Focusing on humans only, here.
I was going to ask how efficient the human body’s digestive system is, but I’m not sure that question is correct. I am wondering how much actual energy or sustenance is left over in the poop that we excrete.
Could one extend their life even a day by eating it if they were starving, or is it more likely to just kill you more quickly?
First off, that’s incredibly stupid. Your body got rid of it for a reason. It’s mostly just fiber, bile, and miscellaneous waste. You know basically every creature with a brain which excretes finds feces gross and shies away from them? Because it’s really unhealthy. It’s swarming with bacteria from digestion (and no, just because they come out one end does not mean they’re any good going in the other).
So, no, there’s virtually no nutritive energy.
Rabbits are different. There are some breeds which have inefficient digestions which, ah, re-use their waste. Although it’s more like eating your own vomit. Which is also really, really disgusting.
Now, if you dry it out and burn it, that’s also a different story. And you could theoretically convert it into enough energy to blow up a city block if you merged it with antimatter.
Pick out the corn. Grind it up (this will increase its digestibility). Goes well with Mexican food.
Ahh, the circle of life.
Hmm… could you gather a bunch of insects that eat poop -dung beetles, flies, etc- and eat the insects?
This assumes those poop eaters like human poop. I don’t think I’ve ever seen any human poop in the wild to observe for signs of life.
A very informative previous discussion on this topic:
How efficient is the human digestive system?
Another discussion (which links to the one above): I eat 2,000 Calories; Am I Necessarily Absorbing All of Them
It seems to me that I once read, here on the SDMB, that most of the solid bulk of human poop is actually the dead remains of gut bacteria.
If that is true, it probably consists largely of bacterial cell wall. The internal contents of a bacterium, once spilled should be easily digestible, but the cell wall of a bacterium contains D-amino acids. Humans, and and other animals, can metabolize only L-amino acids. This is presumably why (or, in lager part why) the dead bacterial cell walls get pooped out rather than digested, and will also mean that the stuff will have very little nutritive value (thank goodness!) (There is polysaccharide as well in the bacterial cell wall, which would probably be digestible otherwise, but I suspect the presence of the D-amino acids prevents this.)
Not a dog owner, are you?
Maybe you should pick different breeds, or just get a mutt.
My dogs would find that statement incredibly stupid. They seem to find their poop perfectly tasty. In fact, most dog owners go through a phase where they have to train their dogs to stop eating their own poop.
ETA, I see Finagle already got to it.
In this video which suggests otherwise, an elephant goes “fishing” (inside another elephant) for tasty dung nuggets. I’ve also seen footage of baby elephants chomping at piles of dung from other elephants; supposedly this is a way to jump-start the bacterial colonization of their guts.
You then go on to point out that rabbits are also part-time coprophiles.
And as Finagle points out, dogs are famous for dining on poop.
I recall hearing a story long ago about starving prisoners at the the Andersonville prison digging through feces to find undigested nutritious matter.
You might wanna take your chances with urine instead
Maybe a friend has diabetes and you’ll get some sugars, too:eek:
Every species has some freaky members. Like labradors. 
In the interests of fighting ignorance: All breeds of rabbits eat their cecotropes, which, while they look like poop and come out of a rabbit’s butt, are not feces. If you have a pet rabbit and are observant, you will soon learn that cecotropes are different from poop nuggets in several observable ways. Both are little brown lumps, but cecotropes are shiny, a little sticky, and have an odor (not a terribly bad one, though) while poops are dry and mostly odorless. Healthy bunnies eat all their cecotropes, often, er, directly from the source; if you feed too rich a diet, though, you may end up with sticky little cecotropes distributed around the house or cage. They will stick to your shoe if you step on one. Poops just get flat.
And it’s not much like eating your own vomit. No acid, no puke smell, etc. It’s a lot like chewing a cud, though. Rabbits who don’t eat their cecotropes may die of malnutrition.
One of our two cockatiels is a coprophage. He eats not only his own poop, but the other bird’s too. Then he tries to eat my pizza, but there I draw the line.
If you manage to convert all the mass in the poop into energy, then, according to e = mc², poop will, just like anything else, contain 9*10[sup]16[/sup] joules of energy per kilogram. You can’t get more efficient than that.
I guess I shouldn’t be surprised by a huitlacoche reference on the Dope.