Which creature has the best ratio of input to output with respect to it’s food? For example, a creature that defecates 1lb of feces for every 10lb of food it ingests would have a ratio of 10:1, which would be better than a creature with a 5:1 ratio.
Are there any creatures that are 100% efficient and manage to make use of all of the food they eat and producing no waste?
I suspect that this would tell you more about the energy potential of the food than it would the efficiency of the creature. You would need to have all the creatures eating the same food day in and day out in order to determine which processed it the best. You would also need to make sure that they were equally active, in respect to their mass, over that time.
Such an organism would have to metabolise the food, then incorporate 100% of the resulting products into its body as structural material; even organisms that feed on simple sugars can’t do this. Besides, excretion is one of the defining characteristics of living organisms (by some definitions, at least).
Not all the content of feces is waste, in the sense of untapped potential. A substantial amount is indigestible material required for healthy digestion (i.e., fiber).
If you believe some of the posters in the Pit thread on fat children, anyone with a thyroid problem fits the bill :rolleyes:
Yet more of it is trashed body parts such as the remnants of dead blood cells. Still more is the remains of gut flora.
I have hypothyroidism so I guess I would qualify as being 100% efficient then 
I was actually thinking of measuring the ratio of each creature in it’s natural habitat eating it’s natural diet and going about it’s daily activities as normal rather than manufacturing a set of circumstances. If you fed all the creatures the same type of food then I would guess that those creatures that naturally eat that type of food would come out on top.
Not much help, but I seem to recall some desert critters that have very little defecation and virtually no urination.
There’s a parallel question about energy efficiency. IIRC, some plants, or plant processes, achieve a level unmatched by animals or man-made things. I want to say it’s 45%, but it’s been decades since I learned it.
My creaky old memory cells are whispering “85%” for that energy efficiency figure. I imagine 45% is not very remarkable.
I’d guess hummingbirds are a strong contender, since they mostly live on sugar water (nectar).
By the way, are we counting liquid waste (including sweat) as “waste”?
Don’t forget that some animals put their food through their systems twice. The first time the food is processed and comes out as waste that is then re-eaten and better absorbed.
Remember, too, that the bowels are not the primary excretory organs, nor are the kidneys. The primary excratory organ is the lungs. Well, for terrestrial vertibrates, at least, but for other critters, it’ll be the structure with analogous function.
I think, too, that I’ve heard of some insect species which don’t excrete (other than gasses) during their entire adult stage. They just accumulate it until they die (which isn’t very long). They do still poop as larvae, though.
Kangaroo rats never drink, as I recall. Their metabolism is high enough to require very little water, and the small amount that they get is from their food.