Can a Dog's Size Predict Its Intelligence?

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It was the lunch break at a dog obedience trial, and a group of competitors had gathered around a notebook computer. One of them had found a website listing the newly-posted national rankings for the past year's dog obedience trial performances.

There was a buzz of conversation as everyone scanned the list looking for familiar names. As was usual for most such rankings, the name of the dog's owner was listed as well as the name and breed of the dog. It was clear that this year, like many others, Golden retrievers, Labrador retrievers, German shepherds, Poodles, Cocker spaniels, Springer spaniels, and, of course, Border Collies dominated the top ranks for obedience.

One of the people scanning the list asked the group, "Isn't it a little bit odd that there are so few small dogs on the list? Yorkshire Terriers, French Bulldogs, and Beagles are in the Top 10 breeds in terms of overall popularity, and yet none of them appear in the list of the top 100 obedience dogs. You find a sprinkling of some smaller breeds, like Shelties [Shetland Sheepdogs], Miniature Poodles, and one Corgi, but in total very few small dogs show up as good obedience competitors."

Another person in the group whose competition dog was a Rottweiler commented, "I think you're right about the small dogs. However I was looking for the giant breeds and there weren't very many of them in the top group. There were a few Rotties, but I didn't see any Great Danes, which rank 15th in popularity this year, or Mastiffs, which are not too far behind." He turned to me and asked, "You know something about this sort of thing: Is there any evidence that a dog's size is associated with its intelligence or its obedience performance?"

It turns out that there actually has been some research on behavior differences between large and small dogs. (Click here for more.) That research has found that there certainly are emotional differences between large and small dogs, with the smaller dogs being more anxious and aggressive, and there was some hint that, perhaps because of these emotional complications, these breeds were not as responsive to obedience commands. But I didn't know of any specific studies targeting the relationship of intelligence to the size of a dog.

The question kept bouncing around in my head, and I wondered if there was an available set of data that might offer an answer. A few days later it occurred to me that I could use the intelligence rankings I obtained for my book, The Intelligence of Dogs. In it, I dealt with three types of dog intelligence:

• Instinctive Intelligence refers to a dog's ability to perform the tasks it was bred for, such as herding, retrieving, and guarding.
• Adaptive Intelligence refers to a dog's ability to solve problems on its own.
• Working and Obedience Intelligence is somewhat analogous to school learning, and represents a dog's ability to learn from humans and perform on command. It is this latter form of intelligence that most people seem to be interested in, and it is certainly this characteristic that dog obedience trials measure.

To gather data on Working and Obedience Intelligence, I sent a series of questionnaires to American Kennel Club and Canadian Kennel Club obedience trial judges, asking them to rank breeds by performance. I received 199 complete sets of responses, representing a bit more than half of all of the obedience judges then working in North America. The results were remarkably consistent and stable and the data ultimately allowed me to create a ranking of the intelligence of 130 breeds.

When my ranking of dog intelligence first came out, it received a lot of media attention and scientific commentary, both pro and con. However, over the years, the ranking of breeds and the methodology I used have come to be accepted as a valid description of differences among breeds, at least in terms of the trainability aspect of intelligence. Further, measurements of canine intelligence coming from other research groups and using other methods have confirmed the general pattern of these rankings. In addition some researchers have used my ranking of breeds as part of other investigations into dog behavior, so I felt that it was sensible to employ this ranking as a way of looking at dog intelligence as it is associated with the size of the dog.

I decided to divide dogs into size groups based on their weight, rather than their height. I used a categorization scheme that divided dogs into five different size groupings:

• Toy Breeds included breeds that averaged from 2 to 9 pounds.
• Small Breeds averaged 10 to 35 pounds.
• Medium Breeds averaged 35 to 55 pounds.
• Large Breeds averaged 55 to 85 pounds.
• Giant Breeds averaged in the range of 85 to 120 pounds or more.

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Each of 128 breeds were then assigned to a grouping and attached to an intelligence score based upon their ranking in my Working and Obedience Intelligence data.

When we analyze the result, we find a statistically significant pattern of differences in Working and Obedience Intelligence showing that there is a relationship between a dog's size and its intelligence. You can see that in the figure below which plots the median ranking for intelligence for each of the five weight groups.

Source: SC Psychological Enterprises Ltd

The pattern of data describes what scientists refer to as an inverted U function, which is another way of saying that the highest values are in the middle ranges and drop off when values become very high or very low. Quite clearly, the lowest Working and Obedience Intelligence scores are found in the toy dogs, and somewhat higher scores, but still low, are found in the small dog group. Likewise, we find lower intelligence scores in the giant breed grouping. So this set of data seems to say that the brightest dogs, at least in terms of working and obedience performance, are found in the medium-to-large dog groups.

What is the basis for this relationship between size and intelligence in dogs? Perhaps a hint to what is going on comes from comments by Theodosius Dobzhansky, a behavior geneticist and evolutionary biologist who was the first president of the Behavior Genetics Association. He said that evolution favored the middle of a population. Individuals at the extremes were never as well adapted as those with average characteristics. This would be consistent with research showing that dogs whose head shape was relatively flat or relatively elongated in comparison to the average head shape of dogs seem to have lower intelligence. (Click here for more about that.) Apparently this same principle seems to hold for dog size: Dogs at the extremes—either excessively small or excessively large compared to the overall population of dogs—seem to be less well adapted in terms of intelligence. Thus the medium-to-large dogs would seem to be the most intelligent, at least in terms of the kind of intelligence that determines how well a dog will perform obedience exercises.

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