Biologists previously believed that the reduction in brain size among ancient wolves began immediately after their domestication by humans. However, computer tomography of fossil skulls has shown that ancient glacial dogs had brain sizes comparable to those of wild wolves from the same era. A significant reduction in neural tissue—about 46%—occurred around 5,000 years ago, when European farmers began selecting smaller, more anxious, and loud-barking dogs to guard their settlements.

Brain Changes in Domesticated Animals

According to a biological principle known as the domestication syndrome, domesticated animals typically have smaller brains than their wild ancestors. In modern dogs, brain volume is on average 20–30% less than that of wolves. It was previously thought that brain degradation happened in two stages: first, the brain shrank due to the loss of the need to survive in the wild, and then, over the last two hundred years, it increased slightly as a result of selective breeding and the need to perceive human social signals. However, these conclusions were based only on studies of modern breeds, leaving the dynamics of changes over millennia unknown.

Research Process

In a study published in the journal Royal Society Open Science, researchers traced the evolution of the dog brain. The analysis used data from 185 modern and 22 archaeological skulls, including wild wolves, Australian dingoes, semi-wild village dogs, and purebred dogs with various specializations. The archaeological samples spanned from the Pleistocene (35,000 years ago) to the late Neolithic (5,000 years ago). The skulls were examined using computer tomography, allowing the creation of virtual 3D models of endocasts—casts of the inner skull cavity that precisely replicate the brain’s volume and surface. The resulting data were compared with skull length to account for the animal’s body size.

Main Findings

The “proto-dogs” of the glacial period, which lived 15,000–35,000 years ago in France and Belgium, had brain volumes indistinguishable from wolves. In the oldest find from Belgium’s Goyet Cave, the relative brain size even exceeded that of wolves. It is suggested that in the early stages of domestication, dogs required behavioral flexibility and cognitive effort, while constant access to calorie-rich food removed energetic constraints on neural tissue growth.

A sharp reduction in brain volume occurred much later. Dogs from the Shalain site in France, which lived about 5,000 years ago, had brains 46% smaller than those of wolves. In terms of head proportions, these animals resembled modern Spitz or terriers, and their brain volume was similar to that of pugs and chihuahuas. Along with the reduction in size, the ratio of brain tissues changed: the share of the cortex decreased, while subcortical structures increased. Such anatomy, as studies of modern breeds show, is associated with increased anxiety, timidity, and a tendency to bark.

Modern Trends

Among modern dogs, the largest brain volumes are found in working breeds—guard and sled dogs. Their measurements even surpass those of Australian dingoes, which had to readapt to a predatory lifestyle. The smallest amount of neural tissue is observed in decorative companion dogs.

Conclusions

The study’s results show that dog evolution is not simply a gradual and inevitable decline in cognitive abilities due to comfortable living conditions. For thousands of years, dogs that accompanied Paleolithic hunter-gatherers retained fully developed brains. Significant changes only occurred with the transition of humans to sedentary agriculture. It is believed that Neolithic villagers deliberately selected timid and noisy dogs, which served as living alarms when predators or strangers approached. However, this status did not protect the animals from being used as food: archaeologists found crushed bones of small dogs from the Shalain site among the remains of consumed meals.