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Biology/Efficiency in extracting energy/calories ranking in animal kingdom


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.

Hi H.P.L
This is an interesting question. We can apply a mathematical number to a machine but not to living organisms. We can state that one type of auto engine is more efficient then another but not that one animal is more efficient then another. That may be the case but we cannot attache a number to it.
 The 2nd Law of Thermodynamics (Entropy )states that with any conversion of energy from one form to another there is a loss of useful energy. Only about ten percent of food consumed by an animal is converted to energy. It takes ten Kg to make 1 Kg of Pig and ten Kg of pigs to make 1 Kg of me if I eat pigs. Therefore 90% of the chemical energy is lost. This is the 2nd Law
It is also a dictum that Nature abhors a waste of energy. This is why some animals eat the excrement. It is why most mammals after the birth of the young consume the placenta. I have observed Blue Jays swallowing the poop of the fledglings
There is no way we can establish an efficiency list about energy with respect to living organisms because all are subject to the Law of entropy. In other words the term inefficiency does not apply. Things are as they are. If an animal metabolism becomes deficient it does not survive.
I summary: An animal is as efficient in energy use as the 2nd law allows and depends upon the evolutionary development of the species.


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Walter Hintz


Science teacher for over 50 years. MSc. in biology. I can answer questions in general biology, zoology, botany, anatomy and physiology and biochemistry.


I have a MSc in biology and have been a science teacher for over 50 years. At present I am a faculty member at a college and a science consultant at seven catholic schools.

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