Before we talk about foods, we have to understand how bunny’s guts work. So, let’s get into it!
Rabbits are herbivores, which means they only eat plants. That means their guts evolved to be able to ingest huge quantities of poorly-digestible fibre.
Their teeth are what is known as hypsodont, and open-rooted which means they have a long crown, rather than a root. Teeth are worn down at the occlusal surface during chewing – bunnies chew 120 times a minute.
Occlusal is the position of teeth when the jaw is closed.
That’s a good thing, because their teeth grow continuously – their maxillary incisors grow at 2mm a week, while their mandibular incisors grow at 2.4mm a week.
A bunny’s gastrointestinal tract takes up 20% of their bodyweight, which has a monogastric stomach that contains 15% of their digesting food at any one time. Their caecum holds 40% of their digesting food and is the largest abdominal organ. Their colon is separated into two parts, distal and proximal, by the fusus coil – which is the thick section that has ganglionic cells and acts as a pacemaker controlling the contractions when they poop (that’s why they make spherical poops!).
Rabbits are classified as hind-gut fermentors, which put simply means they have one stomach that does all the hard work. The microorganisms in the gut make the fermentation occur within the caecum and colon.
They do not completely ferment fibre, their gut separates indigestible fibre and expels it as fast as possible. The sorting mechanism that controls this is in the large intestine. Indigestible fiber is usually pooped out within 4 hours of consumption. The rest is passed backwards into the caecum by muscles that form wave-like contractions, this is known as reverse peristaltic action. You can actually see/feel this occurring when you gently hold your bunny and watch/feel its tummy.
Once in the caecum, the fluid is fermented further by microflora which extracts proteins, amino acids and volatile fatty acids. These particles are smaller than 0.3mm and are digestible/”fermentable”.
Rabbits selectively poop fibre, then re-eat the poop to get the vitamin B out of it. These poops are usually a soft and squishy a grape like cluster surrounded by a gelatinous membrane. They are called caecotrophs. Bunnies usually eat it directly from their anus, but they can be left in the litter box too. A caecotroph is formed and expelled roughly 8 hours after consumption.
Coprophagy/caecotrophy is part of a bunny’s circadian rhythm. Caecotrophs are produced in the early hours of the morning. A strong odor of volatile fatty acids stimulates the bunny to consume it directly from the anus. Once ingested, the caecotrophs remain intact for 6 hours, until their protective mucus layer has dissolved. Bacterial synthesis occurs at this time. Caecotrophs contain high levels of B and K vitamins, as well as twice the protein and half the fibre of hard poops. Redigestion of them helps the gut to absorb previously undigested nutrients because it helps them keep up their high food intake (65-80g/kg bodyweight) and rapid food digestion time (19 hours) allowing rabbits to meet their nutritional requirements.
If bunnies don’t eat their caecotrophs, that means they aren’t consuming enough fibre.
The gut microflora in the large and small intestines produce volatile fatty acids, very similar to how cows do in their rumen. These volatile fatty acids provide limited energy for digestion. The microflora are very sensitive to antibiotics and often leads to an increase of toxins, diarrhoea and enterotoxaemia. That’s why it’s good to give bunnies probiotics after they have been on antibiotics. 80% of rabbits who receive probiotics do not get gut upsets and diarrhoea after antibiotics.
If bunnies eat something that doesn’t agree with them, they can’t vomit, due to their well-developed cardiac sphincter.
The Stomach
A bunny’s stomach is a very important compartment int he digestive system, it has a very weak muscular layer and is always partially filled. Once a caecotroph has been ingested, the fundic region acts as a storage cavity for the caecotroph.
The stomach is continually secreting and the pH is acid which ranges from 1 to 5, depending on whether there is the presence or absence of soft faeces. When the pH is at its lowest, from 1 to 2.5, it shows that the bunny last ate 4+ hours ago and the faeces are limited to none.
The capacity of the stomach is about 0.34 of the total capacity of the entire digestive system.
The stomach is linked with a coiled caecum by a small intestine approximately 3cm long, where the secretion of bile, digestive enzymes and buffers occurs.
The Small Intestine
The pH of the small intestine is close to 7.
It is the site where the majority of the digestion and absorption takes place by transporting food through the mucosa. Once food gets to the ileum, it accounts for 0.8-1 of the total digested amino acid and starch.
The Caecum
The caecum has a weak muscular layer and has only dry matter in it. The contents are slightly acid with a pH of 5.4-6.8. The ceacum is roughly 0.49 of the total capacity of the digestion system.
The population of microbes in the caecum gives the rabbit extra energy, amino aids and vitamins.
The Colon
The colon can be divided into two portions:
- the proximal colon; about 35cm long
- the distal colon; about 80-100cm long
The proximal colon can be further divided into three segments: 1. has three taeniae with haustra between them 2. has a single taenia covering half of the circumference of the digestive tube 3. is called the fusus coli, has no taeniae or haustra, but is very dense. It acts as a pacemaker in the formation in the hard faeces formation
Microbial Community
The entire community of microbes in the digestive process were first classified in 1973. They were studied again in 2003 using molecular techniques, and it was found that the gut was much more complex and there were many more different genus of microbes than first thought. They calculated that only about 40% of microbes were known.
Bacteroides is the main genus of microbes in an adult rabbits caecum. There are usually about 109 – 1010 bacteria – which is 10 with 9 zeros, or 10 with 10 zeros after it. That’s a lot of microbes!
Bifidobacerium, Clostridium, Strepococcus and Enterobacter are also found in the caecum, which increase the count to 1010 – 1012 bacteria.
In young rabbits (18 days old), the microbes importance was far greater in digesting the milk then when bunnies start eating solid food. The molecular studies gave scientists the understanding of the vital importance of the microbes being passed from mother to baby in the milk, and how the nutrients helped fight against digestive diseases. Scientists have concluded that there needs to be much more study into how the bacterial community in the digestive tract functions.
As for poops, the main microbial component is Firmicutes and Bacteroidetes, with a very low concentration of Fibrobacteres which is the group of bacteria that is specific to the fermentation of fibre. When a bunny has consumed a large amount of fibre, there is a greater amount of Bacteroidetes in the gut.
Caecotrophy
A scientific study of rabbits showed that they produce caecotrophs mainly during the morning and day time, while they are active. Most rabbits produced caecotrophs between 08:00 and 17:00, with a maximum at midday (12:00).
Some rabbits in the study were given a reduced amount of day light. These bunnies produced a second round of caecotrophs during the night.
When bunnies are in the caecotrophy period, which can last between 7-9 hours, they don’t produce hard poops and they don’t eat much.
That means, bunnies don’t eat much through the day, but they eat more at night.
Usually bunnies eat an increasing amount between 15:00 and 18:00, and remains high until 02:00. After this time bunnies reduce their food intake when they stop eating at 06:00, and it finishes at 08:00. Hard poops occur between 18:00 – 08:00.
Of course age and health affects this rhythm. So does the amount of food available. The rabbits in the study were given as much food as they wanted. Other similar studies gave bunnies food once or twice a day, and this affects the changes in poops and caecotrophs.
A baby bunny first starts making caecotrophs at 3-4 weeks of age, when they start to eat solid food. Then caecotroph development increases with age reaching a maximum between 63-77 days old.
Caecotrophs contain greater amounts of protein, minerals and vitamins than hard poops, while hard poops are high in fibre.