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What evolutionary pressures would result in metabolic IN-efficiency being a fitness-increasing adaptation in an animal?

By "metabolic efficiency", I mean "percentage of usable energy extracted from ingested energy-containing substances", not "how much energy does this animal use at rest". It's not about how much it uses overall, it's about how much it can extract without wasting. As such "metabolic inefficiency" would mean that the animal extracts little of the energy in the food it eats. For a benchmark of "metabolic inefficiency", let's say that this animal is capable of utilizing, at most, 10% of the energy in the food it ingests.

By "evolutionary pressure", I mean "a reason an animal would evolve such an aspect of its biology".

By "fitness", I mean "the animal's ability to survive long enough to produce offspring".

Generally speaking, evolutionary pressure leads to more metabolic efficiency, not less metabolic efficiency; after all, not having to spend as much time and energy eating means that an animal is more evolutionary fit in other areas.

Note that Kleiber's law is a thing - the idea that basal metabolic rate scales to the 3/4 power of an animal's body mass. In other words, it states that an animal of mass 100M consumes about 32 times more energy than an animal of mass M, with the relevant equation being

[energy used] = roughly [mass^0.75]

However, this is less of an evolutionary adaption and more of a natural constant that appears to have something to do with heat dissipation, efficiency of nutrient distribution, ratio of structural mass to growth mass, or other such reasons (no individual reason has been actually found that fully fits Kleiber's law); nor is it an evolutionary pressure. As such, please do not cite Kleiber's law as an answer.

Additionally, "the animal is small" or "the animal is big" or "the animal is slow-moving" are not properties related to how efficient this animal is at extracting energy from its food, nor are they evolutionary pressures. They might be reasons that an animal uses a lot of energy or not a lot of energy, but they're not related to why the animal can't extract much energy from its food. As such, please do not use the animal's size or activity rate as answers.

Good answers will explain a reason or reasons why an animal would be more evolutionarily fit if it was less metabolically efficient.

The best answers will cite evolutionary pressures that could actually exist on Earth, or a planet relatively similar to Earth in terms of atmospheric pressure and composition, type of life (carbon-based, of course), gravity, composition, and the like - no super-Earths, no living in a gas giant's atmosphere, etc.

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    $\begingroup$ The giant panda Ailuropoda melanoleuca and the red panda Ailurus fulgens are two well-known examples of carnivorans which evolved to eat vegetable food, thus going from very high efficiency at extracting energy from their food to very low efficiency. (And moreover they are not closely related; one is in the bear family Ursidae and the other is in the weasel superfamily Musteloidea.) Their evolutionary histories may be of interest. $\endgroup$
    – AlexP
    Dec 11, 2021 at 22:39
  • $\begingroup$ @AlexP Well, how is it evolutionarily advantageous for them to do that? $\endgroup$
    – KEY_ABRADE
    Dec 11, 2021 at 22:41
  • $\begingroup$ I don't know. But somebody who is interested in such a process should do some reading on their evolutionary histories. $\endgroup$
    – AlexP
    Dec 11, 2021 at 22:44
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    $\begingroup$ Inefficacy is a definitional proposition, and is founded on how you define what efficiency is, so how are you defining efficient? How are you defining a metabolic inefficiency? also size is absolutely related to how effectively you can extract energy, it is a major factor in the length of the gut which limits digestion. Endothermy is a massive metabolic inefficiency, it evolved because the cost of greatly increased food demand is more than offset by greatly increased activity level. $\endgroup$
    – John
    Dec 12, 2021 at 4:41
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    $\begingroup$ Then please ask that. that is a an answerable worldbuilding question, as it stands this is not. there are many many ways to do that many probably better than relying on metabolics. many herbivores need to eat more or less continuously, discounting sleep. $\endgroup$
    – John
    Dec 12, 2021 at 5:12

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Energy goes to feed microbial commensals in the gut.

The microbes get the energy. In return they synthesize vitamins and protein that the host cannot, and those are passed to the host. The host uses its food gathering and chewing ability to support the microbes that live inside of it. In return those microbes support their host with their metabolic and synthetic abilities.

This is not fiction.

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Literally all of digestion.

All of digestion is a trade off being good at extracting somethings means being bad at extracting other things. Lets pick an example cellulose.

You can't digest cellulose, a high energy molecule, because the machinery (enzyme production, larger digestive tract, ect) need to digest it is highly costly and not enough of an advantage to compensate for the that standing cost of supporting that machinery. All of digestion is a trade off, go for low digestive cost but high behavioral cost or low behavioral cost and high digestive cost. There is also a big trade off between time to extract vs preventing the ingestion of more food. If we could digest cellulose doing so would prevent us from filling our gut with easier to digest material we are adapted to find.

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  • $\begingroup$ Similarly, cooking: The process of outsourcing part of digestion to tools and other people so you can get by with a less-efficient, less-resource-intensive digestive system. $\endgroup$
    – notovny
    Dec 13, 2021 at 21:23
  • $\begingroup$ @notovny or use your existing system to gain twice the nutritional value like humans. $\endgroup$
    – John
    Dec 13, 2021 at 21:36
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Poisonous Ecosystem

In your hypothetical system, every potential food source is toxic to other lifeforms.

There are evolutionary workarounds for this but in this ecosystem they all increase metabolic inefficiency or have other negative side effects (similar to the suppression of the immune system when regularly processing alcohol).

Therefore the majority of organisms adopt a cautious digestive strategy whereby they vomit or otherwise expel anything overly toxic and intensively digest everything else to ensure it is safe. This is highly energy intensive and so reduces efficiency but is preferable to the alternative of regular fatal poisoning.

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Inefficiency is relative to the environment. Take for example the people with a high level of pigmentation in their skin. Their pigmentation makes so that of all the sunlight, only a very little passes the skin and helps producing vitamin D. Due to the high level of sunlight in their places of origin, this inefficiency is functional, and not really an inefficiency. When they live in countries like Northern Europe, with a low level of exposure to sun light, the inefficiency becomes visible.

The same motivation can exist in your case: when there is plenty of food that can be eaten with low effort, using only a small fraction of it is not a problem.

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For the Next Generation.

The needs of the very young, once freshly weaned are about re-populating their guts with bacteria suited to their future diet, and plenty of energy and nutrients to grow. They follow the adults around when they graze, hoping to be passed-on presents of pre-digested food and healthy gut-bacteria. (This necessitates lots of energy being put into the digestion of the food, but without the energy and goodness being drained from it).

If you're thinking "yuck, no way" at this point, think on the cuddly and cute Koala which feeds the weaning joey faecal-pap, a pre-digested, bacteria-rich meal in order to wean it on to plant-based food.

Rabbits are similarly coprophageous, termites pass-on their protozoan passengers enabling cellulose digestion, even baby elephants eat their parent's pap.

This has the knock-on effect of enabling the young to accumulate body-bulk and grow faster than without this pre-digestion, thus making them stronger and more likely to pass-on their genes.

Perhaps, rather than the whole group, a particular subset - say the older males might be relegated to the duty of feeding the infants in this way, leaving other adult members of the group strong-enough to protect it.

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