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I thought of this question yesterday and it turns out it's surprisingly hard to Google.

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What kind of waste are you thinking about? Waste as form of undigestable food or broken down protein etc.? –  Chris Mar 25 at 12:43
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Technically, if it's still being recycled internally it isn't waste yet... –  keshlam Mar 25 at 19:07
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This makes me remember a question: why virtually all animals need an excretor system to eliminate nitrogen? Why can't we recycle it, since it's so important for protein building? –  Rodrigo Mar 25 at 21:54
    
@Rodrigo That's an interesting question ! Why don't you create a new post ? –  biogirl Mar 26 at 3:56
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Well, I would, but it's already asked here: biology.stackexchange.com/questions/10737/… –  Rodrigo Mar 26 at 13:01
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Hibernating bears have an ingeneous way of recycling their urine (urea) while they hibernate. Also turtles and frogs in the bottom of ponds deal in unique ways during hibernation.. This should get you started. recycling waste externally is done by rabbits who will pass pellets through their system twice, the firsttime to partially digest, the second time to extract nutrients.

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All organisms recycle their waste internally. Every cell of every living organisms is constantly breaking things down and re-using the components so produced. But you're presumably wondering about things such as carbon dioxide, urine and faeces? These are not recycled because the benefits of doing so are not worth the costs.

Let's consider carbon dioxide as an example. We know that carbon dioxide can be usefully converted into energy-holding molecules using readily available sources of water, oxygen and sunlight. So why do animals** "wastefully" breathe it out? Because the energetic benefits of respiration are high compared to the energetic benefits of photosynthesis and so photosynthesis is unable to make a useful contribution to the overall energy balance of an animal while the costs of the effectively photosynthesising are quite high (e.g. having leaf-like protrusions, synthesizing chlorophyll and so on).

In fact, it must always be true that there are processes which are not energetically worthwhile because of entropy. An organism that could perpetually recycle its waste would be a perpetual motion machine.

** - and many microbes and, in fact, plants - although their intake of carbon dioxide is typically higher over the solar cycle.

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"An organism that could perpetually recycle its waste would be a perpetual motion machine." Hm. Not quite, since there can still be an (almost) non-material energy flow through the system. A little eco system in a closed bottle or a big one like on earth could be considered a single organism which recycles it's "waste" (which immediately stops being waste, as others pointed out). –  Peter Schneider Mar 25 at 23:50
    
@Peter Schneider This only appears that way because there are some 173PW of free energy falling in from a nearby thermonuclear reactor. The moment the energy budget becomes more restrictive, complete recycling of materials suddenly loses its appeal (applies to any system, be it natural or artificial). And your "bottle ecosystem" example would not stand at all - isolated ecosystems tend to lose complexity and accumulate "non recyclable" byproducts. –  oakad Mar 26 at 0:33
    
@oakad The energy flow from the sun was exactly my point, so we agree there. With respect to degradation of isolated ecosystems: do you have pointers to research about that? Sounds plausible but I couldn't immediately find anything. I also could live with a certain degree of degradation -- hell, I have to, considering ;-). Earth itself seems to do ok so far, and this man's bottle has been unopened for 40 years unless he's blatantly lying: sciencethat.com/?p=273 –  Peter Schneider Mar 26 at 0:51
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If an organism recycles some material, by definition, it is not waste.

Though some animals will eat feces.(For eg.Rabbits) It's a way to double-digest something that was not fully broken down or absorbed the first round.

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