Phylogenetic relationships of the enigmatic hoatzin (Opisthocomus hoazin) resolved using mitochondrial and nuclear gene sequences

The hoatzin (Opisthocomus hoazin) is a bizarre, long-tailed, crested bird that inhabits the riparian lowlands of South America. Among its peculiar attributes are (1) microbial foregut fermentation to convert plant cellulose in consumed foliage into simple sugars, (2) a highly modified skeleton to accommodate its large crop, and (3) in the young of this species, wing claws at the wrist joint which are used to climb among the branches of the nest tree. Consequently, the taxonomic position of this unusual bird has perplexed systematists since its description over 200 years ago. Traditionally classified among the fowl-like birds (Galliformes), recent studies have favored its placement with the cuckoos (Cuculiformes: Cuculidae). To help resolve this systematic uncertainty, we sequenced six mitochondrial genes (cytochrome oxidase I, II, and III, ATPase 8, ATPase 6, and cytochrome b) and one nuclear gene (c-mos), totaling 5,487 base pairs. With this large data set and an appropriate range of outgroup taxa, we demonstrate that the hoatzin should not be classified among the cuckoos or Galliformes. Instead, our analyses indicate that the hoatzin is most closely related to the turacos (Musophagiformes: Musophagidae), a small family of arboreal, frugivorous birds inhabiting sub-Saharan Africa. This phylogenetic relationship is also supported by osteological, behavioral, and fossil evidence.