Numerous hypotheses have been advanced to explain relative increases in brain size in primates and other mammals. However, notably less attention has been directed towards addressing the biological limits to increasing brain size. Here we explore variation in brain size in orangutans. We evaluated both raw and size-adjusted cranial capacity (CC) in adult Pongo pygmaeus pygmaeus (n=147), P. p. wurmbii (n=24), P. p. morio (n=14), and P. abelii (n=36). Results demonstrate significant variation in CC among orangutan taxa. Population differences in raw CC are significant for females (p=0.014) but not males. Post-hoc pairwise comparisons among females further reveal that raw CC is significantly smaller in P. p. morio compared to both P. abelii and P. p. pygmaeus. When evaluated for proportionality, geometric equivalence in CC is not maintained in orangutans, as P. p. morio has a significantly smaller CC when compared to one or more other orangutan groups. Even after statistically partitioning size and size-correlated shape, P. p. morio has a significantly smaller CC compared to most other orangutan groups. These observed differences in relative brain size are consistent with known variation in resource quality and life history amongst orangutan populations. Specifically, P. p. morio is characterized by the least productive habitat, the lowest energy intake during extended lean periods, and the shortest interbirth intervals. Our results, therefore, provide conditional support for the hypothesis that decreased brain size is related to prolonged episodes of food scarcity, and suggest a correlation between brain size, diet quality, and life history at the lowest macroevolutionary level. The association of a relatively small brain and poor diet quality in Pongo further suggests that ecological factors may plausibly account for such a reduction in brain size as observed in the recently recovered Homo floresiensis from Indonesia.
Orangutans are amongst the most craniometrically variable of the extant great apes, yet there has been no attempt to explicitly link this morphological variation with observed differences in behavioral ecology. This study explores the relationship between feeding behavior, diet, and mandibular morphology in orangutans. All orangutans prefer ripe, pulpy fruit when available. However, some populations of Bornean orangutans (Pongo pygmaeus morio and P. p. wurmbii) rely more heavily on bark and relatively tough vegetation during periods of low fruit yield than do Sumatran orangutans (Pongo abelii). I tested the hypothesis that Bornean orangutans exhibit structural features of the mandible that provide greater load resistance abilities to masticatory and incisal forces. Compared to P. abelii, P. p. morio exhibits greater load resistance abilities as reflected in a relatively deeper mandibular corpus, deeper and wider mandibular symphysis, and relatively greater condylar area. P. p. wurmbii exhibits most of these same morphologies, and in all comparisons is either comparable in jaw proportions to P. p. morio, or intermediate between P. p. morio and P. abelii. These data indicate that P. p. morio and P. p. wurmbii are better suited to resisting large and/or frequent jaw loads than P. abelii. Using these results, I evaluated mandibular morphology in P. p. pygmaeus, a Bornean orangutan population whose behavioral ecology is poorly known. Pongo p. pygmaeus generally exhibits relatively greater load resistance capabilities than P. abelii, but less than P. p. morio. These results suggest that P. p. pygmaeus may consume greater amounts of tougher and/or more obdurate foods than P. abelii, and that consumption of such foods may intensify amongst Bornean orangutan populations. Finally, data from this study are used to evaluate variation in craniomandibular morphology in Khoratpithecus piriyai, possibly the earliest relative of Pongo from the late Miocene of Thailand, and the late Pleistocene Hoa Binh subfossil orangutan recovered from Vietnam. With the exception of a relatively thicker M(3) mandibular corpus, K. piriyai has jaw proportions that would be expected for an extant orangutan of comparable jaw size. Likewise, the Hoa Binh subfossil does not differ in skull proportions from extant Pongo, independent of the effects of increase in jaw size. These results indicate that differences in skull and mandibular proportions between these fossil and subfossil orangutans and extant Pongo are allometric. Furthermore, the ability of K. piriyai to resist jaw loads appears to have been comparable to that of extant orangutans. However, the similarity in jaw proportions between P. abelii and K. piriyai suggest the latter may have been dietarily more similar to Sumatran orangutans.
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.
Dietary consistency has been shown to influence cross-sectional area and fiber type composition of the masticatory muscles. However, little is known about the effects of dietary consistency on masticatory muscle fiber architecture. In this study, we explore the effects of dietary consistency on the internal architecture of rabbit masseter muscle. Because activity patterns of the rabbit chewing muscles show inter- and intramuscular heterogeneity, we evaluate if alterations in fiber architecture are homogeneous across various portions of the superficial masseter muscle. We compared masseter muscle fiber architecture between two groups of weanling rabbits raised on different diets for 105 days. One group was raised on a diet of ground rabbit pellets to model underuse of the masticatory complex, while the other group was fed a diet of intact pellets and hay blocks to model an overuse diet. In all portions of the superficial masseter, physiological cross-sectional areas (PCSAs) are greater in the overuse compared to underuse diet rabbits. Thus, the mechanical demands for larger muscle and bite forces associated with early and prolonged exposure to a tough diet are met by an increase in PCSA of the superficial masseter. The larger PCSA is due entirely to increased muscle mass, as the two rabbit groups show no differences in either fiber length or angle of pinnation. Thus, increasing pinnation angle is not a necessary biomechanical solution to improving muscle and bite force during growth. The change in PCSA but not fiber length suggests that variation in dietary consistency has an impact on maximum force production but not necessarily on excursion or contraction velocity.
Common marmosets (Callithrix jacchus) and cotton-top tamarins (Saguinus oedipus) (Callitrichidae, Primates) share a broadly similar diet of fruits, insects, and tree exudates. Common marmosets, however, differ from tamarins by actively gouging trees with their anterior teeth to elicit tree exudate flow. During tree gouging, marmosets produce relatively large jaw gapes, but do not necessarily produce relatively large bite forces at the anterior teeth. We compared the fiber architecture of the masseter muscle in tree-gouging Callithrix jacchus (n = 10) to nongouging Saguinus oedipus (n = 8) to determine whether the marmoset masseter facilitates producing these large gapes during tree gouging. We predict that the marmoset masseter has relatively longer fibers and, hence, greater potential muscle excursion (i.e., a greater range of motion through increased muscle stretch). Conversely, because of the expected trade- off between excursion and force production in muscle architecture, we predict that the cotton-top tamarin masseter has more pinnate fibers and increased physiological cross- sectional area (PCSA) as compared to common marmosets. Likewise, the S. oedipus masseter is predicted to have a greater proportion of tendon relative to muscle fiber as compared to the common marmoset masseter. Common marmosets have absolutely and relatively longer masseter fibers than cotton-top tamarins. Given that fiber length is directly proportional to muscle excursion and by extension contraction velocity, this result suggests that marmosets have masseters designed for relatively greater stretching and, hence, larger gapes. Conversely, the cotton-top tamarin masseter has a greater angle of pinnation (but not significantly so), larger PCSA, and higher proportion of tendon. The significantly larger PCSA in the tamarin masseter suggests that their masseter has relatively greater force production capabilities as compared to marmosets. Collectively, these results suggest that the fiber architecture of the common marmoset masseter is part of a suite of features of the masticatory apparatus that facilitates the production of relatively large gapes during tree gouging.