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

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.