THE taxonomic position of the Hoatzin (Opisthocomus hoazin) of South America has long been one of the most debated problems in avian system- atics. It has usually been placed in the Galliformes, but some authors have allied it with the African turacos (Cuculiformes: Musophagidae), the pigeons (Columbidae), or the rails (Rallidae), and others have placed it in its own monotypic order. The Hoatzin occurs in riparian vegetation along the streams of the Orinoco and Amazon river drainages in northern South America. It is a slender bird, about 25 inches in length and generally brownish in color, somewhat resembling a chachalaca (Cracidae). The small head bears a ragged, bristly, reddish-brown crest and the bare facial skin is bright blue. As is apparent from the Frontispiece, the Hoatzin is actually most like the smaller Guira Cuckoo (G. guira) in coloration; in fact, the two species are remarkably similar, except in size. This paper reviews the taxonomic history of the Hoatzin and presents new evidence from a study of the egg-white proteins indicating that the resemblance to Guira is due to a close relationship, not merely to con- vergence or coincidence. The correct position of the Hoatzin is as a genus within the neotropical subfamily Crotophaginae (Cuculidae) and its dosest relatives are Guira and Crotophaga. The long association with the Gal- liformes, we believe, was based upon little more than the prejudice estab- lished by the original description as Phasianus hoazin (Miller 1776) and the inability of subsequent workers to interpret their anatomical data. The link to the Musophagidae seems to be based upon the common possession of a number of anatomical characters (see Verheyen 1956), the vegetarian diets, poor flying ability, general proportions, and similar size. In the turacos and in the Hoatzin the young have wing claws and clamber about near the nest before they can fly. THE TAXONOMIC HISTORY OF THE HOATZIN In general appearance a museum specimen of the Hoatzin resembles a galliform bird and it is not surprising that M[iller (1776) originally named TABLE 1 SOME OPINIONS ON THE SYSTEMATIC POSITION 01* OPISTHO1/2OMUS Evidence of relation- Placed in Galliformes Placed in separate order Placed near Musophagidae ship to Cuculidae Miiller, 1776 Linnaeus, 1788 Illiger, 1811 Cuvier, 1817 Carus, 1868 Reichenow, 1882 Fiirbringer, 18884 Gadow, 1892, 18934 Shufeldt, 1904 Wetmore, 1930, 1934, 1940, 1951, 1960 Peters, 1934 Berlioz, 1950 Mayr and Amadon, 1951 Storer, 1960 Verheyen, 1961 L'Herminier, 18372 Huxley, 18682 Garrod, 1879  Sclater, 1880  Newton, 18842 Elliot, 1885 a Sharpe, 18915 Beddard, 1898  Stresemann, 1927-34, 1959 ø Nitzsch, 1829, 1840 Gray, 1844-49, 1869-71 Cabanis, 1847 ø Barnikol, 1953 Verheyen, 1956 Goodchild, 1886 (secondary coverts) Pycraft, 1895 (pterylography) Beebe, 1909 (behavior) Lemmrich, 1931 (sclerotic ossicles) Sibley and Ahlquist, 1972  Between Galliformes and Cuculiformes. a Between Galliformes and Columbiformes. a Next to Ga]]iformes. a Noted ties to Musophagidae.  Between Co]umbae and Ralli, following Seebohm (1888, 1890, 1895) who believed that Opistlto- comus had gruiform affinities. o Cabanis was apparently the first to note the similarity in co]or and pattern between Opistltocomus and Guira. it Phasianus hoazin. Apparently this was enough to establish a bias in favor of galliform affinities, and in most classifications to the present time, the Hoatzin has been placed in or near the Galliformes. Some of the opinions concerning the classification of the Hoatzin are summarized in Table 1. The first suggestion that the Hoatzin might be related to the Muso- phagidae was that of Nitzsch who found similarities in the carotid arteries (1829) and pterylography (1840). In both papers Nitzsch placed Opistho- comus with Colius and the Musophagidae in the "family" Amphibolae. Gray (1844-49, 1869-71) followed a similar scheme, placing these three groups, along with the Bucerotidae and several passerine families, in the Conirostres. Huxley (1867) first examined only an incomplete skull and the feet of a Hoatzin. He found that the slender vomer bifurcates anteriorly in an unusual way but he considered the palate to be schizognathus, as in the Galliformes, rather than desmognathous, as are the cuckoos. Huxley con- sidered the tarsometatarsus to be galliform and assigned Opisthocomus to a "special subdivision," Heteromorphae, in his large "suborder" Schizog- nathae. He placed the Heteromorphae between the sandgrouse ("Ptero- clomorphae") and the pigeons ("Peristeromorphae"), not far from the galliforms ("Alectoromorphae"). Huxley soon obtained a complete skele- ton and (1868) decided that in the majority of its osteological characters the Hoatzin resembles the Galliformes and the Columbiformes and that where it differs from these it is either unique or like the Musophagidae. He concluded that Opisthocomus is a highly modified form derived from "Gallo-columbine" stock, worthy of distinction in his monotypic "Hetero- morphae." Huxley's prestige and influence were great and these pronouncements must have had a strong effect upon subsequent workers. The galliform affinities of the Hoatzin seemed confirmed, but an alliance to the cuckoos, via the turacos, had been left open. This basic concept has prevailed to the present day, in spite of evidence to the contrary. Garrod (1879) was among the first to follow Huxley's lead by placing the galliform and cuculiform birds together in his Galliformes (which also included the ratites, tinamous, rails, seriemas, and bustards). Garrod expressed his belief that Opisthocomus is an intermediate form, linking the Cuculidae and Musophagidae to the typical galliforms. He thus emphasized that Opisthocomus has a relationship to the cuckoos, although he retained the supposed link to the Galliformes as well. Others who adopted Opisthocomus as the evidence for a galliform-cuculi- form alliance included Ftirbringer (1888) who placed the family Opistho- comidae next to the Gallidae in the "gens" Galli, but also noted a dozen characters linking Opisthocomus with the Musophagidae. Gadow (1892: 229) also considered Opisthocomus to be a connection between the cuckoos and the galliforms but (1893) he placed the Opistho- comi in the Galliformes. Like Ftirbringer, Gadow consistently emphasized the alliance to the cuckoos. Gadow (1893: 177) noted the following char- acters in comparisons among the three groups: I. Opisthocomus agrees with the Galliformes and differs from the Cucu- liformes in: 1) downy precocial young, not blind at hatching, 2) schizognathous palate, 3) fusion of many dorsal vertebrae, 4) syrinx, 5) foot structure, 6) large crop. II. Opisthocomus agrees with the Cuculiformes and differs from the Galliformes in: 1) deep temporal fossae, 2) short mandibular proc- esses, 3) lack of basipterygoid processes, 4) lack of spina interna, 5) metasternal configuration, 6) large procoracoid, 7) spotted eggs, 8) 10 rectrices. III. Opisthocomus differs from both Galliformes and Cuculiformes in: 1) lacking apteria on the sides of the neck, 2) arrangement of the adult downs, 3) number of cervical vertebrae, 4) small thoracic hae- mapophyses, 5) shape of the liver, 6) other specialized characteristics, for example, sternum, stomach, crop. Because virtually all subsequent classifications, including those of Strese- mann (1927-34), Wetmore (1930, 1934, 1940, 1951, 1960), Peters (1934) and Mayr and Amadon (1951) are based upon Gadow and F[irbringer, it is easy to understand why the Hoatzin has been placed so consistently in the Galliformes. The anatomical evidence indicating a relationship to the Cuculidae includes a few additional points. Pycraft (1895: 362) found a general resemblance in the pterylography of the spinal tract among Opisthocomus, Tauraco, and Centropus but declined to offer an opinion on their relation- ships. Lemmrich (1931) studied the bony platelets of the sclerotic ring in several groups of birds. He found that all of the 10 species of galliforms he examined had 13 to 15 plates but that Opisthocomus has only 12. Lemmrich noted (p. 534) that such a difference is "very remarkable" because there tends to be little variation within a group. Among the few other birds with 12 plates is Cuculus. The skull musculature and its innervation in Opisthocomus were studied by Barnikol (1953). He compared the Hoatzin with representative species of galliforms, turacos, and pigeons and summarized the study in a table (p. 520) listing 40 anatomical characters. Of these Opisthocomus shared 8 with the Galliformes, 9 with the Columbidae and 13 with the Muso- phagidae. Barnikol concluded that Opisthocomus is an isolated species deserving recognition as the sole occupant of the order Opisthocomae. Verheyen (1956) defined 66 osteological characters of the Hoatzin and found that 50 of them are shared with the Musophagidae. This led him to combine the two groups as the order Musophagiformes, placed near the Galliformes and the "Anhimiformes." However, in his later (1961) clas- sification Verheyen included Opisthocomus next to the Cracidae in his Galliformes, "owing to new information" (p. 21). Some especially convincing sets of data arguing against a galliform alliance for Opisthocomus have appeared in recent years. In 1959 Hudson, Lanzillotti, and Edwards found 13 aspects of the hind limb musculature in which the Hoatzin differs from all galliforms and in 1964 Hudson and Lanzillotti listed 21 important differences between the pectoral musculature of Opisthocomus and that of the galliforms. They concluded (p. 111 ) that "Opisthocomus has either been erroneously associated with the Galliformes, or has diverged so far from the original ancestral condition, that there is little or no justification for retaining it in that order? Because of his long interest in the Opisthocomus problem, the opinion of E. Stresemann is of particular significance. In his classifications of 1934 and 1959 he followed Fiirbringer, placing the Hoatzin with the Galliformes and attributing resemblances to the tufacos to convergence. But in recent studies of pterylosis and molt patterns E. Stresemann (1965) and E. and V. Stresemann (1966) discovered that Opisthocomus differs from all galli- forms in four ways: 1) the nestling plumage lacks flight feathers; 2) the first flight feathers grow to nearly adult size and are not molted before the bird reaches maturity; 3) the two outer secondaries are not shorter than the adjacent ones and all develop at the same time; and, 4) the primaries are replaced in a continuous, stepwise process, not in the galliform sequence. These findings prompted E. Stresemann (1965: 64) to conclude (transl.): "If Opisthocomus is related to the gallinaceous birds it is only by way of Adam and Eve." LIFE HISTORY AND BEHAVIORAL EVIDENCE Accounts of the natural history of the Hoatzin have been published by Goeldi (1886), Quelch (1890), Beebe (1909), Chubb (1916) and Young (1929). A popular article by Grimmer (1962) is one of the most in- formative, including excellent photographs, and Sick (1964) presented a summary. From these accounts a fairly complete life history can be constructed. The Hoatzin lives in the permanently flooded forests along the banks of some of the larger rivers and their tributaries in northeastern South America. The birds feed primarily, and perhaps exclusively, on the leaves, flowers, and fruits of certain marsh plants, including the genera Mon- trichardia (Araceae) and Avicennia, the arboreal White Mangrove. Grim- mer found them feeding only on plants; others have recorded small animals (fish, crabs) as part of the diet. When feeding the birds fill the large, specialized crop, the resultant weight of which makes balance difficult and the bird often rests the sternal area on a branch to support the heavy crop. A special epidermal callosity has evolved in the area of contact. Hoatzins are usually described as clumsy, poor fliers, their feet not well adapted for grasping branches and their weak wings restricting flight to short distances. The wings are used in clambering about in the vegetation and the weak flight is correlated with the reduction in the sternum and pectoral musculature associated with the exceptionally large crop. When not breeding the Hoatzin forms flocks of up to 40 individuals which break up into smaller groups of 2 to 6 birds when nesting. The discovery (Grimmer 1962) that the Hoatzin forms communal nesting associations is of particular interest because the three species of anis (Cro- tophaga) and the Guira Cuckoo also build communal nests (Young 1929, Skutch 1935, 1966, Chapman 1938, Davis 1940a, b). With the first rains the members of a nesting group build a flat nest of loosely organized dry twigs in branches overhanging the water. Mating seems to be indiscriminate and may be polygamous, and all members of the group participate in incubation and care of the young. The 2 to 5 buff-colored eggs have pink, brown, or bluish spots, and incubation takes about 28 days. The young have two successive coats of down and are fed from the crops of the adults, apparently on plant material. The young remain in the nest for a considerable time but soon begin to undertake climbing excursions nearby, using the well-known wing claws, as well as the feet and beak. When frightened the young I-Ioatzin drops into the water and either dives or swims into the vegetation and hides. Later it climbs back to the nest. The young of the Groove-billed (Crotophaga sulcirostris) and Smooth- billed (C. ani) Anis have a similar pattern of behavior according to Skutch (1959, 1966). The flightless ani youngster also uses the beak, feet, and wings (which are clawless) in clambering about near the nest. When frightened it either moves away through the branches or drops to the ground and hides in the vegetation, later returning to the nest. Both Beebe (1909) and Skutch (1959: 294) commented upon the behavioral sim- ilarities between the Hoatzin and the anis. THE EGG-WHITE PROTEIN EVIDENCE Egg white of the Hoatzin was first received in 1961 through the kindness of J. Lear Grimmer and Woodbridge Williams. Additional material came from Ram E. Singh in 1972. It was obvious from the first comparisons using starch gel electrophoresis (Sibley and Ahlquist, 1972, but see Sibley 1970 for a description of the technique) that Opisthocomus is not a galliform and is probably a cuckoo, but the resolving power of the starch-gel technique was insufficient to provide an unequivocal answer. A study of the literature on the natural history of the I-Ioatzin and of museum skins caused us to feel fairly certain PENELOPE PURPURASCENS CRAX BLUMENBACHII OPISTHOCOMUS HOAZIN GUIRA GUIRA CROTOPHA6A ANI COCCYZUS AMERICANUS . GEOCOCCYX CALIFORNIANUS ; MOROCOCCYX ERYTHROPYGI]S PH 3,0 PH 10,0 Figure 1. The egg-white proteins of Opisthocoraus compared with those of some New World cuckoos and cradds using the technique of isoelectric focusing in acrylamide gel. The similarities among Opisthocomus, Guira, and Crotophaga are greater than between any one of these and the other genera compared. The patterns were produced using an Ampholine range of pH 3-10. that the neotropical crotophagine cuckoos (Guira, Crotophaga) were the most likely relatives. In 1972 we felt confident enough to suggest such an alliance, but proof still seemed elusive. During 1972 we began work with the electrophoretic technique of isoelectric focusing in acrylamide gel (IFAG) (see Sibley and Frelin 1972, for a description of the technique), which has much greater resolving power than that of starch gel electro- phoresis. Using the IFAG technique we compared the egg-white proteins of Opisthocomus with those of Guira, Crotophaga, other cuckoos, various galliforms, pigeons, and many other groups of nonpasserine birds. It quickly became dear that the patterns produced by Hoatzin egg white were most similar to those of the Crotophaginae. We now feel confident in stating, without qualification, that Opisthocomus is a dose relative of Guira, which it resembles in plumage coloration, although it is a much larger bird. If the two had been more alike in size, or if the Hoatzin had possessed the typical cuculiform foot, we feel confident that this relation- PENELOPE PURPURASCENS CRAX BLUMENBACHII OPISTHOCOMUS HOAZIN GUIRA GUIRA CROTOPHAGA ANI : COCCYZUS AMERICANUS PH 6,0 PH ,0 GEOCOCCYX CALIFORNIANUS MOROCOCCYX ERYTHROPYGU$ Figure 2. The same species as in Figure 1 but using an Ampholine range of pH 6-4. ship would long ago have been recognized. The egg-white data confirm the relationship, but ample additional evidence from the plumage, anatomy, life history, and behavior supports this conclusion. A detailed description of the IFAG patterns follows. Figures 1 and 2 depict those egg-white proteins that have isoelectric points between pH 3 and 10. Because no egg-white proteins are known to be isoelectric below pH 3 and only lysozyme may be isoelectric above pH 10, nearly all components are resolved in these patterns. In these figures the egg white of Opisthocomus is compared to that of several species that have been suspected of being related to it. The pattern of Opistho- comus differs from those of the curassow Crax and the guan Penelope in nearly all aspects. In the cracids the ovalbumins and most other com- ponents are isoelectric near pH 5. This produces a "short" pattern with many bands close together. In Opisthocomus the ovalbumins are isoelectric near pH 4 and many components, mainly the ovotransferrins, are isoelectric at pH 6 or above. Crax and Penelope have a prominent lysozyme near pH 10 that Opisthocomus lacks. In those features where the Opisthocomus pattern differs from those of the cracids it is like those of Guira and Crotophaga. This resemblance is CRINIFER AFRICANUS OPISTHOCOMUS HOAZIN RALLUS LON6IROSTRIS . COLUIBA LIVIA PH $,0 PH 10,0 Figure 3. The egg-white proteins of Opistho1/2omus compared with those of some Old World cuculiforms, a rail, and a dove using isoelectric fuscusing with an Ampholine range of pH 3-10. The similarities among Opistho1/2omus and the others are less strik- ing than those among Opistho1/2omus, Guira, and Crotophaga in Figure 1. most striking in the ovalbumin region around pH 4, but there are other components as well that have identical isoelectric points. The general pattern of Opisthocomus is similar to those of the musophagid Crini]er and the coucal Centropus, but few components have identical isoelectric points. The pattern of Opisthocomus differs considerably from those of the New World cuckoos Coccyzus, Geococcyx, and Morococcyx. The patterns of Coccyzus and Morococcyx in turn resemble those of the Old World Cuculus and Clamator. Finally there are no similarities between Opisthocomus, RaNus, and Columba. Figures 3 and 4 depict the egg-white proteins of the same species in the pH range 6 to 4. In these separations greater resolution of those com- ponents in the ovalbumin region is achieved. For example, in the pH 3-10 range four major bands and an additional four minor bands can be seen in the ovalbumin region of Opisthocomus. In the pH 6-4 patterns Opis- thocomus has 18 components. Opisthocomus again bears the greatest resemblance to Guira and Crotophaga, although small differences in the CRINIFER AFRICANUS ' i_= OPISTHOCOMUS HOAZIN :, .- . ,...  .. CUCULUS CANORUS   ....  ,,,. CHALCITES MAYUS ,.  CMATOR LEVAILLANTII  RALLUS LONGIROSTRIS COLUMBA LIVIA PH 6,0 PH ,0 Figure 4. The same species as in Figure 3 but using an Ampholine range of pH 6-4, isoelectric points of a number of proteins can be discerned. The Opistho- comus pattern shows some similarity to that of Crinifer, but less to those of Centropus and the other cuckoos, and is unlike those of the cracids, Railus, or Columba. ACKqOWLEDGIEIqTS $Ve are especially grateful to J. L. Grimmer, Woodbridge Williams, and Ram E. Singh for providing the specimens of Hoatzin egg white. Other specimens used in the study were provided by A. Baddokwaya, F. Contino, P. Davis, R. Dean, J. Doebell, A. Garza, M. Goddard, J. Griffith, H. Hendrickson, L. Kiff, P. Miles, C. Perrins, N. Proctor, G. Ragless, J. Robjent, S. Rothstein, A. Ruschi, F. Sibley, W. Spofford, F. Todd, D. $Vells, and A. $Villiams. Dorothy J. Moore and Karen Heckmann provided laboratory assistance and A. H. Coleman prepared the photographs. The National Science Foundation provided support under GB-6192X. SUMMARY The systematic relationships of the Hoatzin (Opisthocomus hoazin), long classified with the Galliformes, have been re-evaluated in the light of data from electrophoretic comparisons of the egg-white proteins. All evidence has been found to be consistent with an assignment to the subfamily Crotophaginae of the Cuculidae. 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