Humans have systematically altered the brains of dogs through selective breeding for favoured behavioural traits over hundreds of years, according to a study. Erin Hecht, from Harvard University in the US and her colleagues investigated the effects of this kind of selective breeding on the brain structure of canines by analysing magnetic resonance imaging (MRI) scans of 33 dog breeds.
"Most modern dog breeds were developed in an intentional, goal-driven manner relatively recently in evolutionary time; estimates for the origins of the various modern breeds vary between the past few thousand, to the past few hundred years,” researchers wrote in the study published in the journal JNeurosi.
The team observed vast differences in the brain structure, not only related to the body size or head shape of dogs, but to the patterns in neural networks associated with different brain functions.
"There is a hundred-fold difference between the body mass of a Chihuahua (about one kilogramme) and the body mass of a Great Dane. However, we found that dog brain sizes do not scale commensurately to dog body sizes,” the researchers noted. For instance, by comparing dachshund and golden retriever brain scan images, the team found that the dachsund's brain takes up most of the available endocranial space, while the golden retriever showed noticeably larger sinuses.
By comparing the evolutionary lineages of the dog breeds, the team revealed that changes in relative brain size are not predicated by relatedness. Instead, the study notes that these are more likely caused by human selection for specific traits in dogs -- occurring more recently across the tree of dog evolution.
When the team examined the areas of the brain with the most variation across breeds, they generated maps of six brain networks, with proposed functions varying from social bonding to movement. Each of these was associated with at least one behavioural characteristic, the researchers said. The differences in behaviours across breeds was linked to the anatomical differences in the six brain networks.