Evolutionary significance of foregut fermentation in the hoatzin (Opisthocomus hoazin; Aves: Opisthocomidae)

The hoatzin (Opisthocomus hoazin) is the only folivorous bird known to possess extensive fermentation in the crop by mixed bacterial populations. In this work, the digestive tract of the hoatzin was studied morphometrically and microbiologically, and its significance in the evolution of herbivory in vertebrates is discussed. The crop of the hoatzin is already formed in newly hatched chicks, and acquires microbial populations of bacteria and protozoa within the first 2 weeks of life, presumably by inoculation during feeding by adults. Numbers of bacteria and protozoa resemble numbers from the rumen of Ruminantia. The presence of foregut fermentation in this bird demonstrates that this strategy is not an exclusive feature of mammals. Herbivorous mammals developed foregut structures without homologues among other mammals, while the crop of the hoatzin, being homologous to that in other birds, is analogous to the mammalian foregut fermentation chambers. Thus, evolution of foregut fermentation in mammals and birds might be a case of evolutionary convergence.