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Dog Behavior Varies With Height, Bodyweight and Skull Shape

How does a dog's physical traits relate to his behavior? By Claudia Bensimoun

According to new research from the University of Sydney and published in Plos One, the size of a dog and skull shape of a dog are important factors in a dog's behavior. Dr. Paul McGreevy from the University of Sydney's Faculty of Veterinary Science explains that certain types of canine physical characteristics can contribute to a dog's behavior. More than 8300 dogs of over 80 different breeds were described in the dog owner reports used in this study. Dr. McGreevy and colleagues used the Canine Behavioral Assessment and Research questionnaire (C-BARQ) and they also examined the sizes of over 960 dogs. Dr. McGreevy explains how smaller dogs seemed to have more aggression issues, and that dog behavior norms will depend on more than a dog's breeding. According to the research published in Plos One, certain physical characteristics in dogs such as height, body weight and skull proportions (length and width of the skull) are linked to certain types of behavior. 

Skull and Eye Size
A dog's skull length can vary from 2 ¾" to 11". With a wolf's skull measuring 11 ¾", a dog's skull can be similar in size. 

Despite this variation, there's not much variation in dogs' eye size across different breeds. (Although Chihuahuas have a large eye size compared to skull size. Usually dogs with larger skulls will have larger eyes.) Earlier studies have shown that dog's eyes usually measure with a radius of 43". 

Cells in the retina of an animal's eyes are all arranged differently so as to give different and most appropriate types of vision for the animal. Eye cells are concentrated horizontally across the retina. This is known as a "visual streak," and it gives animals more sensitivity to movement in their visual field. (Humans, on the other hand, have eye cells concentrated in the area called area centralis, and this allows for us to get up close to something and focus on it.) 

Pug photo courtesy of Kelly McFaul-Solem.
In past research, it was also demonstrated that dogs with long noses have more of a visual streak than those with short noses. Thus, the German Shepherd would have more sensitivity to movement for longer distances around him than a Pug would (Pugs have almost no visual streak). Scientists say that the variety of the distribution of eye cells in a single species like a dog is unique.
As a result of selective breeding, many dogs (like Boxers and Labradors) have retained puppyhood features. This new study demonstrates that a dog's eye and brain structure relates to his skull size. The smaller the dog, the more behavioral quirks a dog owner may face like humping, urinating indoors or excessive barking.
Canine size and head shape have been shown to influence behavior. Photos courtesy of Katie McKewen (left) and Carla Pavaravani (right).

The study demonstrated that 33 behavioral traits in dogs correlated with height alone. Some of these are:
  • mounting persons or objects
  • touch sensitivity
  • urination when left alone
  • dog-directed fear
  • separation-related problems
  • non-social fear
  • defecation when left alone
  • owner-directed aggression
  • begging for food
  • urine marking
  • attention seeking behavior
In earlier studies from Coppinger & Schneider (1995, Evolution of working dogs, Cambridge University Press, Cambridge, 21-47), it was demonstrated that the way working dogs' heads were shaped was according to their breed's original purpose. The ratio of a skull's width to skull's length (known as the CI) was described as follows:
  • CI is linked to how the retinal ganglion cells are concentrated in the form of an area centralis instead of a visual streak. 
  • Thus, small-skulled dogs like the Yorkie breeds would have more visual acuity in the center of their visual field, but less visual acuity in their peripheral field of vision (around them or far ahead).
  • Breeds that have a large CI have a larger ability to detect movement around them (peripheral vision). These dogs would have the ability to follow a pointing finger. Scientists say that this indicates that the way that a dog's retinal cells are arranged may very well be closely related to that dog's social cognition skills.

Small-skulled dogs have more visual acuity in the center of their visual field. Photo courtesy of Traci Murdock.Dogs with long noses have good peripheral vision. Photo courtesy of Julie Weir.

What Do Canine MRIs Reveal?
Different dog breeds with different-sized skulls had MRIs. Here's what they found:
  • Smaller skull size compared to skull width meant that the brain was "pitching," as well as having a "downward" shift in the position of the olfactory lobe, which handles the sense of smell.
  • CI is indicative of how dogs perceive stimuli and how they process information.
  • The huge diversity in all the breeds' body shape, brain shape and skull shape have led to many changes in a dog's brain. These changes are human-induced.
This new study examines all correlations between height, body weight and skull size using a survey called C-BARQ. The scientists researched certain correlations such as skull measurements of 588 dogs and breed-specific profiles. They also used the database of 8,301 dogs.

Testing Methods
  • 12 show dogs tested (6 females, 6 males), and the study was approved by the New South Wales Animal Ethics Committee. None of the dogs were distressed.
  • Dogs were 2 years and older, of show quality, or from show dog pedigree lines. All pups from the show dogs that had already been measured were not taken into consideration, so that data was not used twice.
  • Dog breeds had to be ANKC (Australian National Kennle Club) recognized.
  • Dogs had to be owned by registered breeders only.
  • Dogs have had to have had over 30 puppies registered yearly with the ANKC in 2009. These dogs were then photographed and measured. The camera was held horizontally. This allowed for a true skull measurement that included the skull's length and width.
  • Heights and weights were also used from the ANKC breed standards. When males and females of the same breeds had different heights, the scientists used the mean height.
There were 6 breeds that had no ANKC preferred height standard, so the scientists went to and drew information from there. Weight information was drawn from the C-BARQ database.

What is C-BARQ?
C-BARQ is a survey completed by dog owners which is designed to provide quantitative assessments of many common behavioral traits in dogs. Read more details.
  • The questionnaire has 100 questions using a 5-point rating scale (0 meaning the behavior is non-existent, and 4 indicating that the behavior is the most intense).
  • Questions relate to a dog's usual responses to many everyday situations like fear, excitability and aggression.  
  • There are 14 behavioral factors tested with an additional 22 traits tested. The higher the scale rating result, the less favorable, except for the trainability of the dog, in which case the higher scale score the better.
  • Scientists were wary of the biases associated with dog owners and questionnaires such as behaviors they didn't like in their dogs. Other behaviors may be overlooked, like defecating in the home, as opposed to excessive barking when left alone. They also took into consideration that, in some cases, attention-seeking behaviors had been triggered by the dog owners themselves.
There seems to be an interesting correlation between behavior and morphology in the canine species.
  • "Undesirable" behaviors increase as the size and height of a dog decreases.
  • Short-skulled dogs had an increase in grooming and compulsive staring.
  • Long-skulled dogs had an increase in stranger-directed fear, persistent barking, and food stealing.
  • More stranger-directed aggression was seen from smaller breeds similar to the terrier breeds. Researchers questioned whether there was simultaneous selection for aggressiveness in smaller breeds related to their past history. Terriers were used for chasing and hunting prey underground. Smaller breeds that have short legs may have inherited aggression.
  • This study also questioned the behavioral responses of smaller breeds with owner-directed aggression, and linked that to attachment and attention-seeking behaviors in smaller breeds. Researchers say that, according to behaviors in C-BARQ, attention-seeking behaviors in dogs are linked to jealousy and pushiness when a dog's owners attention is given to someone else. Scientists refer to this as resource guarding.
  • Larger breeds descended from smaller breeds that were originally meant to be companion dogs (like breeds that come in multiple sizes) may display behaviors that do not match up with their body size. 
  • Dogs with a higher CI had greater central visual field acuity. These dogs would be more able to see and focus on objects in front of them, instead of objects in their peripheral vision. These dogs would then not be able to scan objects in the distance, thus resulting in a reduced chasing response.
  • Fly-snapping, a compulsive behavior, according to an earlier study done by Dr. McGreevy (2009, A Modern's Dog's Life, UNSW Press, Sydney), may indicate stress factors. The researchers found that fly-snapping was not related to the height of the dog, but more so to the weight. That said, shorter and stockier dogs, like the Bulldog, were more at risk for displaying coping behaviors. It is unclear as to why this is so.
  • Obsessive tail-chasing behavior was not linked to size or breed. Tail-chasing, according to a study done by Dr. Osmo Hakosalo and colleagues and published in Plos One (2012, Environmental Effects on Compulsive Tail Chasing in Dogs) is associated other factors.
  • Trainability was the only behavioral trait associated with increasing height in dogs.
Short, stocky dogs may be


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