Dietary Correlates of Primate Masticatory Muscle Fiber Architecture

Abstract

Analyses of masticatory muscle architecture-specifically fascicle length (FL; a correlate of muscle stretch and contraction speed) and physiological cross-sectional area (PCSA; a correlate of force)-reveal soft-tissue dietary adaptations. For instance, consumers of large, soft foods are expected to have relatively long FL, while consumers of obdurate foods are expected to have relatively high PCSA. Unfortunately, only a few studies have analyzed these variables across large primate samples-an order of particular interest because it is our own. Previous studies found that, in strepsirrhines, force variables (PCSA and muscle masses; MM) scale with isometry or slight positive allometry, while the body size corrected FL residuals correlate with food sizes. However, a study of platyrrhines using different methods (in which the authors physically cut muscles between fascicles) found very different trends: negative allometry for both the stretch and force variables. Here, we apply the methods used in the strepsirrhine study (chemical dissection of fascicles to ensure full length measurements) to reevaluate these trends in platyrrhines and extend this research to include catarrhines. Our results conform to the previous strepsirrhine trends: there is no evidence of negative allometry in platyrrhines. Rather, in primates broadly and catarrhines specifically, MM and PCSA scale with isometry or positive allometry. When examining size-adjusted variables, it is clear that fascicle lengths (especially those of the temporalis muscle) correlate with diet: species that consume soft, larger, foods have longer masticatory fiber lengths which would allow them to open their jaws to wider gape angles. Anat Rec, 301:311-324, 2018. © 2018 Wiley Periodicals, Inc.

Bite Force and Masticatory Muscle Architecture Adaptations in the Dietarily Diverse Musteloidea (Carnivora)

“…This study aims to examine the masticatory apparatus of a taxonomically diverse sample of Musteloidea in relation to two dietary factors: food item size and mechanical resistance. Following previous work on the relationship between masticatory anatomy and diet in both carnivores (Hartstone‐Rose et al, ) and other mammalian taxa (Santana et al, ; Perry et al, ; Hartstone‐Rose et al, ), we predict that the architecture and mechanical efficiency of the jaw adductor muscles within Musteloidea will be heavily influenced by their dietary category.Hypothesis PCSA and Bite Force will scale with positive allometry relative to body size, such that larger species will be able to produce relatively larger masticatory forces. In other carnivoran lineages (Felidae and Canidae), PCSA and bite force were observed to scale with positive allometry relative to body size across a broad interspecific sample (Hartstone‐Rose et al, ).…”

“…FL was used in conjunction with muscle mass (MM) to calculate the PCSA of each muscle portion using the modified formula from Schumacher (): where q is PCSA (cm 2 ), m is MM (g), l is average FL (cm), and p is the specific density of muscle (1.0564 g/cm 3 ; Murphy and Beardsley, ). Although some colleagues “reduce” PCSA by the angles of pennation with the muscles (=RPCSA), there are several reasons that this is inadvisable in the masticatory apparatus (see Hartstone‐Rose et al, for a complete discussion of masticatory PCSA vs. RPCSA). Total MM and total PCSA across the adductor complex were also calculated by summing all adductor muscle portions from each specimen.…”

“…Total MM and total PCSA across the adductor complex were also calculated by summing all adductor muscle portions from each specimen. Additionally, a weighted average of FL across the adductor musculature was calculated using the formula from Hartstone‐Rose et al (, ): where FL X is the average FL for the jaw adductors, FL MS , FL TMP , and FL PT are the average FLs and m MS , m TMP , and m PT are the MMs for each respective muscle group (masseter, temporalis, and medial pterygoid respectively).…”

Behavioral Correlates of Cranial Muscle Functional Morphology

“…In the end, Fabre and colleagues find that muscle architecture has a stronger impact on cranial shape than mandibular shape, and, to no one's surprise, aye-aye's are weird! Next comes the contribution by Hartstone-Rose et al (2018) examining the muscle fiber architecture across a broad sample of primates. This paper follows similar work that Hartstone-Rose published with Jonathan Perry on strepsirrhines and felids.…”

Functional Morphology and Behavioral Correlates to Postcranial Musculature

“…Traditional approaches to muscle functional morphology research focus mainly on the gross anatomy of muscles and their attachment to bones, while recently researchers are exploring muscle functional morphology using new approaches, in particular, analyses of fiber architecture—more elaborate analyses of the arrangements and dimensions of the fascicles and fibers present in a muscle. A growing body of studies is coupling traditional gross anatomical dissections with chemical dissection (Perry et al, ; Hartstone‐Rose et al, ), Diffusible iodine‐based contrast‐enhanced computed tomography (diceCT) and virtual dissection (Gignac et al, ; Dickinson et al, ; Santana, ) providing deeper information on muscle fiber architecture.…”