Dogs’ shrinking brain size was partially caused by humans 

Humans may be partially responsible for the reduction in brain size observed in domestic dogs. Researchers in Hungary have discovered that domestic dogs possess significantly smaller brains compared to their wild ancestor, the gray wolf. 

This change is attributed to domestic dogs living in less complex environments, which require less cognitive effort than the wild environments gray wolves inhabit.

Dog domestication occurred passively 

Genetic analyses of the oldest known dog remains indicate that dogs were domesticated in a single event by humans living in Eurasia approximately 20,000 to 40,000 years ago. 

According to Krishna Veeramah, an assistant professor of evolution at Stony Brook University, this process has probably been very complex, involving many generations where distinctive dog traits evolved gradually.

“The current hypothesis suggests that dog domestication likely occurred passively. A population of wolves likely lived on the outskirts of hunter-gatherer camps, feeding on human refuse. Tamer and less aggressive wolves would have been more successful in this environment,” explained Professor Veeramah.

“While humans initially did not gain any direct benefit, over time, a symbiotic [mutually beneficial] relationship developed between the humans and these animals, eventually leading to the dogs we see today.”

Brain size in domestic dogs 

The new study, led by László Zsolt Garamszegi from the Institute of Ecology and Botany at the Center for Ecological Research in Hungary, suggests that other factors might also influence the reduction in brain size in domestic dogs, although these factors are not clearly identified. 

The researchers noted that in the wild, phenomena like hibernation could contribute to brain size reduction in other canid species.

“The dog is a canid species that was domesticated from its ancestral species the gray wolf at least 15,000 years ago,” the experts wrote. 

“Over 400 breeds of dogs exist today, and the overall pattern is that relative brain size is dramatically reduced in dogs as compared with the ancestral species, the gray wolf.” 

This idea, that domestication has led to smaller brains in dogs, is widely accepted among veterinarians and researchers.

Domestic dogs require less brain capacity

Domestic dogs face fewer cognitive challenges, such as foraging for food, avoiding predators, and finding mates, compared to their wild counterparts. This reduced need for brain capacity is believed to have led to a gradual decrease in brain size. 

According to the researchers, this phenomenon is thought to be a result of the decreased necessity for metabolically costly brain tissue in a domesticated environment.

Dog brain and body size

To investigate whether the brain size of domestic dogs is particularly small relative to their body size when compared to wild canids, the researchers analyzed the brain and body sizes of 25 canid species, including the domestic dog (Canis familiaris), the gray wolf (Canis lupus), and the common raccoon dog (Nyctereutes procyonoides). 

The experts found that domestic dogs have experienced a significant reduction in brain size compared to gray wolves. However, the domestic dog’s brain is still relatively large compared to some other canid species studied.

The role of hibernation 

An interesting finding of the study is that the common raccoon dog stands out as a “more pronounced outlier” in terms of brain size reduction, having an unusually small brain for its body size. The researchers believe this is due to the raccoon dog’s hibernation habits. 

“Prolonged periods of food shortage, such as during hibernation, prevents the evolution of large brains due to their constantly high energy demands,” explained the study authors. 

The experts conclude that while domestication plays a role in the reduction of brain size in dogs, hibernation also contributes to this phenomenon in wild canids. 

The authors caution against viewing domestication as a “uniquely powerful evolutionary force” responsible for shrinking dog brains. 

The research has been published in the journal Biology Letters.

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