The digestive systems of rabbits and guinea pigs have adapted to a herbivorous diet high in fibre. High-quality indigestible fibre assists the movement of food through the digestive system.
Lack of fibre can cause the slowing of the digestive tract, leading to GI stasis. Additionally, both guinea pigs and rabbits have continuously growing teeth. The process of eating hay helps to grind down their teeth preventing overgrowth and disease.