Rabbits ferment fibre in an enlarged caecum located after the small intestine, which means the nutrients their gut bacteria produce are formed downstream of the absorptive surface. Caecotrophy solves the problem. Soft, mucus-coated clusters are expelled separately from the hard faecal pellets, usually during rest periods, and are consumed immediately without chewing.
The second pass through the stomach and small intestine allows absorption of microbial protein, volatile fatty acids and B-group vitamins, and recovers water. The mucus coat protects the bacteria from gastric acid long enough for fermentation to continue in the stomach. A rabbit denied access to its caecotrophs, for instance by obesity or spinal pain preventing it reaching around, becomes malnourished on an otherwise adequate diet.
The behaviour is often described as coprophagy, which blurs an important distinction: rabbits do not eat ordinary faeces, and hard pellets left in the hutch are genuine waste. Uneaten caecotrophs smeared around a rabbit's rear are a clinical sign, usually of excess dietary starch, dental disease or reduced mobility.