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 populat ions. In this work, the digestive tract of the hoatzin was studied mor phometrically and microbiologically, and its significance in the evolu tion of herbivory in vertebrates is discussed. The crop of the hoatzin is already formed in newly hatched chicks, and acquires microbial pop ulations of bacteria and protozoa within the first 2 weeks of life, pr esumably by inoculation during feeding by adults. Numbers of bacteria and protozoa resemble numbers from the rumen of Ruminantia. The presen ce of foregut fermentation in this bird demonstrates that this strateg y is not an exclusive feature of mammals. Herbivorous mammals develope d foregut structures without homologues among other mammals, while the crop of the hoatzin, being homologous to that in other birds, is anal ogous to the mammalian foregut fermentation chambers. Thus, evolution of foregut fermentation in mammals and birds might be a case of evolut ionary convergence.