Jaw‐Muscle Fiber Architecture and Leverage in the Hard‐Object Feeding Sooty Mangabey are not Structured to Facilitate Relatively Large Bite Forces Compared to Other Papionins

Abstract

Numerous studies have sought to link craniofacial morphology with behavioral ecology in primates. Extant hard-object feeders have been of particular interest because of their potential to inform our understanding about the diets of early fossil hominins. Sooty mangabeys (Cercocebus atys) are hard-object feeders that frequently generate what have been described as audibly powerful bites at wide jaw gapes to process materially stiff and hard seeds. We address the hypothesis that sooty mangabeys have features of the masticatory apparatus that facilitate this feeding behavior by comparing fiber architecture and leverage of the masseter and temporalis muscles between sooty mangabeys and three papionin primates that do not specialize on hard objects. Contrary to predictions, sooty mangabeys do not have relatively larger muscle physiologic cross-sectional areas or weights compared to other papionins, nor do they consistently display improved leverage. In this regard, sooty mangabeys differ in their morphology from other hard-object feeders such as tufted capuchins. However, males of all four papionin species converge on a shared pattern of relatively longer anterior superficial masseter fibers compared with female conspecifics, suggesting that males are likely prioritizing muscle stretch to improve gape performance as part of a behavioral repertoire that includes agonistic social interactions and intense male-male competition. These findings strengthen support for the hypothesis that gape display behaviors can exert a strong selective influence throughout the musculoskeletal masticatory apparatus. Results also raise questions about the morphological suitability of extant cercopithecines as models for interpreting feeding behavior and diet in fossil hominins with limited jaw gape capacity. Anat Rec, 301:325-342, 2018. © 2018 Wiley Periodicals, Inc.

Diet, feeding behavior, and jaw architecture of Taï monkeys: Congruence and chaos in the realm of functional morphology

“…At least one contributing factor is the reflexive assumption that the soft tissue aspect of the masticatory mechanical environment also reliably tracks exceptional dietary demands. Taylor and coauthors discovered that relative to other papionins, sooty mangabeys enjoy no clear mechanical advantages in terms of skull architecture (relating to adductor leverage) or fiber architecture (relating to muscle force‐generating capacity) …”

“…A fair criticism of much past comparative work is that feeding behavior is often summarized in terms of a simple dietary metric . In this scheme, folivores are challenged by tough foods, durophagous primates must contend with an obdurate diet, and frugivores face relatively minor mechanical challenges.…”

“…Nevertheless, we will speculate in the tradition of teleological thinking as the hallmark of functional morphology. Hylander surmised that the most important mechanical threat to mandibular integrity was fatigue failure . Holes in bone, such as alveoli, are nominally stress raisers, and high stresses in combination with frequent loads of even modest severity are a recipe for damage accumulation.…”

“…Taylor and coauthors discovered that relative to other papionins, sooty mangabeys enjoy no clear mechanical advantages in terms of skull architecture (relating to adductor leverage) or fiber architecture (relating to muscle force-generating capacity). 18 A fair criticism of much past comparative work is that feeding behavior is often summarized in terms of a simple dietary metric. 8,15,16,18 In this scheme, folivores are challenged by tough foods, Larger when scaled to a measure of mandibular length.…”

Behavioral Correlates of Cranial Muscle Functional Morphology

“…One such study is the next paper in the issue, submitted by Taylor et al (2018), in which they examine similar questions of how diet correlates with masticatory muscle architecture in the durophagous sooty mangabey. While many studies of masticatory architecture sacrifice intraspecific depth of analysis for interspecific breadth (e.g., the two papers that we lead in this issue), Taylor et al use an admirable sample of their target species and relatively large samples of a few other species for comparative purposes.…”