Dogs brains shrank by around half at certain stages during their domestication, a study shows.
Early ancestral dogs still had brains of a similar size to their wolf relatives, say scientists.
But by 5,000 years ago, their brains had shrunk dramatically by almost half (46%), to a size comparable only to that of today’s small terrier and miniature dog breeds, according to the study published in the Royal Society Open Science.
That may also have altered dogs’ temperament, suggests the team led by Thomas Cucchi from the Museum national d’Histoire naturelle (MNHN) in Paris.
The reduction in brain size may have made the animals more alert and cautious, traits that could have been valuable in ancient human societies.
For their study, the researchers analysed 207 skulls, 185 from modern dogs, dingos and wolves, and 22 prehistoric ones. The oldest specimens were 35,000 years old.
They used CT scans to create virtual impressions of the skull cavities. In fossil skulls, their volume is considered a reliable indicator of brain size.
Challenging coexistence
The findings suggest the Ice Age ancestors of modern dogs show no signs of reduced brain size compared to wolves of that era. In one specimen found in Belgium, the internal skull volume was even slightly larger.
Living in close proximity to humans may have created new cognitive challenges, such as more complex social interactions and the use of new food sources, the scientists suggest.
Some 30,000 years later, we see a different picture, with dogs' brain volume having shrunk significantly. Furthermore, the animals had become smaller, the researchers say.
Dogs reached a shoulder height of 35 to 45cm. Their skulls would have resembled those of modern herding dogs in shape and size, according to the scientists' estimates.
Needing dogs for safety
That change came as humans in Central Europe, who had previously lived as hunter-gatherers, became increasingly sedentary, switching to farming and rearing livestock.
As agriculture spread, settlements emerged and that meant new roles for dogs, the scientists say. The animals may have lived near human settlements as scavengers, been used as guards and may have served as a source of meat.
The team suggests humans may have played a direct role in altering brain size through targeted behavioural selection, breeding for docility. Similar patterns are also evident in other farm animals, though to a lesser extent.
Scientists are still trying to work out how far the shrinking of the brain affected the animals’ intelligence.
Brain size alone is not the decisive factor, but rather, above all, how efficiently the brain is organised. – dpa
