival advantage.  I'll discuss the reasons for this at greater length in next week's article.

It's very interesting that you're attempting to expand the ETH to apply to all mammals, not just the great apes, and it's a worthwhile exercise.  Unfortunately, evaluating your hypothesis is beyond the scope of this series at this time…that's a whole another series right there.

And while it's true that the ancestors of modern humans only left Africa c. 60 KYA, it's also true that Pleistocene climate shifts dramatically affected Africa even if there were no glaciers there.  There were permanent snowfields in Morocco even during the Little Ice Age, and there is snow on Kilimanjaro now.  More importantly, Ice Age climate is much cooler and drier than interglacial climate, and it would have featured much more savanna, grassland, and temperate forest than tropical forest and desert.  (We find all kinds of archaeological remains in the Sahara.)

Neal:

Absolutely correct, and that's a big piece of the puzzle.  In addition, larger predators spend more time scavenging, while smaller predators spend more time hunting.  I'll bring that up next week.

Anand:

That's one problem with Navarrete et.al.  There are a lot of problems with it, and I'll discuss them in the future when I devote an entire article to the subject.

Also keep in mind Kleiber's Law is both an empirical observation and an approximate rule of thumb.  There is still much debate as to the best value of the exponent (2/3 or 3/4?), and as to which biological constraints cause it to be a useful rule of thumb.

Fmgd:

Yes, spotted hyenas are awesome creatures, especially the bipedal ones…hence the name of this site! I'm glad you found TGC worthwhile.

I'll make fun of the lazy, stupid prejudice of Hemingway and Disney all day, but I understand the sex role confusion: when females are just as well-hung as males and socially dominant, it's easy to make that mistake.

jesse:

The ETH tradeoff doesn't necessarily go the other way.  Assuming constant income, if your rent goes up you have to spend less money on something else, like food…but if your rent goes down, you're not required to spend the surplus on anything in particular.

You're absolutely correct that aminos can enter directly into the Krebs cycle and be burned for energy: though each amino enters at a different place, it averages out to the published energy figure of about 4 kcal/gram for "protein".  This pathway is unjustifiably ignored by most because it's difficult to measure the degree to which it's occurring.

I'm looking for papers on carnivore physiology, but they're rather rare!

jesse, anand:

The "inefficiency" is more in the fact that we don't have a dedicated storage organ for protein as we do for fat and (limited amounts of) glucose: building lean tissue is much slower than fat storage in adipocytes or glycogenesis in muscles and the liver.  So it's not that protein is an intrinsically poor energy source, it's that we have a limited ability to store and retrieve it.

More soon!

JS

Vizeet:

Gorillas, chimpanzees, bonobos, and human hunter-gatherers all have approximately four-year interbirth intervals.  Having more than 5-6 offspring is a primarily Neolithic behavior, because it's only made possible by sedentism.  (If you're a hunter-gatherer mother, you can only realistically carry around one small child at a time.  H-G mothers nurse for several years vs. the one year or less common with agriculturalists.)

Also, none of the great apes have seasonal estrus, though it isn't concealed to the degree it is in humans.

However, you are correct AFAIK that human babies are indeed 'premature' (helpless for longer than chimp/gorilla babies), most likely because it's difficult to fit that big head through the hips.  Thus the flared hips of the human female.

JS