"Little brainiacs and big dummies: Are we selecting for stupid, stout, or small dogs?"
Intriguing study. The title is catchy, so I read it.
A general scaling quotient related to brain size, or encephalization, for all mammals is defined as the amount of brain mass exceeding that related to an animal's total body mass. Brain size non-linearly scales with body weight between species within mammals to around the 0.67 power. However, within species, this scaling exponent appears to be much smaller. An early study compares dogs with non-canid species. According to this study, we are breeding dogs to have smaller brains.
Dogs today have smaller relative brain size than dogs 100 years ago. That may not be an intention, but we do selectively breed them for certain behavioral traits as well as well as physical appearance. The famous Russian Silver Fox experiment confirms that (6 generations of breeding and choosing individuals for tameness resulted in a domestic fox). Keep in mind that genes linked to behavior and physical appearances may be linked to brain size.
This encephalization quotient generally estimates 'evolved intelligence' or other behavioral traits, "under the assumption that the bigger the brain per kilogram of bodyweight beyond what would be required for basic neural functions, the greater the intelligence." But have we humans also bred for lower brain function? aka 'stupid' dogs?
My interest was more in how this compares to non-dog canids than within domestic dogs. A phenomenal earlier study* (1986) compared brain size and weight between all families within the carnivore order (all carnivore families, genera and most species). The study also used metrics including ecology and behavior as well as body and brain size, developing an encephalitic quotient (EQ) range. Canids placed in between the families Ursidae (bear family) and felids (cats).
What is interesting is the EQ within domestic dogs compared to the within species of non-dog EQ. The current study found that EQ decreases with increasing body weight in dogs: "Small dogs had higher EQ than their non-dog canid counterparts of comparable size, but large dogs had lower EQ than similarly sized non-dog canids."
Encephalization quotient decreases with increasing body weight in dogs. The dashed black line represents the regression line for dogs, the dark gray solid line represents the regression line for dogs from the Richet dataset, and the light gray solid line represents the regression line for canids from the Gittleman dataset. The regression line for canids crosses the regression lines for dogs at approximately 10–15 kg.
In other words, domestic dogs have a brain-size-to-body-size relationship that markedly differs from the general rule between-species relationship (power function of 0.26 rather than 0.65–0.67). That's quite a difference.
But this is even more impressive: modern dogs have lower relative brain sizes than dogs of a century ago. There are two possible biological explanations. First, humans purposely select for smaller heads, which includes the size of the cranium. This is unlikely.
The second and more likely explanation is that modern dogs are fatter than their lighter counterparts from a century ago: "...plentiful evidence exists that dogs, like people, in the United States are experiencing an increasing prevalence of obesity."
Authors in the recent study recognize an important methodological criteria related to the above explanation: "Researchers a century ago recognized the importance of examining brain-size-to-body-size relationships in animals that are in 'optimal condition,' and cautioned including data for domestic animals that often differ in body condition from their wild counterparts." Definitely true. Regardless, the prevalence of obesity in dogs (and humans) is concerning.
What about intelligence? Are bigger dogs smarter than small dogs due to brain size, or vice versa?
Despite common beliefs we all hear, the authors conclude "no behavioral evidence exists that small breed dogs are more intelligent than large breed dogs." Most of us know that behavioral studies are fraught with discrepancies in methodology and interpretation. Additionally, several studies failed to observe a relationship between brain size and cognition. Some studies have reported that certain breeds are more "trainable" or perform better at certain skills. Similar reports are associated with horse breeds.
The authors offer a more plausible explanation; what these studies and anecdotal claims more correctly identify is a "selection of specific morphotypes for specific tasks, and certain cognitive skills that underpin those tasks, rather than selecting for or against overall intelligence." Specific physical attributes are more suited for specific tasks and skills than others, and behavior is a result of genes and learning.
"Such a hypothesis finds support in studies that identify “trainability” with working breeds vs. non-working breeds, rather than size. Given that working breeds are generally larger than non-working breeds, size might simply be a poor surrogate for “working breed.”
I enjoy good challenges (and controversies) in biology. They force people to critically think (and they should). The authors in this study challenge existing and generalized assumptions about interpretations and conclusions relating to differences in body sizes with a subtle hint of well-deserved sarcasm.
"This observation flies in the face of assigning an encephalization quotient to dogs—in our study, small breed dogs had a relative brain size far exceeding their “expected” brain size, while large and giant breed dogs had relative brain sizes of mental midgets. Therefore, applying an encephalization quotient to dog breeds appears nonsensical, and challenges the entire anthropocentric notion of encephalization as a measure of intelligence."
This, to me, is the coup d'état challenge for scientists that arrive at assumptions and generalizations that persist on shaky evidence. It also represents a challenge to those that choose and pick their data or facts to support their preconceived assumptions. In this case, the challenge is how we perceive intelligence in dogs, and perhaps all animals. Including our own genus, Homo spp.**
Read the last two paragraphs of the paper where the authors discuss "What makes a dog a dog?" and how selecting for smallness has a limit (which it does, so stop it).
"....regardless of how small domestic dogs become, brain size must be conserved to accommodate sufficient neuronal complexity for the dog to maintain its “dogness.”
In all, excellent discussion and writing for a published scientific paper.
* Gittleman J.L. Carnivore brain size, behavioral ecology, and phylogeny. J. Mammal. 1986;67:23–36.
** A recent article (sometime in March 2021) challenged our modern assumptions about the inferior intelligence, or lack of, in Neandertals and Denisovans. You'll have to do a google search for that article.
