Re: Which Animals Can Digest Cellulose?

Thank you for your question? Its a quite complex one and my answer will be 
only very limited.

Which animals can digest cellulose?
In short: all that eat mainly cellulose. So cow, deer, elephant, also some ants 
and termites ... All need bacteria or fungi to do this. Their own capabilities 
in digesting cellulose are very limited. For some termites I know, that they are 
able to survive quite a while without symbiontic bacteria.

Isn't it simply waste of energy, for those that cant do it?
It is not that easy, it is a sort of balance.
You have to inhabit baceria in your body (or in the case of some of the 
termites and ants to keep a fungus garden). For this you have to have 
special structures within your digestve system. Building up such structures
and keeping such strucures consumes lots of energy. You always have to keep 
the bacteria and fungy under control, that they only digest what they should digest, 
and not your own body.

Additionally: optimal milieu for cellulose digestion means far suboptimal
milieu for protein digestion. You loose lots of energy if you try to digest
proteins in your "cellulose optimized" stomach.
See following page on ruminants .
text_link.htmlblue underlined text for some details, references and an address
to ask if you want to know more.

Finally WHY?
This question can not be answered scientifically. Evolution does not know 
any reasons. Its simply and only by chance. No reason for any structure,
no reason for any behaviour, no reason for life itself, just statistics and 
chances.
Of course, if you believe in a God, who created us all, then God has 
planned it this way and therefore it is this way.

For a few more links see a former answer to a very similar question
What makes a cow produce methane gas in it system. 

Viele Gruesse und bis zur naechsten Frage
Jurgen Ziesmann