Re: How can rabbits eat only plants?

Rabbits are herbivores and have the same type of gastrointestinal system 
as horses do.  They have a very well developed cecum (the equivalent to 
your appendix but it's gigantic).  The cecum contains bacteria which break 
down (by a process called fermentation) the cellulose fiber found only in 
plants (as opposed to animal cells).  The large intestine absorbs much of 
the nutrients broken down by the bacteria.  However, because this 
digestive stomach is located further down the gastrointestinal track than 
the actual stomach and the small intestine where crucial amino acids are 
absorbed, the amino acids are excreted by default.  Because nature doesn't 
like to waste things, both rabbits and horses exhibit a behavior called 
coprophagy, meaning "excrement eating." They will eat their own excrements 
to catch the critical amino acids that bypassed the site of absorption on 
their first run through the digestive track.  On the second passing-
through, these amino acids are "caught" in the small intestine where they 
are absorbed.
Why don't rabbits and horses just eat meat to get these essential amino 
acids?  In fact, rabbits can and do. Rabbits are lagomomorphs, not 
rodents.  But like their rodent cousins, they exhibit cannibalistic 
behavior when their babies'nest is disturbed by a predator.  Again, nature 
selected for this type of behavior.  Perhaps because of their large 
incisives, rabbits cannot pick up their young by the scruff of the neck 
(unlike dogs and cats). When a predator comes, the mother has to save 
herself. She cannot pick up her babies and  go.  Rather than letting her 
babies be eaten by a predator, she will eat them herself and save for 
herself the fruits of her labor (28 days of gestation to produce a litter 
of 5-8 babies).  
So while rabbits can digest proteins quite well, they resort to acquiring 
their nutrition from plants because their bodies are better designed to 
absorb nutrients from fermented plant products than from a meat source.