Scaling of chew cycle duration in primates

Abstract

The biomechanical determinants of the scaling of chew cycle duration are important components of models of primate feeding systems at all levels, from the neuromechanical to the ecological. Chew cycle durations were estimated in 35 species of primates and analyzed in conjunction with data on morphological variables of the feeding system estimating moment of inertia of the mandible and force production capacity of the chewing muscles. Data on scaling of primate chew cycle duration were compared with the predictions of simple pendulum and forced mass-spring system models of the feeding system. The gravity-driven pendulum model best predicts the observed cycle duration scaling but is rejected as biomechanically unrealistic. The forced mass-spring model predicts larger increases in chew cycle duration with size than observed, but provides reasonable predictions of cycle duration scaling. We hypothesize that intrinsic properties of the muscles predict spring-like behavior of the jaw elevator muscles during opening and fast close phases of the jaw cycle and that modulation of stiffness by the central nervous system leads to spring-like properties during the slow close/power stroke phase. Strepsirrhines show no predictable relationship between chew cycle duration and jaw length. Anthropoids have longer chew cycle durations than nonprimate mammals with similar mandible lengths, possibly due to their enlarged symphyses, which increase the moment of inertia of the mandible. Deviations from general scaling trends suggest that both scaling of the jaw muscles and the inertial properties of the mandible are important in determining the scaling of chew cycle duration in primates.

A Preliminary Analysis of Correlations between Chewing Motor Patterns and Mandibular Morphology across Mammals

“…Timing of peak muscle activity is quantified by taking the difference in time of peak activity for a muscle relative to the time of peak activity of the working-side superficial masseter (as a reference muscle) (Hylander and Johnson 1994;Hylander et al 2000). Because the length of the chewing cycle changes with body size (Fortelius 1985;Druzinsky 1993;Gerstner and Gerstein 2008;Ross et al 2008), we scaled the timing of peak muscle activity by the total amount of time between the first and last jaw adductors to reach peak activity during a chewing cycle. Future work should consider variation in timing relative to duration of the chewing cycle, but this variable was not available for most species in this analysis.…”

Mammalian Oral Rhythms and Motor Control

“…The development of X-ray reconstruction of moving morphology (XROMM) has been used to study oral movements and promises to revolutionize comparative biomechanical studies of oral function (Brainerd et al, 2010). It is also possible to track certain jaw movement features with marker-less methods (Gerstner & Goldberg, 1994;Ross et al, 2009). This is possible, because most species remain relatively motionless while masticating.…”

“…3 only shows spindle afferents in the masseter, a jaw closer (A S ), and high (A HM ) and low (A LM ) threshold mechanoreceptors from the periodontal ligament of a lower molar. The spindle and mechanoreceptor afferents are believed to play important roles in minimizing tooth breakage and wear during mastication among other functions (Ross et al, 2009). Feedback from afferents is carefully modulated by CPG circuitry throughout the masticatory cycle (for review, see Lund, 1991).…”

“…Chewing cycle duration, T C , scales with body mass across mammalian species (Druzinsky, 1993;Gerstner & Gerstein, 2008;Ross et al, 2009). The scaling takes the mathematical form:…”

“…Among mammals, the scaling exponent, b, ranges from 0.14 -0.20, when y = T C and M = M B (Druzinsky, 1993;Gerstner & Gerstein, 2008). Among primates, the scaling exponent ranges from 0.514 -0.583, when y = T C and M = L J (Ross et al, 2009). The slope or scaling exponent is important for several reasons.…”