Foregut Fermentation in the Hoatzin, a Neotropical Leaf-Eating Bird

Abstract

The only known case of an avian digestive system with active foregut fermentation is reported for the hoatzin (Opisthocomus hoazin), one of the world's few obligate folivorous (leaf-eating) birds. Hoatzins are one of the smallest endotherms with this form of digestion. Foregut fermentation in a flying bird may be explained by increased digestive efficiency by selection of highly fermentable and extremely patchy resources, coupled with microbial nutritional products and secondary compound detoxification. This unexpected digestive system gives a new perspective to the understanding of size limitations of vertebrate herbivores and to the evolution of foregut fermentation.

Rumen-like methanogens identified from the crop of the folivorous South American bird, the hoatzin (Opisthocomus hoazin)

“…However, the mean density of methanogens in the hoatzin crop was in agreement with real-time PCR densities of methanogens from cattle from Canada using 16S rRNA (7.93 Â 10 4 to 1.62 Â 10 9 ; Hook et al, 2009), and cattle from Australia using the mcrA genes (9.8 Â 10 8 , Evans et al, 2009;1.34 Â 10 9 , Denman et al, 2007). The mean density of hoatzin crop bacteria (7.93 Â 10 12 ) from the present study was in agreement with previously published studies from the hoatzin, having viable cell counts ranging from 1.1 Â 10 9 to 4.97 Â 10 12 (Grajal et al, 1989;Dominguez-Bello et al, 1993;Garcia-Amado et al, 2007), and with a study on another avian species, the green-rumped parrotlet, having 1.1 Â 10 5 to 7.3 Â 10 12 colony-forming units in the crop (Pacheco et al, 2004). In addition, the mean density (3.31 Â 10 5 ) of ciliate protozoa was also in agreement with previously published microscopic protozoal cell counts of 10 4 per gram (Dominguez-Bello et al, 1993).…”

“…The increased size of the foregut allows a long retention of the food and microbial digestion of the folivorous diet. As a result, mean retention times for the hoatzin are among the longest recorded for a bird (Grajal et al, 1989), and digestion efficiencies are very similar to those of ruminant animals (Grajal and Parra, 1995). Short-chain fatty acids, a by-product of microbial fermentation, are also present in the crop in high concentrations similar to that of ruminants.…”

“…Nearly 20 years after Grajal et al (1989) first reported foregut fermentation in a nonmammalian animal, information on the microbial consortia resident in the hoatzin's crop is now being elucidated using molecular approaches (Godoy-Vitorino et al, 2008). In the present study, 197 clones were examined and 24 different phylotypes were grouped into nine OTUs based on a similarity criterion of 98% (Table 1).…”

“…As a result, the muscular crop has displaced the flight muscles and keel of the sternum, making the hoatzin a poor flyer (Grajal et al, 1989). The increased size of the foregut allows a long retention of the food and microbial digestion of the folivorous diet.…”

Comparative analyses of foregut and hindgut bacterial communities in hoatzins and cows

“…Strict herbivory is rare amongst birds, presumably due to mass tradeoffs associated with flight; however, a few birds such as the South American hoatzin (Opisthocomus hoazin) feed on terrestrial plants. The hoatzin is a folivorous bird, unique in possessing an enlarged crop with microbial fermentation (Grajal et al, 1989). This bird is a browser that feeds primarily on tender young plant leaves, from B17 plant species (dominated by the plant genus Coccoloba).…”