Being herbivores, specifically, folivores, rabbits depend on succulent green plants that are high in fiber. As with any other small animal, they have a higher metabolic rate. Also, their small size makes them sought for by most predators, and thus they need to be agile and outrun them. To survive, bunnies require high energy diets as foods with low energy will not sustain them.
For rabbits to be able to deal with these challenges, their digestion system is much evolved and better than gastric fermenters such as ruminants or colon fermenters such as horses. Here is how its digestive system is adapted:
- It permits high food intake (consequently high protein and energy foods intake).
- It sorts out fermentable and easily digestible foods. They have a hindgut fermentation
- It removes slow fermentable foods fast to avoid carrying them around for a longer time.
To deal with the problem of requiring a high absorption surface area in their colons, it separates feces from products for cecal fermentation. The latter often referred as cecotropes are re-ingested and absorbed in the small intestines.
We have noted that they need to remove slow fermentable foods fast. This is achieved with the help of the availability of a lot of these indigestible fibers. Therefore, “lack of this fiber is the most common cause of gastrointestinal disturbance in the rabbit.” Lack of fiber can also cause GI stasis which can further complicate into intestinal stasis, impaction or cecal stasis.
Finally, do not forget that some of the foods may be having microorganisms that are harmful such as bacteria as well as some toxins. Their digestive system must handle against them all.
Here are the main parts of a rabbit’s digestion system.
- Oral cavity
- Small intestines
- Large intestines – cecum and appendix
The oral cavity helps in ingestion, chewing and produces some essential salivary secretions or enzymes that aid in digestion.
Wild rabbits they eat mainly succulent young shoots using their incisors that are chisel-like. Their highly mobile and prehensile lips have sensitive vibrissae that help it locate food. In captivity, since their diets are mainly long-stemmed hay, pellets, and chopped vegetables, they incisors are not used much.
Once ingested, the premolars and molars will break down or grid the food to small pieces to enable swallowing. These cheek teeth are close to each, and the continuous tooth-tooth and tooth-food attrition together with coordinated jaw and tongue movement break down the food into smaller pieces.
However, when ingesting cecotropes, they do not masticate them. They swallow them while they are intact.
Finally, since plant materials can be abrasive and coarse, and the grinding can wear their teeth out, their teeth are adapted to grow continuously. On the positive side, the plant material also helps to stop overgrown teeth.
They have four main salivary gland pairs, i.e., the mandibular, sublingual, zygomatic and parotid glands. The mandibular glands produce galactosidase and amylase.
Besides aiding in digestion, these secretions also help in moistening the food and allow easy swallowing.
However, unlike humans and ruminants, rabbits there is a small amount of urea and lipase. Their saliva also has potassium and bicarbonate ions which are essential.
It has little digestive or no role and only transports food from the oral cavity into their stomach for further digestion.
The stomach, which is pouch-like in appearance has thin walls and accounts for almost 15% of the GI tract volume. On the entrance to the stomach, there is a cardiac sphincter which stops true vomiting. The stomach has the cardia, fundus, and pyloric (has muscular and thicker walls) areas all with gastric glands.
The cardia region of a rabbit has a thin wall, is immobile and non-glandular. It is the bunny movement as well as colon movement that makes food churning possible at this part.
A rabbit’s stomach is hardly empty even after a long day fast (24hours) it will be half full. You will find a mass of food and some ingested fur or hairballs. Hairballs reduce gastric movement and are therefore unwanted. They can also cause a blockage if your furry friend swallows so much of the hair.
The fundus secrets pepsinogen which is a precursor for pepsin and the parietal cells secrete hydrochloric acid that kills bacteria and create a sterile condition while pepsin helps in protein breakdown. On the other hand, mucus is also secreted to help protect the stomach lining against the hydrochloric acid.
While still suckling, the gastric pH will be much higher, about 5 to 6.5 and the ingested milk become semi-solid while in the stomach and slowly passes to the ileum in about 23.5 hours. The rennin-like enzymes help the milk to curd.
The mother’s ‘milk oil’ or ‘stomach oil’ which is an antimicrobial fatty-acid help combat bacterial activities. Also, the kits have a maternal antibody that is passive, and it is obtained via first milk and through the placenta. These two makes GI to nearly in a sterile state.
Until they are ten days, kits depend on their mother’s milk. After that, they can be taking their mother’s cecotropes besides the milk. Cecotrophs have a mucous coating that keeps the bacterial intact until they pass to the stomach.
At day 20 they begin taking solid foods, and cecotrophy, and after 30 days, the milk intake usually reduces, their cecotrophy is now fully developed. After that, the ‘milk oil’ production decreases, and their gastric pH falls to 1-2.
During the weaning period, intestinal problems such as coliform infections, diarrhea from rotaviruses, and mucoid enteropathy syndrome are common immediately after weaning.
Postweaning or adult rabbits
The pH is much lower, about 1-2 due to acid secreted from the fundic area. There is also a lot of water secretion. The pH destroys any microbial organisms, and the pepsin-hydrochloric complex begins the hydrolysis of proteins. Food takes about 3 to 6 hours to pass through the stomach.
However, cecotropes are not digested since as we mentioned, they do not get masticated in the mouth, and their mucinous coat protects cecal material them from this low pH conditions for about 6-8 hours.
Since fermentation continues in the cecotropes, lactic acid is produced, and it will consequently reduce the pH of the stomach to about three due of the buffering effect of the lactate, a conjugate base of the lactic acid.
From the stomach, the digestion mixture often known as chyme or digesta enters the duodenum part of the small intestines via the pyloric sphincter. Much of the digestion and nutrients absorption occurs on this part of their gut.
The small intestines are divided into two sections, the duodenum, and ileum. At the beginning of the duodenum, there is the bile duct while towards its ascending area there is there is the pancreatic duct. There may be other minor pancreas duct opening into the ileum as some sources state.
The pancreas releases proteolytic enzymes including trypsin, carboxypeptidases, and chymotrypsin into the intestinal lumen and they work together with intestinal aminopeptidases to complete the digestion of proteins into peptides and amino acids.
Also, the pancreas produces various kinds lipases that help in fatty acids and glycerol digestion. These lipases work with the help of bile enzyme. It also releases bicarbonate ions that will neutralize the hydrochloric acid in the chyme from the stomach.
Bile acids are drained into the duodenum immediately after pylorus via the cystic duct and the microbial activities convert it into deoxycholic acid. This bile acid helps in the breakdown of fats into small absorbable micelles. This also allows the absorption of fat-soluble vitamins.
Not that bile also has Biliverdin which is from various products breakdown including hemoglobin.
Finally, there is also the amylolytic secretion enzymes which will play an important role digesting some starches.
Small intestine motility
Motility is stimulated by motilin secreted in the duodenum and jejunum, and the presence of fats in the intestinal content stimulates it while carbohydrates inhibit it. The macrolide antibiotics also play a role as motilin receptor agonists.
Motility is achieved through the help of various process. The first one is segmentation that occurs in duodenum involving the mixing of the different contents in the small intestines during a static constriction that arises on the intestinal walls.
The second process is peristalsis where there is a ring of contractions that occur along their length towards the colons. Gastrointestinal peptides and hormones aid in this movement. In the jejunum, they occur after 10-20 minutes while in the ileum after 30-60 minutes.
At the distal end of ileum, on the upper side of the large intestines, there is the is the sacculus rotundus only found in lagomorphs. This is where most obstructions occur. There is an ‘ileocecal valve’ between sacculus rotundus and the ileum that stops backflow of fluids into the ileum. The valve redirects the chyme into the cecum via sacculus rotundus.
Small intestine absorption
After the chyme is neutralized chyme by bicarbonates in the duodenum, most of the simple proteins and carbohydrates will have been digested in the duodenum and jejunum into monosaccharides and amino acids before they get absorbed on the jejunal brush border.
Cecotroph materials are also digested duodenum and jejunum (between duodenum and ileum) and absorbed at jejunal brush border. Some of the nutrients absorbed include vitamins, amino acids, digested microbial organisms, and volatile fatty acids.
The lysozyme added in the cecotropes aids as digesta pass through the colon in the digesting cecotropes microbial proteins. The lysis that occurs and microbes in the cecotropes release enzymes including amylase which helps the rabbit with further starch digestion.
Finally, the rabbit’s ileum helps in recycling and regulating some of the electrolytes that the stomach secrets. This is achieved by bicarbonate ions reabsorption.
Large intestines comprise the cecum, appendix, and colon. Their cecum is proportionally more significant compared to that of any other mammal.
Cecum and appendix
The cecum is double the length of the abdominal cavity and accounts for about 40 to 60 percent of the GI tract. This blind sac or pouch has folds into four gyri with the first three having translucent walls while the final fold forms the vermiform appendix.
The cecum is located at the place where the small and large intestines meet, and this is where the rabbit sorts indigestible material with those that can be digested. The indigestible ones pass to the colon to form feces.
The appendix secrets bicarbonate ion utilized in the cecal lumen to help in buffering volatile fatty acids. Rabbit’s whose diets have high fermentable low fiber carbohydrates tend to have a more massive appendix.
This larger appendix is due to the needed more secretions from the appendix to offset the various products formed due to increase fermentation of carbohydrates. The other explanation of why it is bigger than rabbits that do not have such diets is the more need for lymphoid tissue since such bunnies have altered populations of microorganisms.
To make the cecotropes soft, water, which is secreted from colon and appendix is added to the cecal content, and some of it is absorbed across the wall of the cecal to make these contents consistent soft.
Inside the cecum, anaerobic fermentation place for various organisms including Bacteroides spp., and besides the food ingested, the mucosa goblet cells secrete mucopolysaccharides which act as a carbohydrate source for cecal fermentation. There are many other bacterial species, and nonpathogenic protozoa often found on the cecal content.
The various microbial flora breaks down ammonia, urea, enzymes, and proteins coming from small intestines as well as cellulose. They can metabolize pectin and olize xylan to give enzyme structures of these microorganisms as well as proteins, (which will later be digested as cecotropes) and volatile fatty acids ((VFAs) such as formic, propionic, butyric and acetic acids.
Some nutrients including the VFAs are later be absorbed across the colon and cecal epithelium and used as a source of energy. However, some nutrients including some amino acids, vitamins as well as minerals are passed out as cecotropes.
On how fiber affects GI motility, an increase in fiber in bunny’s diets and fasting increases acetic acid which will alter the amount of butyric acid that inhibits peristalsis and hence you expect fiber to promote GI motility.
Colon and fusus coli
It has the ascending colon which is long, divided into various parts which have taeniae and haustra but the fusus coli (muscular thickening) has not haustra. At the end of this section, you will be able first to identify fecal pellets. After the ascending colon, you go into the transverse colon which is short and then finally to the descending colon.
However, it is the fusus coli that separates that colon into parts that have different physiological and anatomical parts, i.e., the proximal and distal colon. The fusus coli acts as a differential pacemaker for the start of peristaltic waves in these two parts of the large intestines (proximal and distal).
The fusus coli has other roles including regulating separation of large indigestible fiber from the fermentable ones via the contractions of the haustrae/ taeniae among many other functions. After the fusus coli, the distal colon and rectum become tubular and have a thicker wall.
The goblet cells in fusus generate mucus which is used to coat cecotropes and fusus does not expel most fluids from them (through mild contractions) making them remain soft whereas it removes most water from the fecal pellets making them dry through stronger contractions.
The distal colon absorbs water, electrolyte and volatile fatty acids forming fecal pellets while for cecotropes, it adds lysozyme.
Since most of the various nutrient’s absorption occurs in the small intestines, some of the nutrients formed during fermentation cannot adequately be absorbed. Instead, they will be released as cecotrophs which the bunny ingests to give the small intestines a chance to absorb these nutrients.
Cecotropes are not feces although some sources call them ‘soft feces’ and they are supposed to be ingested directly from the rabbit’s rectum, an act that is triggered by neurological licking and they are ingested without being chewed.
Cecotrophy often occurs eight hours after your rabbits and will be high when your furry friends eat foods that are high in nondigestible fiber.
Hard and soft feces phases
One of the essential steps in digestion in large intestines is the cecal and colonic motility that allows a rabbit to separate contents it has into fermentable substrates and indigestible wastes.
Water secreted from the proximal colon plays a vital role in mixing and separating the intestinal content, and the fusus coli controls various types of contractions that separate the indigestible pellets with large indigestible fibers into feces and digestible to cecotropes.