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For my story I have an alien species who are detrivore. They are also an intelligent species on par with humans. But many people have pointed out to me that intelligent detrivores don't work, because their diets don't support a large brain or a large body.

My question is therefore whether intelligent detrivores that are human sized could exist on a planet with the following characteristics:

  • super sized fauna being super common (for average size think titanosaur)
  • most fauna having super inefficient digestive systems so that their waste is rich in calories
  • detritus covering the floors of almost all the land on the planet
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    $\begingroup$ so... You want the megafauna to kind of 'cook' the plant life and then excrete it? $\endgroup$
    – Tim B II
    Apr 19, 2018 at 1:47
  • $\begingroup$ @TimBII not cook it just turn it into calories rich waste more or less $\endgroup$
    – Amoeba
    Apr 19, 2018 at 1:49
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    $\begingroup$ Cooking is the process of making food easier to digest and extract calories from, so it still kind of fits (I know this is an oversimplification, but it still works). To expand on this however, your fauna won't be able to add more calories into the food; they'll just concentrate the remaining calories from large amounts of consumed food into a single place and even then, your dentrivores aren't really dentrivores anymore, they're Corophages. $\endgroup$
    – Tim B II
    Apr 19, 2018 at 1:53
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    $\begingroup$ Understood. It's actually not a bad idea, but I think you'll find that the best possible way to maximise calorie intake has already been discovered by evolution; creatures that eat fresh vegetation and meat. $\endgroup$
    – Tim B II
    Apr 19, 2018 at 1:56
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    $\begingroup$ Actually usually the biggest animals are the ones feeding on low-energy food. So it makes sense to support a human-sized brain with a supersized body. $\endgroup$
    – Daniel
    Apr 19, 2018 at 13:03

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The answer is: Possible, but contrive

Let's face it, detrivore is not a good option for the kind of energy needed to support a large brain / body ratio. By the time they eat, their food have already pass through at least 2, probably more trophic level (plant => herbivore => carnivore (continue for more carnivores)). Combine that with the fact that what the detritivore eat is waste (ie. not as good as the organism itself, and laden with substances that is at least harmful to the excreting organism), there is not much left for them to pick over here. On the other hand, a big, well-developed nervous system is an important foundation for intelligence.

Enter the slime mold, eater of various organic scrap, and surprisingly intelligent (not human level though). They can solve mazes, choose healthy foods, and plan ahead of time (so at the very least, they are smarter than me :o). Currently still under study, so we don't quite know how they do those things yet, but I think being a collection of multi-nuclei cell with capability for cooperation as a colony play a big part in it.

Another thing you might want to know about intelligence is why the organism develop it in the first place. Plant can gain lots of energy from the sun, which means they only need to be smart enough to search for light, water, and nutrients and out-compete other plant for it, so they don't need much intelligence. On the other end of the spectrum, carnivore need to compete with both their prey, and other carnivores of the same trophic level or above it, so they tend to be more clever. The only main challenge for detrivore are environmental conditions and predators, so not a lot to go on here (even the slime molds above only become multicellular if the condition are harsh, which affect everyone). The good news is intelligence reward having more intelligence, so as long as you make the first step, things are looking up from there.

So sure, if the planet you are imagining is ridiculously abundant, but only in a few specific spots, and harsh in everywhere else, with energetic sun to support a lot of energy going through different energy level, but damaging in places without plantlife (not impossible), and somehow the carnivores don't develop intelligence first (that will be a problem), then yeah, you can have an intelligent detrivore that's as smart as human (their expression of it will be different though), but bigger in size, and probably composed of many less intelligent smaller organism that might bail if the condition become too plentiful though.

There is a simple, hand-wavey solution to those problem by just making the detritivore organism have extremely different metabolism than the rest of the biosphere, so that a normal, plentiful, temperate environment for everyone else is a harsh place with bountiful spots to them, and somehow their vital nutrients are indigestible to everything else. But then you have to explain why they are the odd one out in the entire biosphere.

Additional information: There is an inherent problem with the intelligent detrivore though, depend on their tech-level in your story. If they have advance enough to overcome other flora and fauna of you world, then there is a good chance that they won't stay detrivore, but develop agriculture and animal domestication, and eventually become omnivore (like us) instead. Therefore take their intended culture into account before you make the species.

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    $\begingroup$ I think you have a logical flaw in your answer: The carnivore does not eat the herbivores waste, but the herbivore itself, which is made of the good stuff found in plants - so if you will eating herbivores is a energy level up, from eating plants. $\endgroup$
    – Daniel
    Apr 19, 2018 at 12:53
  • $\begingroup$ @Daniel yes, but; the higher the trophic level the less dense the area is with the animal, so the less common is its poop. There will be km between polar bear scats, but you can't walk across a field goats have been in with clean shoes. $\endgroup$
    – user25818
    Apr 19, 2018 at 19:20
  • $\begingroup$ @Daniel, yeah, but it's kind of hard to express that the detrivore strategy is to glean a bit of energy (and the bad stuff, even) from a lot, if not all tropic level $\endgroup$
    – Nam Nguyen Hoang
    Apr 20, 2018 at 4:11
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If you stick with your idea of huge mega-fauna with vastly inefficient digestive systems to provide the nutrition then it might just be possible...

As mentioned in the other answers intelligence requires a reason to evolve and eating faeces doesn't exactly require a huge amount. But if you consider that most of the herbivores are colossal giants then it makes sense that carnivores may have a hard time feeding on them. Even more so if you make them somewhat toxic (let's say they feed off a plant that's toxic but they are immune to it and the toxins build up in their bodies). So your intelligent detrivore is specially evolved to maximise the now safe calorific content of these beasts waste, and as such are the perfect target for carnivores.

The detrivores then evolve intelligence to evade their predators.

I'm imagining they might be a long lived but less numerous species so you don't need quite so much waste to sustain the population and there is a drive for each individual to survive which leads to the development of intelligence. And if you say their digestive system has evolved to only be able to process the waste from your megafauna (or the rest of the possible food on the planet is highly toxic in some way) then they can stay detrivores past the point where intelligence might make them carnivores or omnivores.

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  • $\begingroup$ Great answer but one thing, how long lived do you think is likely? $\endgroup$
    – Amoeba
    Apr 21, 2018 at 2:28
  • $\begingroup$ @user45751 I'm not a biologist so this is just guessing, but I'm thinking something along the lines of a Galapagos tortoise so maybe 200 years? $\endgroup$
    – adaliabooks
    Apr 21, 2018 at 8:44
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Intelligence only evolves if it is needed. I can't really imagine detrivores needing intelligence to hunt down their food, and that's what intelligence is for. But intelligence is also easier to evolve if communication between a group of individuals is needed for daily survival, again, I cannot imagine they would need to utilize teamwork if their food just lay there "covering the floors of almost all the land on the planet". They would probably jut evolve a shell for defences and stick to the ground because they don't really need to work for their food.

So even if they have all the nutrition required to evolve a big brain, it would be much more efficient not to have one, which means intelligence in this species would not naturally evolve.

ETA months later: I'm surprised I haven't thought of this first but human diet also contains excrement. This food item is called "wine" or "beer" or, indeed, any fermented beverage or food. Although humans are not evolved to make much use of alcohol, it contains a lot of calories - as evidenced by its use as biofuel. The yeast or some sort of bacteria that consume sugars like yeast would need to live inside the bowels of your super-sized fauna and the droppings would need to be at least gelatinous for this to work. I'm no biologist though, so not sure how realistic this would be.

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  • $\begingroup$ That's why environment is so important. Yeah, if you have a homogenous nutritional environment, then intelligence won't evolve. On the other hand, an environment devoid of nutrient except for a few extremely fertile places will reward intelligence. Once you pass the threshold, intelligence reward having greater intellect, so it will be easier from there $\endgroup$
    – Nam Nguyen Hoang
    Apr 19, 2018 at 8:19
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While intelligent detrivores are not impossible, it may be improbable. One way I can think of to support your idea is to have the plant life or the fauna or both, excrete waste products that are actually highly beneficial to your detrivores and chemically alters their intelligence and consciousness over time to improve and expand it. Like magic mushrooms but increases your IQ, EQ etc. Hope this gives you a germ of an idea on how to proceed.

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  • $\begingroup$ good answer, might be better if you add that even then, the detritivores will need a massively different metabolism than the rest of the lifeforms, otherwise other organisms will utilise it first $\endgroup$
    – Nam Nguyen Hoang
    Apr 19, 2018 at 7:02
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    $\begingroup$ Yeah, the waste they eat could be too highly toxic to other life forms, but due to their specialized evolved digestive system and metabolism, only the detrivores can safely consume it. $\endgroup$
    – Arkhaine
    Apr 19, 2018 at 7:04
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The only way I can guess it can work is a particular combination of high solar energy and a full symbiotic cycle.

Too much light: This planet receives an awfull amount of energy and thus anything living under the sun must be sturdy to survive it. Plants are bold and with a rich soil and lots of umidity even the grass of this planet looks like a tree. Those big tree-grass are food for species of big scarabs like insects, they sawrn like a plague and eat like termites (maybe even using bacteria to digest de celulosi) but they life cycle is short. They eat a lot and die full off eggs but not before tranforming a big area of grass in bug-poo. The slime-mold-intelligent detrivore will come at night, maybe pouring from subterran to avoid the sun light. They will feast in the sugar rich bug-poo and bug-corpses but symbiotic nurture the bug eggs. After million years the slime mold start to become more smart and develops a efficient bug farming. They can start to develop grass plantations to feed the scarab-cattle. From farming follows all the traits of a cvilization.

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I've read that there are kinds of trees that harbor ants who protect the tree. In the extreme case the tree provides places for the ants to nest, and feeding stations.

Also there are trees that are "intentionally" hollow, to provide places for bats etc to live in relative safety, and the bats spending their days inside the tree naturally excrete there which provides minerals the tree needs.

You could combine those ideas, directly or not.

Giant herbivore. It nurtures a colony of intelligent human-size detrivores who protect it in various ways, and it feeds them. They can ride the herbivore and it gives them various kinds of protection also. Rabbits etc excrete their partially-digested food and eat it again. This herbivore could feed that -- or part of it -- to its commensals.

Giant plant. It harbors giant birds that bring it minerals. The birds need lots of energy to fly, and they strip out the easiest-to-digest parts from their food, and dump the rest inside the tree. The intelligent detrivores care for the plant and the birds (and might sometimes kill or drive off a bird that behaves wrong). They eat the food which the birds haven't digested, and perhaps ferment it further to get the digested cellulose etc. This doesn't seem completely plausible to me. I'm imagining flying elephants that carry lots of elephant shit home. But consider panda bears that eat lots of bamboo they can't digest well.... Still pandas don't have to fly carrying the excess weight. Something like this seems possible but not the default.

Now I'm imagining these creatures thinking about space travel, and the necessity to take their trees and elephant-birds with them....

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