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Skull morphology of the domestic dog in relation to cephalic index.

Authors:
  • Hartpury University

Abstract

There are 210 dog breeds recognised by the UK Kennel Club, showing a huge diversity in skull shape. Dog skulls have been sorted by Cephalic Index (CI) into three distinct categories (dolichocephalic, mesocephalic and brachycephalic). However, it is becoming increasingly evident that this is overly simplistic and a more detailed classification is needed. Aim/s To investigate the suitability of CI for domestic dogs. Method Skull measurements of individual canines (n = 107) were taken to calculate their CI, and they were grouped into skull category by breed. CI values were correlated using a Spearman’s rank test to both height : length ratio (HLR) and cranium : muzzle ratio (CMR). Results Results suggest a large discrepancy between expected CI ranges and breed level classifications. A significant positive correlation was found between CI and HLR (r=0.691, p<0.001; figure 1), and between CI and CMR (r=0.980, p<0.001; figure 2). Conclusion The discrepancies between breed skull classifications and CI measurements demonstrate that current systems are not effective for studying dog skulls. Muzzle length in relation to the cranium is responsible for most of the change seen in CI. However, this is not the only morphological feature changing, with skulls becoming deeper as they shorten. Possible Applications or Implications The current CI system is unsuitable for the diverse range of dog skull shapes. Changes in muzzle length between skull shapes implies that changes to bite and mastication pattern would be expected. This could have relevance to size and texture elements of diet design.
Moulton College • Moulton • Northampton • NN3 7RR • www.moulton.ac.uk
Results
Introduction
References
• Clutton-Brock, J. (1995) “Origins of the domestic dog: domestication
and early history In Serpell, J. (ed) The domestic dog: its evolution,
behaviour and interaction with people. Cambridge University Press:
Cambridge.
• Bierer, T.L. (n.d.) The Role of Biometrics in Canine Oral Health and
Nutrition. Pedigree.
The dog is the oldest domesticated species (Clutton-Brock, 1995; Vila et al., 1997).
It has evolved from the grey wolf to accommodate changes in diet quantity and quality
leading to a smaller body size and reduction in tooth size (Clutton-Brock, 1995), as well
as undergoing selective breeding for a range of phenotypes. There are now 210 dog breeds officially recognised by
the UK Kennel Club and in the region of 400 worldwide (Clutton-Brock, 1995; The Kennel Club, 2014). Once all
breeds and crossbreeds are accounted for, there is potential for an extensive range of skull shapes and sizes. Dog
skulls have traditionally been sorted into three distinct categories: dolichocephalic, mesocephalic and
brachycephalic. This can be done using a Cephalic Index (CI) or more normally by simple subjective sorting (often
at breed level). However, it is becoming increasingly evident that three categories is overly simplistic for the diversity
of skull shapes and a more detailed classification is needed (Georgevsky et al., 2014). The aim of this study was to
investigate the suitability of CI for domestic dogs.
Results suggest a large discrepancy between
CI ranges and breed level classifications
as well as between individuals (see table 1).
A significant positive correlation was found between CI and HLR
(r=0.691, p<0.001; figure 1), and between CI and CMR (r=0.980,
p<0.001; figure 2).
Skull morphology of the domestic dog in
relation to cephalic index.
K Andrews, J Lowe & WD McCormick
wanda.mccormick@moulton.ac.uk
• Georgevsky, D.; Carrasco, J. J.; Valenzuela, M. & McGreevy, P. D.
(2014). Domestic dog skull diversity across breeds, breed groupings,
and genetic clusters. Journal of Veterinary Behaviour 9(5): 228-234.
doi:10.1016/j.jveb.2014.04.007.
• Vila, C., Savol ainen, P., Maldonado, J. E., Amorim, I. R., Rice, J. E.,
Honeycutt, R. L., Crandall, K. A., Lundeburg, J. and Wayne, R. K.
(1997) Multiple and ancient origins of the dom estic dog. Science. 276,
1687–1689.
Acknowledgements
The authors would like to thank the
owners of all dogs who participated in the
study.
Methods
Discussion
Skull measurements of individual dogs (n=107;
male=56, female=51) were taken from a variety
of breeds (n=25). Measurements taken were:
Skull length: tip of the nose to the external occiput.
Skull width: from each side end of the zygomatic arch.
Skull height: underneath the jaw bone to the highest point of the
cranium.
Muzzle length: tip of the nose to the stop between the eyes
The CI was calculated as (skull width/skull length) x 100, and they
were grouped into skull category by breed according to literature. CI
values were correlated using a Spearman’s rank test to both height :
length ratio (HLR) and cranium : muzzle ratio (CMR).
The discrepancies between breed skull
classifications and CI measurements
demonstrate that current systems are not
effective for studying the diverse range of dog skull shapes. Muzzle
length in relation to the cranium is responsible for most of the
change seen in CI. However, this is not the only morphological
feature changing, with skulls becoming deeper as they shorten.
Changes in muzzle length between skull shapes implies that
changes to bite and mastication pattern would be expected. Skull
size and shape has profound effects on chewing capabilities and
characteristics of different species due to a wide range of factors
(tooth position, musculature, pressure application, gape and bite)
(Jaslow, 1987 & Van Valkenburgh, 1989, cited in Bierer, n.d.).
Better understanding of dog skull functional morphology could have
relevance to size and texture elements of diet design as well as to
further understanding of health conditions relating to the skull.
Table 1. CI values for breeds with multiple
individuals according to subjective scores:
dolichocephalic, mesocephalic, brachycephalic.
Breed n mean SE min max
Greyhound 4 43.7 2.3 40.3 50.2
Welsh Springer
Spaniel
4 48.1 0.3 47.6 48.6
Cocker Spaniel 5 50.8 3.5 45.9 61.4
Golden Retriever 3 50.9 2.5 46.1 56.0
Cockapoo 4 53.2 3.7 42.3 57.9
Labrador 10 54.3 2.1 49.0 62.2
Labradoodle 3 58.7 2.7 54.4 64.7
Pomchi 4 65.8 3.0 58.6 72.2
Staffordshire Bull
Terrier
11 67.0 2.9
57.5 76.0
French Bulldog 2 86.1 14.4 65.8 106.4
Bulldog 2 88.3 12.5 75.8 100.8
Figure 1. Correlation between the cephalic index and
the height : length ratio of canine skulls (n=107).
Figure 2. Correlation between the cephalic index and
the cranium : muzzle ratio of canine skulls (n=107).
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