Size and shape dimorphism in great ape mandibles and implications for fossil species recognition

Abstract

Sexual dimorphism is an important source of morphological variation, and species differences in dimorphism may be reflected in magnitude, pattern, or both. While the extant great apes are commonly used as a reference sample for distinguishing between sexual dimorphism and intertaxic variation in the fossil record, few studies have evaluated mandibular dimorphism in these taxa. In this study, percentage, degree, and pattern of mandibular dimorphism are evaluated in Pongo, Gorilla, and Pan. Mandibular dimorphism patterns are explored to determine the extent to which such patterns accurately track great ape phylogeny. Pattern stability is assessed to determine whether there are stable patterns of mandibular size and shape dimorphism that may be usefully applied to hominoid or hominid fossil species recognition studies. Finally, the established patterns of dimorphism are used to address recent debates surrounding great ape taxonomy. Results demonstrate that mandibular dimorphism is universally expressed in size, but only Pongo and Gorilla exhibit shape dimorphism. Pattern similarity tends to be greater between subspecies of the same species than between higher-order taxa, suggesting that within the great apes, there is a relationship between dimorphism pattern and phylogeny. However, this relationship is not exact, given that dimorphism patterns are weakly correlated between some closely related taxa, while great ape subspecies may be highly correlated with taxa belonging to other species or genera. Furthermore, dimorphism patterns are not significantly correlated between great ape genera, even between Gorilla and Pan. Dimorphism patterns are more stable in Gorilla and Pongo as compared to Pan, but there is little pattern stability between species or genera. Importantly, few variables differ significantly between taxa that simultaneously show consistently relatively low levels of dimorphism and low levels of variation within taxa. Combined, these findings indicate that mandibular dimorphism patterns can and do vary considerably, even among closely related species, and suggest that it would be difficult to employ great ape mandibular dimorphism patterns for purposes of distinguishing between intra- and interspecies variation in fossil samples. Finally, the degree of pattern similarity in mandibular dimorphism is lower than previously observed by others for craniofacial dimorphism. Thus, the possibility cannot be ruled out that patterns of craniofacial dimorphism in great apes may be associated with a stronger phylogenetic signal than are patterns of mandibular dimorphism.

Variations of the mandibular shape in extant hominoids: Generic, specific, and subspecific quantification using elliptical fourier analysis in lateral view

“…Our results parallel previously well-established differences in patterns of mandibular and craniofacial dimorphism in hominoids, (Wood et al, 1991;Humphrey et al, 1999;Daegling and Jungers, 2000;Taylor, 2006). According to Taylor (2006), only Pongo and Gorilla exhibited a significant mandibular shape dimorphism, while Pan showed no significant differences. O'Higgins and Dryden (1993) similarly demonstrated significant sex differences in craniofacial shape reflected in Gorilla and Pongo but not in Pan.…”

“…The significant shape dimorphism depicted in P. p. pygmaeus is consistent with the marked dimorphism already described for the mandibles (Weidenreich, 1936;Humphrey et al, 1999;Taylor, 2006), the cranium (Hens, 2002(Hens, , 2003, and the facial complex (Guy et al, 2003). Given that P. p. abelii also evidences a marked sexual dimorphism (Plavcan and van Schaik, 1997;Plavcan, 2002;Taylor, 2006), the lack of significant sexual differences of the mandibular profile may be explained by sampling error and lower statistical power due to the small sample size (Table 1).…”

Using photogrammetry and color scoring to assess sexual dimorphism in wild western gorillas (Gorilla gorilla)

“…Here we could demonstrate that measures that revealed the highest ISD also showed the highest within-sex-variation. Facial height dimorphism reported here (ISD: 21.3) is similar to findings of other studies (ISD: 19.9 (Lockwood, 1999;Schaefer et al, 2004); ISD: 20 (Schaefer et al, 2004)), but slightly larger compared with mandibular size dimorphism (ISD: 18.3 (Taylor, 2006)). Interestingly facial height dimorphism revealed in our study is very similar to our estimate of body length dimorphism and could indicate isometric scaling, although within silverbacks there was no influence of body length on facial height and cranial height measures.…”

Sexual Dimorphism Using Geometric Morphometric Approach

“…In contrast, sexual shape dimorphism (SShD) has been much less investigated [17][18][19]36]. From those studies that considered SShD, most have discussed it as a diagnostic trait for diverse purposes, such as sex identification or the analysis of ontogeny [37][38][39][40]. Nevertheless, some other authors have considered sexual dimorphism evolution covering only some aspects of a limited number of taxa, such as: the evolution of cranium in primates [41][42][43][44]; the proportions and dimensions in lizard bodies [45,46]; newts [47]; or in flies [48]; and variation of shape in insect heads [49]; and variation of sexual dimorphism in Drosophila wings [36].…”