Alterations to vestibular morphology in highly bred domestic dogs may affect balance

The modern domestic dog (Canis lupus familiaris) provides an excellent model to examine the effects of cranial modification. Extreme variation in skull length among dog breeds due to high levels of selective breeding is known to be linked to disorders of the head and neck. Such alteration may also influence sensory organs including those of the vestibular system (VS), one of the most fundamental sense organs, essential in maintaining balance. Studies in mammals have shown that orientation of ipsilateral semicircular canals (SCCs) of the VS at right angles (orthogonality) is related to angular acceleration sensitivity. Due to their considerable variation in craniofacial form while exhibiting similar locomotion, domestic dogs provide an excellent natural experiment to examine if cranial alteration influences VS functional morphology. Our methods examine how change in cranial base length across dog breeds relates to SCC orthogonality using linear modeling and analyses of variance. The sample studied (29 bony labyrinths of 17 dog breeds) was obtained from a previous study on canid inner ear metrics. Results support the hypothesis that orthogonality between the anterior and posterior SCC + ampulla significantly correlates with cranial base length. This suggests a close relationship between the orientations of SCCs with their ampullae and cranial structure among dog breeds. Specifically, highly derived breeds, such as the brachycephalic pug, have anterior and posterior SCCs and ampullae that deviate the most from orthogonality. Therefore, such highly bred domestic dogs may also have altered vestibular function due to compressed cranial form.