Anatomical Contributions to Hylobatid Taxonomy and Adaptation

Abstract

Compared with the great apes, the small-bodied hylobatids were treated historically as a relatively uniform group with 2 genera, Hylobates and the larger-bodied Symphalangus. Four genera are now recognized, each with a different chromosome number: Hoolock (hoolock) (38), Hylobates (44), Nomascus (crested gibbon) (52), and Symphalangus (siamang) (50). Previous morphological studies based on relative bone lengths, e.g., intermembral indices; molar tooth sizes; and body masses did not distinguish the 4 genera from each other. We applied quantitative anatomical methods to test the hypothesis that each genus can be differentiated from the others using the relative distribution of body mass to the forelimbs and hind limbs. Based on dissections of 13 hylobatids from captive facilities, our findings demonstrate that each of the 4 genera has a distinct pattern of body mass distribution. For example, the adult Hoolock has limb proportions of nearly equal mass, a pattern that differentiates it from species in the genus Hylobates, e.g., H. lar (lar gibbon), H. moloch (Javan gibbon), H. pileatus (pileated gibbon), Nomascus, and Symphalangus. Hylobates is distinct in having heavy hind limbs. Although Symphalangus has been treated as a scaled up version of Hylobates, its forelimb exceeds its hind limb mass, an unusual primate pattern otherwise found only in orangutans. This research provides new information on whole body anatomy and adds to the genetic, ecological, and behavioral evidence for clarifying the taxonomy of the hylobatids. The research also underscores the important contribution of studies on rare species in captivity.

Functional Anatomy and Adaptation of Male Gorillas (Gorilla gorilla gorilla) With Comparison to Male Orangutans (Pongo pygmaeus)

“…Orangutans do not resemble the African apes; they show correspondences with the siamang in having heavier forelimbs than hindlimbs, but curiously, not the gibbons (Hylobates) (Schultz, 1933;Zihlman et al, 2011). Overall, the orangutan hind limb is unique among catarrhine primates in its extreme flexibility and in the mass and length of the foot that extends its reach.…”

“…For example, addition of segment mass for comparison to linear dimensions of ape fossils can take the evidence beyond an exclusive dependence on osteology and expand potential interpretations. Furthermore, application of soft tissue data can serve as another line of evidence along with molecular, dental-cranial, and skeletal to contribute to phylogeny (Pilbeam, 2000;Gibbs et al, 2000Gibbs et al, , 2002Zihlman et al, 2011).…”

“…Few comparative studies focus on joints and the associated soft tissue; both are important, but the opportunity for their combined study is rare (e.g., WrightFitzgerald et al, 2010;Davis, 1961;Zihlman et al, 2011). Whole animals provide multiple ways to analyze and compare, for example body segments relative to total body mass (TBM), forelimb to hind limb mass, joint configuration, and corresponding muscles.…”

Nomascus leucogenys (Primates: Hylobatidae)

“…For example, mean of 6 wild-shot males was 7.27 kg ± 0.44 SD and mean of 2 wild-shot females was 7.65 ± 0.21 kg (Geissmann 1993). However, in captivity mass may be larger (e.g., a 9.5-kg female- Zihlman et al 2011). In N. siki and N concotor males had slightly larger body masses than females, suggesting that the apparently smaller body masses in males of N. /eucogenys were due to variation in a small sample size.…”

“…Shoulder joints are always flexed during takeoff (Channon et al 2010). Zihlman (2011) found that N. leueogenys has the heaviest body and limb proportions among the Hylobatidae (other genera of Nomascus were not included in the analysis), suggesting greater power in shoulder, hip, and knee motions. These capabilities allow N. leueogenys to "progress, like gymnasts, by means of their long and powerful arms, and throw themselves from one branch to another for distances of thirty feet and more" (Delacour 1933:72).…”