American Journal of Primatology 5:83-87 (1983)

TAXONOMIC NOTE Bunopithecus: A Genus-Level Taxon for the Hoolock Gibbon (Hylobates hoolock)

LEONARD A PROUTY', PHILIP D. BUCHANAN', WILLIAM S POLLITZER3, AND ALAN R. MOOTNICK4 'Department ofAnthro ology, University of North Carolina, 'Genetics Associates of North

4Gihbon and Gallinaceous Bird Center, Saugus, California

The recent discovery that the hoolock gibbon (Hylobates hoolock [Harlan, 18343) has a karyotype distinct from all other hylobatids provides a new and strong motive for revising gibbon taxonomy and establishing hoolocks in a separate, higher taxon. Revising Groves's taxonomy of 1972, we propose that hoolock, along with the fossil species sericus, occupy a subgenus, Bun- opithecus. With the newly added taxon, the genus Hylobates would thus contain four subgenera: Bunopithecus, Hylobates, Nomascus, and Symphal- angus.

Carolina, Chapel Hill, g Department of Anatomy, Uniuersity of North Carolma, Chapel Hill and

Key words: gibbon classification, Hybbates hoolock, Hybbates sen'cus, Bunopithecus, hoolock gibbon

INTRODUCTION The complexity of resemblances among hylobatid species is reflected in the

variety of groupings of these animals in modern classifications. Above the species level, the long-recognized differences between the siamang and other hylobatids have supported classifications containing two genera, Symphalangus and Hylobates [Schultz, 1933; Napier & Napier, 19671. Other taxonomists have emphasized conti- nuity in major traits among hylobatids and have grouped all species into the single genus Hylobates [Kloss, 1929; Miller, 1933; Simonetta, 19571. Groves [1968, 19721 found the concolor as divergent from the lar-group as the siamang, and following Ellerman and Morrison-Scott [ 19511, has provided a genus Hylobates with three subgenera: Symphalangus, Nomascus for the concolor, and Hylobates for the rest [Groves, 19721. This classification is the most nearly comprehensive to date. On the basis of new and old information, we propose revising the classification by splitting the species hoolock from the subgenus Hylobates and placing it in a fourth subgenus with a fossil species resembling hoolock.

Received December 13, 1982; accepted March 18,1983.

Address reprint requests to Dr. William Pollitzer, Department of Anatomy, Medical Research Bldg. D 331H, University of North Carolina, Chapel Hill, NC 27514.

0 1983 Alan R. Liss, Inc.

84 Prouty et a1

While placing the hoolock in the subgenus Hylobates, Groves recognizes the exceptional nature of certain of its morphological traits in comparison with those of the Kloss and lar-group gibbons [Groves, 19721. The hoolock female undergoes a color change late in maturation, while no other females of the subgenus do. The low hoolock brachial index contrasts with the long forearms possessed by the Kloss and agile gibbons [Groves, 19721. Data on vocalizations further distinguish the hoolock from the subgenus Hylobates and ally it variously with the siamang and the concolor gibbon. For example, in the great call sequences of the siamang and hoolock, male and female parts are interspersed, with moments of simultaneous vocalization. In all other species, either the male’s part is concentrated at the end, or the great call is a solo [Marshall & Marshall, 1976; Haimoff et al, 19821.

The hoolock gibbon differs from all other hylobatids in its body weight, which is greater than any gibbon species except the siamang [Schultz, 19731. Its third molars are unreduced relative to those of the other hylobatids, although it expresses the cingulum less [Frisch, 19731. A multivariate study of the cranial features of hyloba- tid has shown the hoolock to be one of five distinct morphological entities, the other four being the siamang and the concolor, Kloss, and lar-group gibbons [Creel & Preuschoft, 1976; cf. Corruccini, 19811.

In addition to its distinguishing anatomical and vocal traits, the hoolock gibbon has a karyotype of unique form [Prouty et al, 19831. Three animals from the Greater Los Angeles Zoological Gardens on breeding loan to the Gibbon and Gallinaceous Bird Center, Saugus, California, were identified as hoolocks by pelage, dental mor- phology, and outer body measurements. The last were made in the manner of Schultz [1929,1933] by Dr. Gene Albrecht, Department of Anatomy, University of Southern California. Sonographs of their vocalizations were made by James Whitehead of the Department of Biology, University of North Carolina, and compared with sono- graphs made from tapes supplied by Dr. Joe Marshall, U S Fish and Wildlife Service. All three animals possessed the distinctive karyotype. Its diploid number of 38 contrasts with that of the siamang (50), the concolor gibbons (521, and the lar-group and the Kloss gibbons (44). The earlier attribution of a karyotype of 44 chromosomes to the hoolock [Chu & Bender, 19611 was doubtless based on a misidentification of the donor animal.

Chromosomal evolution among the subgenera Hylobates, Nomascus, and Sym- phalangus has been extensive. Van Tuinen & Ledbetter [1982, and personal com- munication] compared the banded karyotypes of these subgenera, using trypsin- Giemsa preparations of high quality, and found, on the average, a mere 50% homol- ogy among them, where homology is defined as identity between chromosomes or segments of chromosomes. The chromosomal rearrangements producing the kary- otypes of modern hylobatids are extremely complex, if descent from a common ancestor is assumed. This condition contrasts with the nearly complete homology and simpler rearrangements found in the karyotypes of the great apes and man [Yunis & Prakash, 19821.

At a resolution of about 300 bands per karyotype, the hoolock karyotype follows the pattern of previously known hylobatid karyotypes in showing poor homology with the other karyotype of the genus. The new karyotype cannot be derived by simple rearrangements from the karyotypes of the lar-group gibbons or from those of the other subgenera. Based on this evidence, the evolutionary separation of hoolocks from other hylobatids is comparable to the separation between the three currently recognized subgenera.

On vocal, anatomical, and karyotypic grounds, we propose that the species hoolock be detached from the subgenus Hylobates [Groves, 19721 and placed in a separate subgenus. The name Bunopithecus is available for this taxon because of the following history.

Classification of the Hoolock Gibbon 85

In 1923 Matthew and Granger described a mandibular fragment from a site near the Yangtse River in China, which retained the second and third molars. Believing that the specimen did not show complete affinity with modern hylobatids, they proposed a new genus name for it, Bunopithecus, and a new species name, sericus. Colbert & Hooijer [1953] reexamined the fossil, found that all its traits are present in extant hylobatids, and grouped sericus in genus Hylobates while retaining Bunopithecus as its subgeneric name. Frisch [1965] observed that the size and cusp pattern of the molars resembled those of modern hoolocks, and Groves [ 19721 condi- tionally listed sericus as a subspecies of hoolock.

One of us (Prouty), with the assistance of Gary Rusch-Behrend of the Anthropol- ogy Department, Indiana University, examined the fossil and a large sample of modern hylobatid mandibles, including those of some twenty hoolocks. The resem- blance that Frisch [1965] pointed out is genuine; therefore Bunopithecus is a valid name for a subgenus containing hoolock. While the fossil contains enough informa- tion for us to make this assignment, we prefer to retain sericus as a species, pending information from further fossil finds.

The new taxon, then, in genus Hylobates, consists of subgenus Bunopithecus, containing species sericus and hoolock, the latter consisting of subspecies hoolock and leuconedys [Groves, 19671; type species: sericus; type specimen: mandible frag- ment AM 18534 in the Department of Vertebrate Paleontology, American Museum of Natural History; type locality: Yen-ching-kao near Wan-hsien in Szechuan. Simo- netta [1957, p 681 has provided a synonymy of the name hoolock. Groves [1972] has described the three subgenera he recognized and included diagnostic traits. The following diagnosis differentiates four subgenera.

Subgenus Bunopithecus Diploid chromosome number: 38; cranial length is moderate (a mean of about

85 mm [measurements here and below are derived from Groves, 19721) nasal bones are convex; a prominent laryngeal sac is possessed by neither sex; intermembral index is low (the mean of the skeletal index being approximately 129); number of thoracic vertebrae averages 13; a late-maturation color change occurs in female; male and female vocalizations are interspersed in the great call.

Subgenus Hylobates Diploid chromosome number: 44; cranial length is small (means for different

species and subspecies range from about 75 to 82 mm, with most around 80); nasal bones are flat; a prominent laryngeal sac is possessed by no species; intermembral index is low (skeletal index: means for different species and subspecies vary from about 124 to 131, with most being around 130); number of thoracic vertebrae aver- ages 13; a late-maturation color change does not occur in the female; when the great call is not a solo, the male part is concentrated at the end.

Subgenus Nomascus Diploid chromosome number: 52; cranial length is moderate (subspecies vary

from about 83 to 87 mm, with most being around 85); nasal bones are flat; a prominent laryngeal sac is not possessed by both sexes; intermembral index is high (skeletal index: mean is approximately 141); number of thoracic vertebrae averages 14; a late-maturation color change occurs in the female; the male part in the great call is concentrated at the end.

Subgenus Symphalangus Diploid chromosome number: 50; cranial length is great (a mean of about 91

mm); nasal bones are flat; a prominent laryngeal sac is possessed by both sexes;

86 Prouty et a1

intermembral index is high (skeletal index: mean is approximately 145); number of thoracic vertebrae averages 13; a late-maturation color change does not occur in the female; male and female vocalizations are interspersed in the great call.

ACKNOWLEDGMENTS We would like to acknowledge the generous technical assistance of Susan Max-

well, Gibbon and Gallinaceous Bird Center; James Whitehead, Department of Biol- ogy, University of North Carolina, Chapel Hill; Dr. Gene Albrecht, University of Southern California; and Miriam McCollom, Chapel Hill, NC. We have profited from the advice and encouragement of Dr. Joe Marshall, US Fish and Wildlife Service, Washington, D C; Drs. Reinhard Rieger and Alan Feduccia, Department of Biology, University of North Carolina; Gary Rusch-Behrend, Indiana University; and Dr. Elliott Haimoff, Los Angeles, CA. Our work was supported in part by a grant from the North Carolina Department of Human Resources, a medical faculty research grant, University of North Carolina at Chapel Hill, and a grant from Sigma Xi.

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