How Does Masticatory Muscle Architecture Covary with Mandibular Shape in Domestic Dogs?

Abstract

Despite the considerable scientific interest in the variability and patterns of integration in the dog skull, how these patterns impact or are driven by function remains largely unexplored. Since the mandible is directly involved in mastication, it can be expected to be directly related to the development of the adductor and abductor muscles. Here, we explore whether variation in the architecture and size of the masticatory muscles is associated with the variation in mandibular shape in dogs. We obtained muscle data from the dissection of 48 dogs from different breeds and morphotypes to explore the architecture of the muscles and used 3D geometric morphometric approaches to quantify the shape of the mandible. Covariations between the masticatory muscles and mandibular shape were explored using two-block partial least square analyses (2B-PLS). Our results show there is a strong covariation between mandibular shape and masticatory muscles mass (rPLS from 0.70 to 0.74 for the first axis representing more than 90% of the total covariance) and physiological cross-sectional area (rPLS from 0.64 to 0.73 for the first axis representing more than 80% of the total covariance), irrespective of whether size is taken into account or not. These results suggest muscle size and thus attachment area requirements for individual muscles are likely drivers of mandibular shape. Moreover, mandible shape is likely to be a good predictor of muscle force. Finally, it appears that domestication of dogs has not resulted in a disuse phenotype characterized by a decoupling between form and function.

Bite force and its relation to jaw shape in domestic dogs

“…We focused on the adductor muscles of the jaw only, because they are involved in mouth closing and bite force generation. The m. pterygoideus medialis and lateralis were considered together because the m. pterygoideus lateralis is very small and difficult to clearly distinguish (Brassard et al, 2020). We considered the constituent bellies of the following jaw adductor muscles (following Penrose et al, 2016): m. masseter pars superficialis (MS), m. masseter pars profunda (MP), m. zygomaticomandibularis anterior (ZMA), m. zygomaticomandibularis posterior (ZMP), m. temporalis pars suprazygomatica (SZ), m. temporalis pars superficialis (TS), m. temporalis pars profunda (TP) and m. pterygoideus (P).…”

“…We considered the constituent bellies of the following jaw adductor muscles (following Penrose et al, 2016): m. masseter pars superficialis (MS), m. masseter pars profunda (MP), m. zygomaticomandibularis anterior (ZMA), m. zygomaticomandibularis posterior (ZMP), m. temporalis pars suprazygomatica (SZ), m. temporalis pars superficialis (TS), m. temporalis pars profunda (TP) and m. pterygoideus (P). The mass and the PCSA were measured from dissections (Brassard et al, 2020). In a previous study, we measured muscle mass using a digital scale (Mettler Toledo AE100) and then we sectioned the muscle along its long axis to measure fibre lengths and pennation angles directly on the muscle.…”

“…R version 3.6.0 (2019-04-26; http://www.R-project.org/) was used for all statistical analyses. For the lower jaw, we considered 25 landmarks, 190 sliding semilandmarks on curves and 185 sliding semi-landmarks on surfaces that were placed on 3D reconstructions of the lower jaw obtained using photogrammetry (Brassard et al, 2020; the 3D models of the mandibles are available on request from the corresponding author). A 3D sliding semi-landmark procedure was performed to transform all the landmarks into spatially homologous landmarks (Bookstein, 1997;Gunz et al, 2005).…”

“…rostral border of the right mandible, just medially to the first incisor, and the y-axis is directed towards the top of the skull and perpendicular to x. The approximate centroid of the origin and insertion areas of the muscles were used, based on observations from our dissections (Brassard et al, 2020). We also recorded the 3D coordinates of three possible points of application of the food reaction force: at the incisor teeth, at the lower canine tooth and at the lower carnassial tooth ( point between the fourth upper premolar tooth P 4 and the first lower molar tooth M 1 ).…”

“…The extraordinary variability in the size and shape of the head (Brassard et al, 2020;Coppinger and Coppinger, 2001; Drake and Klingenberg, 2010;Miller et al, 1965;Selba et al, 2019;Wayne, 1986Wayne, , 2001, and jaw muscle architecture (Brassard et al, 2020) between dog breeds raises questions about the impact of this variability on the function of the masticatory system and bite performance. Differences in skull shape between breeds have been suggested to be associated with differences in jaw strength (Case, 2013) and bite force (Ellis et al, 2008(Ellis et al, , 2009 as the shape of the neurocranium drives the size of the jaw muscles and the length and shape of the jaws determine the out-and in-lever arms of the system.…”