Masticatory Loading and Ossification of the Mandibular Symphysis during Anthropoid Origins

Jaw-Muscle Structure and Function in Primates: Insights Into Muscle Performance and Feeding-System Behaviors

The jaw-adductor muscles drive the movements and forces associated with primate feeding behaviors such as biting and chewing as well as social signaling behaviors such as wide-mouth canine display. The past several decades have seen a rise in research aimed at the anatomy and physiology of primate chewing muscles to better understand the functional and evolutionary significance of the primate masticatory apparatus. This review summarizes variation in jaw-adductor fiber types and muscle architecture in primates, focusing on physiological, architectural, and behavioral performance variables such as specific tension, fatigue resistance, muscle and bite force, and muscle stretch and gape. Paranthropus and Australopithecus are used as one paleontological example to showcase the importance of these data for addressing paleobiological questions. The high degree of morphological variation related to sex, age, muscle, and species suggests future research should bracket ranges of performance variables rather than focus on single estimates of performance.

The effects of feeding frequency on jaw loading in two lemur species

OBJECTIVES

Studies on oral processing are often snapshots of behaviors that examine feeding through individual bouts. In this study, we expand on our previous work comparing bite/chew variables per feeding bout to summed daily biting, chewing, and food intake to interpret loading that could have potential morphological effects.

MATERIALS AND METHODS

We observed sympatric Lemur catta and Propithecus verreauxi over two field seasons in the dry forest of Bezà Mahafaly Special Reserve in southwestern Madagascar. Bite and chew rates determined from videos filmed during observations were multiplied with time spent feeding on specific foods during focal follows to calculate daily values for each feeding bout. Food mechanical properties (FMPs) were tested on dietary items with a portable tester. We contrasted daily bite/chew numbers and intake with FMPs, species, season, and food shape.

RESULTS

Daily bite and chew numbers increased with maximum, but not average, food toughness. Daily intake decreased with average and maximum toughness. Season had a strong effect on daily bites and chews, but not on intake. Food shape influenced intake and total bite and chew numbers. The lemur species did not differ in our models.

DISCUSSION

Maximum food toughness impacted feeding behaviors and intake, which is consistent with higher loads having a greater effect on morphology. In contrast to feeding per bout, cumulative biting and chewing did not differ between species; taking feeding frequency into consideration affects interpretation of jaw loading. Finally, biting, as much as chewing, may generate strains that impact morphology.

Symphyseal morphology and jaw muscle recruitment levels during mastication in musteloid carnivorans

In studies of mammalian mastication, a possible relationship has been proposed between bilateral recruitment of jaw adductor muscle force during unilateral chewing and the degree of fusion of the mandibular symphysis. Specifically, species that have unfused, mobile mandibular symphyses tend to utilize lower levels of jaw adductor force on the balancing (nonchewing) than the working (chewing) side of the head, when compared to related species with fused symphyses. Here, we compare jaw adductor recruitment levels in two species of musteloid carnivoran: the carnivorous ferret (unfused symphysis), and the frugivorous kinkajou (fused symphysis). During forceful chewing, we observe that ferrets recruit far more working-side muscle force than kinkajous, regardless of food toughness and that high working-to-balancing side ratios are the result of increased working-side force, often coupled with reduced balancing-side force. We propose that in carnivorans, high working-to-balancing side force ratios coupled with an unfused mandibular symphysis are necessary to rotate the hemimandible for precise unilateral occlusion of the carnassial teeth and to sustain laterally oriented force on the jaw to engage the carnassial teeth during shearing of tough foods. In contrast, the kinkajou's flattened cheekteeth permit less precise occlusion and require medially-oriented forces for grinding, thus, a fused symphysis is mechanically beneficial.