# Why can't animals digest cellulose?

**URL:** <https://boards.straightdope.com/t/why-cant-animals-digest-cellulose/48768>\
**Category:** Factual Questions\
**Created:** [January 6, 2001, 10:28pm UTC](https://boards.straightdope.com/t/why-cant-animals-digest-cellulose/48768 "2001-01-06T22:28:07Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Bob\_Scene](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bob\_Scene](https://boards.straightdope.com/u/Bob_Scene)\
**Post date:** [January 6, 2001, 10:28pm UTC](https://boards.straightdope.com/t/why-cant-animals-digest-cellulose/48768/1 "2001-01-06T22:28:07Z")

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As far as I know, all the animals that can digest cellulose (like cows and termites) have to do it with the help of symbiotic bacteria that produce enzymes that break down the cellulose. This is surprising because cellulose is just an isomer of starch that has the glucose monomers in a slightly different configuration. Cellulose is one of the most abundant biochemicals in the world, has a huge amount of energy in it, and it seems that it would be a great food resource for anything that could eat it. So how come humans and other animals never evolved an enzyme that could break it down? Also, how come cellulose digestion produces methane and starch digestion doesn’t?

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**Author:** ![Shiva](https://avatars.discourse-cdn.com/v4/letter/s/85e7bf/32.png) [@Shiva](https://boards.straightdope.com/u/Shiva)\
**Post date:** [January 7, 2001, 2:42am UTC](https://boards.straightdope.com/t/why-cant-animals-digest-cellulose/48768/2 "2001-01-07T02:42:18Z")

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Good question.

According to this site: [http://www.furman.edu/~dhaney/feed.htm](http://www.furman.edu/~dhaney/feed.htm)  
There _are_ a few animals that can digest cellulose.

> [@](#):
>
> Very few animals have developed a true cellulase enzyme. It is seen in a few animals - e.g., longhorn beetles, shipworms, silverfish, etc.

Going to look for cellulase info. BBL maybe.

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**Author:** ![dasmoocher](https://avatars.discourse-cdn.com/v4/letter/d/dbc845/32.png) [@dasmoocher](https://boards.straightdope.com/u/dasmoocher)\
**Post date:** [January 7, 2001, 4:07am UTC](https://boards.straightdope.com/t/why-cant-animals-digest-cellulose/48768/3 "2001-01-07T04:07:02Z")

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Off the top of my head, I would imagine that most animals have evolved to eat or be symbiotic with organisms that can break down cellulose. Why spend the energy making an enzyme yourself when you can take advantage of organisms that have done it for you.

As to methane production, this (IIRC) is a end product of an anaerobic metabolic pathway which is characteristic of the microorganisms that can utilize cellulose. Animals metabolize glucose in starch aerobically.

The linkage between the glucose monomers is key: alpha (1-\>4) vs. beta(1-\>4) (or beta (1-\>6), it’s been awhile, that may be the branching linkage of starch).

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**Author:** ![Bob\_Scene](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bob\_Scene](https://boards.straightdope.com/u/Bob_Scene)\
**Post date:** [January 7, 2001, 10:36am UTC](https://boards.straightdope.com/t/why-cant-animals-digest-cellulose/48768/4 "2001-01-07T10:36:13Z")

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Thanks, Shiva! I had no idea that all the silverfish running around in my book-filled apartment had their own cellulase! Now I’m wondering if they are expelling dangerous quantities of methane. Could I save the planet from global warming just by fumigating the place?