Structure and function of the digestive tract of the Hoatzin (Opisthocomus hoazin): A folivorous bird with foregut fermentation

The Hoatzin (Opisthocomus hoazin) is a unique obligate folivorous bird with a well-developed foregut fermentation system. Its relative gut capacity is equivalent to 9% of the adult body mass (ca. 680 g). The large crop and lower esophagus represent 77% of the total gut capacity. The crop is folded into two interconnected chambers, and the lower esophagus is a multichambered organ. Both are unusually muscular with constrictions between chambers. The interior lining of the crop and esophagus has longitudinal ridges covered by cornified epithelium. The crop and esophagus are the main fermentation organs,with pH and volatile-fatty-acids levels equivalent to those found in mammals with foregut fermentation. The proventriculum and gizzard are much reduced in capacity. A combination of abrasion and microbial action effectively reduces particle size along the gut. A trial with markers made out of thin (0.6-mm) plastic film demonstrated that large particles (10 mm²)are retained longer than medium (4-mm²) or small (1-mm²) particles at the anterior fermentation sites. The extreme gut adaptations in the Hoatzin are more similar to those of small mammals with foregut fermentation than to any known bird. This suggests that a similar set of evolutionary constraints may affect the evolution of foregut fermentation in vertebrates.