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

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.