One of this species' many peculiarities is that it has a digestive system unique amongst birds. Hoatzins use bacterial fermentation in the front part of the gut to break down the vegetable material, much as cattle and other ruminants do. Unlike ruminants, however, which have the rumen (a specialized stomach for bacterial fermentation), the hoatzin has an unusually large crop, folded in two chambers, and a large, multi-chambered lower oesophagus. Its stomach chamber and gizzard are much smaller than in other birds.