Rabbits eat their own poo. While some people might find that practice distasteful and say "Yuck", biologically it represents a highly efficient and successful solution to a digestive dilemma.
Grass is a hugely important source of food for many animals worldwide, but it has a downside in that it is difficult to digest. Grass is high in fibre and plant fibre is indigestible to mammals. Mammals never achieved digestive enzymes to cope with fibre, so a number of different strategies have evolved to try to get the best out of grass as a food.
Cows and other ruminants like sheep, goats, deer, camels, giraffes, yaks, antelope and llamas 'chew the cud'. They eat grass, chew it and swallow it. The stomach of ruminant animals has four compartments. Cud is a portion of food that returns from the first stomach compartment to the mouth to be chewed for the second or subsequent time before passing on down the system.
Rabbits don't chew the cud. They graze and process the grass and other herbage in much the same way as we digest our food. The grass passes quickly through their systems.
Since mammals don't have digestive enzymes to break down fibre, they, ourselves included, employ bacteria to do the job for them. Large colonies of bacteria live in the gut earning their keep by providing a valuable service to their host. Our gut bacteria are hugely important to both our physical and mental health.
In ruminants, the large colonies of gut bacteria are concentrated at the front end of the digestive system and the animal facilitates their work by chewing the cud. In rabbits the large colonies of gut bacteria are concentrated at the rear end of the digestive system and the animal gains benefit from their work by eating its own droppings.
Rabbits produce two kinds of droppings. The first pellets are larger, softer, moister, mucous-coated and greener. We hardly ever see these as they are normally excreted during the day when rabbits are lying up and are eaten immediately to run the material through the digestive system a second time.
The second processing of the grass allows for nutrients to be absorbed and the animal to gain benefit from its grazing. The pellets produced second time around are smaller, harder, drier and browner and are the rabbit dropping that we are all familiar with seeing in areas where rabbits abound or in a hutch where pet rabbits are kept.