The Impact of Selection for Facial Reduction in Dogs: Geometric Morphometric Analysis of Canine Cranial Shape

Abstract

Through artificial selection, humans have altered the morphology of domestic dogs and created the range of morphological traits present in the diverse dog breeds seen today. This article tests how artificial breeding for extreme facial reduction affects the craniodental complex in domestic dogs (Canis lupus familiaris). These brachycephalic dog breeds allow for a natural experiment on facial reduction, providing additional insight into this well‐studied phenomenon associated with the evolutionary history of other animals including primates. We used an existing set of clinically obtained CT scans from dogs representing three brachycephalic breeds and a comparative sample. We recorded three‐dimensional coordinate data for 62 cranial and mandibular landmarks that allowed us to analyze differences in shape in the neurocranium, basicranium, and viscerocranium. Through geometric morphometric analysis, we confirmed that most of the cranial shape variance in our sample was associated with cephalic index (cranial width‐to‐length ratio) and reflects the effects of facial reduction. Shape changes in the cranium and mandible co‐vary. We also identified several areas of localized shape change. Whereas the primary shape change seen in the mandible is a uniform shift in proportion involving rostral‐caudal shortening and medial‐lateral widening, the majority of the shape change in the palate is localized to the maxilla. Here, a pronounced difference in maxillary carnassial orientation relative to the mandibular carnassials results in disruption of the functionally important carnassial complex. These results support previous studies showing integration within the skull and highlight the deleterious effects of artificial selection for extreme facial reduction in domestic dogs. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc. Anat Rec, 303:330–346, 2020. © 2019 American Association for Anatomy

Facial changes related to brachycephaly in Cavalier King Charles Spaniels with Chiari‐like malformation associated pain and secondary syringomyelia

“…Canine brachycephalic conformation typically includes foreshortening of the facial skeleton (muzzle) and not necessarily incudes the cranium . The results of discriminant analysis gave 75.8% correctly classified grouping with SM‐S highest separation of 92.5%, together with the predictive statistics of ML, consolidate the concept that a deep stop is a risk factor for SM‐S and CM‐P.…”

From head to hind: Elucidating function through contrasting morphometrics of ancient and modern pedigree dogs

“…In general, dogs with short faces have spheroid‐shaped vaults. Conversely, dogs with longer faces such as the borzoi have neurocrania that are elongated along the rostrocaudal axis (Drake, 2011; Marchant et al, 2017; Schoenebeck et al, 2012; Selba, Oechtering, Heng, & DeLeon, 2019). The Warmington Roman dog's positive ncPC1 and negative ncPC2 scores places it within morphospace that is also occupied by medium‐sized herding or hunting dogs.…”

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

“…Extreme variation has already been demonstrated for the skull (Drake and Klingenberg 2010;Selba et al 2019) and is generally considerd to be the result of intensive dog breeding and artificial selection for aesthetic reasons. Our study demonstrates that the masticatory muscles and the shape of the mandible also show important variation related to variation in the size of the individuals and the type of breed.…”

“…As a consequence of several thousand years of artificial selection and inbreeding, the domestic dog has the highest variability in skull shape within the Carnivora (Drake and Klingenberg 2010;Selba et al 2019) and encompasses over breeds according to kennel clubs. The shapes extend beyond the variability of wild species (Drake and Klingenberg 2010), varying from elongated and narrow skull shapes (dolichocephalic) to short and wide (brachycephalic) skulls.…”

“…For example, the mutation of BMP3 has been shown to be involved in brachycephaly (Schoenebeck et al 2012). Integration and modularity have also been extensively studied within the skull and even the mandible (Drake and Klingenberg 2010;Meloro et al 2011;Curth et al 2017;Curth 2018;Machado et al 2018;Selba et al 2019). However, the functional impact of this extraordinary variability in shape has received less attention in dogs (but see Ström et al 1988;Endo et al 1999;Koch et al 2003;Ellis et al 2008Ellis et al , 2009.…”