I was recently asked by a vet friend of mine for a list of all the pedigree breeds that are ‘brachycephalic’. This should have been a simple task,
I was recently asked by a vet friend of mine for a list of all the pedigree breeds that are ‘brachycephalic’. This should have been a simple task, which I expect she thought someone researching dog skull morphology would be able to send across in a few minutes. But, despite the term being widely used in the veterinary world and ever growing in popularity, a single list has yet to be established. Largely, this is due to the discrepancy among veterinary scientists and researchers as to what exactly brachycephalic should mean for a dog, how we should go about measuring it and hence classifying dogs by this term. The word brachycephalic has recently become a more familiar term outside of the field of skull morphology. This is likely due to the combination of increasing popularity of dog breeds with very short noses (pugs and French bulldogs) as well as the serious health conditions associated with these breeds.
Originally, a term used to define the skull shape of humans, brachycephalic (meaning broad headed in Greek, βραχυκεφαλικό where βραχύς=Broad and κεφαλή=head) is categorised when the breadth is at least 80 per cent of the length. Brachycephalic e.g. bulldogs, mastiffs, comes as the first of three skull shape categories, the others being mesocephalic or sometimes mesaticephalic (Greek for middle headed, Μεσοκεφαλικο where μέσος=middle and κεφαλή=head) e.g. Labradors, Dalmatians and dolichocephalic (Greek for long headed, Δολιχοκεφαλικο where δόλιχος=long and κεφαλή=head) e.g. rough collies, greyhounds, each defined by the proportion of skull breadth to length. There is much disparity as to the precise separation between each of these categories (Hussein, Sullivan and Pendris, 2012). Most use the rule that width to length ratio of the head should be the guide. If this is to be considered the defining criterion, using data collected by Carrasco et al. (2014) of the cephalic indices of pedigree dogs, the following list would be created:
- Shih Tzu
- Tibetan Spaniel
- British Bulldog
- Boston Terrier
- Griffon Bruxellios
- French Bulldog
The data collected by these authors however, only lists 80 dog breeds (around 65% of the total UK Kennel Club list) so could have omitted influential breeds. The study was also carried out using the Australian Kennel Club breed standards, which could differ from those in the UK. Also, as already stated, this method of categorising dogs as brachycephalic has been criticised as insufficient.
Originating in human medicine, this technique is not designed for categorising dog skulls. The initial method of calculating the ratio of breadth to width includes only measurements of the cranium itself. The cephalic index, as it is known, neglects to include any aspect of the rostrum, a fundamental region for distinguishing dog skulls since they can vary so greatly. It is not completely clear from the methods of the study used for the CI data whether rostrum length was included. This could obviously have a substantial impact on the results. It is therefore becoming increasingly evident that three categories are overly simplistic for the diversity of dog skull shapes and a more detailed morphometric classification is required (Georgevsky et al., 2014).
At present, there is a great deal of research interest in the potential relationship between a dog’s skull shape and its health. Studies have investigated various disorders such as Syringomyelia, as well as bite force, breathing and behaviour (Ellis et al., 2009; Koch et al., 2003; Knowler et al., 2014; McGreevy et al., 2013) and the relationship of these to skull shape. There is currently a lack of evidence exploring the possible relationship between this and periodontal diseases or dental disorders and skull shape. Even though each of the studies looking into skull morphometrics intends to do so reliably, there appears to be a lot of disparity within the field as to which anatomical regions should be included in the measurements. None of the measures currently in use are scientifically validated and results may therefore be inconsistent. Without a clear system to cover all exhibited phenotypes, any further investigation of dog skulls becomes severely hindered. With the creation of a scientific measure of canine skull morphometry and distinct set of categories, correlation between skull shape and health-related factors would be simpler