Microbial Digestive Symbionts of the Crop of the Hoatzin (Opisthocomus hoazin): An Avian Foregut Fermenter

The hoatzin is the only bird known to have a well-developed system of microbial fermentation of plant material (mostly young leaves) in its crop. In this study, we describe the microbial inhabitants of the crop and discuss their nutritional significance. Bacteria, which were mainly Gram negative anaerobes, were found in counts of 10⁹ per gram of crop contents. Strains resembling bacteria from the genera Clostridium, Bacteroides, Fusobacterium, Peptostreptococcus, and Veillonella were tentatively identified by morphological and substrate-utilization studies. Although no cellulolytic activity was demonstrated among isolated bacteria, cellulose degraders were found in a mixed culture by enrichment. No anaerobic fungi were found. Ciliate protozoa were found in the crop in counts of 10⁴ per gram of contents. Protozoa have not been found in the crop of other birds. These microbial symbionts appear to play an important role in the nutrition of this bird.