I would like to know either (a) what animal(s) is/are most efficient in extracting energy or calories from food, (b) how one would go about finding an answer to that question, (c) the terminology one would use to search the literature for an answer to the question.
Regarding (a), I've been considering cows or camels at least somewhat inefficient because their feces and be dried and burned (hence the animal does not convert all the energy in its food to energy for its own use); it is likewise with whales, the evidence being that other creatures feed on their excrement. I assume all animals have guts inefficient to some extent but would like to know how animals rank.
Regarding (b), my guesses include subtracting calories in excrement from calories in food or, perhaps, comparing weight of food and weight of excrement (as I have seen done with coproliths).
Regarding (c), as I have never studied any field of biology, I do not know what terms would be applicable ("metabolic efficiency"?) or even what specialty would be relevant to such a question.
Study Food Chain.