Cecotrope

Cecotropes (also caecotropes, cecotrophs, cecal pellets, soft feces, or night feces) are a nutrient filled package created in the gastointestinal (GI) tract, expelled and eaten by rabbits and guinea pigs (among other animals) to get more nutrition out of their food. The first time through the GI tract, small particles of fiber are moved into the cecum (at the small intestine/colon junction), where microbes ferment them. This creates useable nutrients which are stored and expelled in cecotropes. The cecotropes are eaten and the nutrients are absorbed in the small intestine.

Terminology
Cecotropes are not fecal material but rather nutrition, so terms such as "soft feces" and "night feces" are technically incorrect. Similarly, though cecotropes are sometimes called "night feces," they are in fact produced throughout the day and night. The act of eating cecotropes is known as cecotrophy (caecotrophy, cecophagy, pseudo-rumination, refection), again as distinct from coprophagy (the eating of feces proper). Rabbits (and other animals discussed here) will occasionally engage in coprophagy.

Cecotropes are a group of small balls clumped together that look like a thin blackberry, which exit the anus all at once. They are very dark (almost black) and smelly, look wet, are sticky (as they are covered in mucus), are very soft, and are full of nutrition.

Cecotropes differ from regular feces (dry feces, hard feces, or fecal pellets), which are larger, single balls, exit the anus one at a time, are dark brown/dark gray, smell only slightly, have very little moisture, are harder and are a waste product.

It is well known that lagomorphs (rabbits, hares, pikas) eat cecotropes. Some rodents do the same, including the beaver and probably all in the suborder Hystricomorpha (e.g., capybara, guinea pig, chinchilla).

Other animals also eat cecotropes, for example marsupials (common ringtail possum, coppery ringtail possum). While the information in this article is specific to rabbits, much of the information also relates to all animals that eat cecotropes.

These animals are hindgut fermenters. This means fibrous food material is fermented after the small intestine (in the cecum and/or colon). Small animals (discussed in this article) are classified as cecal fermenters while large animals are colonic fermenters. They all have one stomach (monogastric). Not all hindgut fermentators have ceca that make material for cecotropes that are reingested.

Overview
The stomach and small intestine digest food material. The small intestine also absorbs some digested material. The rest of the material then moves into the colon. Here material is separated according to size. The large particles continue to move down the colon while the small particles move back up the colon and into the cecum. The large particles are formed into regular feces and expelled. Meanwhile, the material in the cecum is fermented by microbes, producing nutrients that can later be absorbed. The material is then passed down the colon, formed into cecotropes, expelled and eaten. When the cecotropes move into the small intestine, the nutrients are absorbed.

Before the cecum
Animals discussed in this article are herbivores. Adults eat more than 30 meals a day. The incisors cut the food (e.g., grass and forbs) while the premolars and molars grind it into smaller pieces. The organized tongue movements help to thoroughly chew the food. Saliva (containing the enzyme amylase) starts the digestion process. The food material is swallowed down the esophagus and then reaches the stomach, where digestion continues. The stomach is 15% of the total volume of the GI tract and has a pH of 1-2 (which is very acidic).

Rabbits cannot vomit. This is because where the esophagus meets the stomach, rabbits have a massive muscular sphincter (with both smooth and striated muscles) along with a mucosal membrane flap that produces a watertight one-way seal. They also lack the nerves necessary to vomit.

Moving down to the small intestine (12% of GI tract), digestion continues. Most absorption occurs in the small intestine, where the nutrients move through the walls and into the bloodstream. Some absorption also occurs in the stomach, cecum and colon.

The material then reaches the proximal (upper) colon. Between the proximal and distal (lower) colon is a small segment called the fusus coli, which is unique to lagomorphs. It regulates the separation of the material. Particles greater than 0.3-0.5 mm (mainly non-fermentable material) move to the center of the colon and then peristalsis moves them down the colon. Particles less than 0.3-0.5 mm (mainly fermentable fiber and proteins) move to the sides, and then retrograde peristalsis moves them back up the colon and into the cecum.

The ileocecal valve (at the end of the small intestine) ensures the material goes to the cecum and not the small intestine.

At the cecum
A rabbit's cecum is proportionally the largest of any mammal. It is 40% of the total volume of the GI tract. The cecum is a blind sac coming off the small intestine/colon junction. At the end of the cecum is the vermiform appendix.

Mammalian enzymes cannot break down fiber. However, microbes in the cecum have enzymes that are capable of breaking down fiber. The microbes include bacteria (mainly Bacteroides and also Bifidobacterium, Clostridium, Streptococcus, Enterobacter, Endophorus, Acuformis, Peptococcus, Peptostreptococcus, Fusobacterium, Coliform (usually in small numbers, e.g., Escherischia coli) and others), protozoa (ciliated and flagellated), an amoeboid organism (Entamoeba cuniculi) and a rabbit-specific yeast (Cyniclomyces guttulatulus).

It is estimated that more than 50% of the microbes are not known. Note that no Lactobacillus species are found in the microbiome of the rabbit and thus using L. acidophilus as a probiotic has unknown value.

The anaerobic fermentation in the cecum breaks down the fiber into useable food for the animal. It is also used as food for the proliferating microbes. The results of the fermentation are volatile fatty acids (VFAs) (mostly acetic, butyric and propionic acids), all of the B vitamins, vitamin K, microbial proteins, essential amino acids and minerals.

The cecum has a pH of 5.4-6.8, which does not harm the microbes. Most of the VFAs are absorbed through the walls of the cecum.

Some of the other nutrients are also absorbed by the cecum and the colon. Four to nine hours after a meal, the cecum empties and the contents (containing the results of fermentation, along with microbes) continue down the colon.

After the cecum
Microbes also live in the colon. Cecotropes and regular feces pass through the colon at different times. Regular feces are formed in the fusus coli, continue through the colon and rectum and are expelled through the anus, about 4 hours after eating. They contain mainly insoluble fiber which is used for motility, not for nutrition.

The material from the cecum is formed into cecotropes in the fusus coli, where it contracts more gently than when forming regular feces. Goblet cells in the fusus coli secrete mucus which covers the cecotropes, which will protect them when they get into the acidic stomach. An enzyme (lysozyme) is also added that aids digestion of microbial proteins. Cecotropes (pH about 7) continue through the colon and rectum and are expelled thru the anus, about eight hours after eating.

Reingestion
Cecotropes are eaten directly from the anus. They usually do not touch the ground. They are not chewed (even though it looks like it). They are swallowed whole so the mucus is not disturbed. They are held in the fundic region of the stomach (at the top) for 3 to 6 hours where they continue to ferment.

Then they move into the small intestine where the nutrients are absorbed, about 17 hours after the original meal.

The gastrointestinal tract of newborn rabbits is sterile and contains no microbes. The babies eat cecotropes and regular feces from the mother in order to obtain microbes that are needed for the cecum. Young rabbits start eating their own cecotropes at about day 20.

The stomach of the babies has a pH of 5 to 6.5, which does not kill the microbes. However, when the mother's milk combines with an enzyme in the baby's stomach, it produces an antimicrobial fatty acid (octanoic and decanoic acids) (milk oil) which kills microbes. Only after the milk oil decreases and before the stomach pH gets too low (as the baby ages) can the microbes survive the stomach conditions and pass into the cecum to proliferate.

Why cecotrophy
Many herbivores have a diet that is low in nutrition and high in fiber (which is a non-starch polysaccharide carbohydrate). Fiber can be either soluble (pectins and gums) or insoluble (cellulose, hemicellulose and lignocellulose). A simple gastrointestinal tract is not capable of extracting enough nutrients for these animals.

One strategy to get the needed nutrition is used by ruminants (e.g., cows). They chew the cud in order to process their food a second time to extract more nutrients. Another strategy (e.g., used by horses) is to have a very long colon to aid in digestion and absorption. Both of these strategies add substantial bulk to the animal.

Since the rabbit is at the bottom of the food chain, it must be nimble in order to out run its many predators. Creating cecotropes is a way to get more nutrients out of their food without adding a lot of bulk to their GI tract (which is 10% - 20% of their body weight). Since their colons do not absorb the nutrients in the cecotropes, they reingest them so they can be absorbed in the small intestine.

Disorder
It is essential to maintain a balanced microbiome in the gastrointestinal tract, especially the cecum. If beneficial microbes decrease and harmful microbes proliferate, the microbiome becomes unbalanced (called dysbiosis). The cause of this includes a diet too high in carbohydrates and/or too low in indigestible fiber (which slows down gut motility and changes the pH of the cecum, among other things); toxins; some medications (especially some antibiotics); dehydration; extreme stress; dental disease and other systemic diseases (e.g., liver or kidney disease). If the balance is not maintained, there can be multiple health issues, including GI stasis, which can lead to pain, stress and death.

A few cecotropes left on the ground of the living area of the animal is not cause for concern. However, if a large amount is found on the ground or stuck to the fur, a veterinarian should be consulted. Possible causes are poor diet, dental issues, arthritis, very large dewlap, obesity, or too-long fur in the anal region.

If the cecotropes are like pudding, it is called intermittent soft cecotropes (ISC). This is different from true diarrhea, which has no form, is completely watery and is very serious. If regular feces are also produced, it is not diarrhea.

ISC will stick to the hindquarters and feet of the animal and to places in the living area. Causes are a poor diet (too many carbohydrates, too little fiber) or inappropriate antibiotics. Treatment is to feed unlimited grass hay, greens and limited pellets and to stop giving inappropriate antibiotics.