Most animals can’t digest fiber well, and it travels through them without much happening. Rabbits, however, have this nifty way of breaking down fibrous plant material and getting to all the nutrition stored inside.
In the rabbit GI system, food goes into the mouth, down the throat, through the stomach, through the intestines, then the colon and out the other end, just like it does for most mammals. But...the enzymes present during that process can’t break down fiber well at all. So rabbits divert much of the fiber to the caecum. The caecum is where the extra steps in digestion start. In the caecum, there are special bacteria that ferment and break down digestible fiber, getting all the nutrients locked inside. Some of the nutrients can be digested right there in the caecum, but most need another pass down the small intestines for absorption, so the food has to pass through your rabbit a second time.
How does this second trip through the system work? The nutrients needing another pass move from the caecum back into the colon, and exit your bun as caecotrophs. These are larger, softer, and usually a slightly different color than the other litter box contents (they are usually together in a cluster, like a little bunch of grapes), and are a very important part of your rabbit’s nutrition. Your rabbit eats these caecotrophs, giving the GI tract another chance to absorb all nutrients that are now all broken down and ready for use. Herbivores get so much more out of their food this way! And since they already spend about 75% of their waking hours eating, getting all they can our of their food is essential. They simply couldn’t eat enough to get all the necessary nutrients if it weren’t for this cool caecum/bacteria thing they’ve got going on.
All of this has to happen at a particular rate, in order to keep things working properly. If there is a lack of fiber slowing down the movement rate through the system, the bacteria in the caecum can get out of balance. Fur your rabbit has swallowed during grooming can begin to form a blockage. Gas can build up and cause pain and bloat. All kinds of bad things happen when fiber isn’t moving through your little friend at the right speed.
So big shout of appreciation for lots of hay! Let’s keep it moving, everyone.