It has long been known that dogs have less between their ears than wolves, but now research has suggested their brains started to get smaller at least 5,000 years ago.

Experts say the results offer fresh insights into the domestication of our canine companions. However, the findings are unlikely to explain why your spaniel will only drink from a muddy puddle: the researchers say a reduction in brain size does not mean dogs are dafter than their wolf-like ancestors.

“The way our dogs live nowadays doesn’t give them the opportunity to always express most of their intelligence,” said Dr Thomas Cucchi, first author of the study from the French National Centre for Scientific Research.

“But they are extremely clever and domestication didn’t make them stupid, but made them really capable of reading us and communicating with us.”

The relationship between humans and canines is ancient, with research revealing the oldest direct genetic evidence for domestic dogs dates back more than 15,000 years.

But while a reduction in brain size is typically considered a hallmark of domestication, there has long been debate over exactly when dogs ended up with smaller brains than wolves, with some experts suggesting this may have occurred early in the dog-human relationship.

However, others argue smaller brain size is not a hallmark of domestication but instead reflects the emergence of pedigree breeds in the last 200 years.

Writing in the journal Royal Society Open Science, Cucchi and colleagues studied CT scans of the skulls of 22 prehistoric wolves and dogs, dating from 35,000 to 5,000 years ago, as well as CT scans from the skulls of 59 modern wolves and 104 modern dogs. The latter included different modern breeds as well as stray or “village” dogs, and dingoes.

The researchers used the scans to track how brain size changed over the evolutionary history of dogs.

The results reveal that, taken together, modern dog breeds, dingoes, village dogs and Late Neolithic dogs had a 32% smaller brain than ancient and modern wolves.

More specifically, dogs that lived in the Late Neolithic period – about 5,000 to 4,500 years ago – had brains 46% smaller in size than wolves from the same period, with brains of a similar size to those of pugs today. Further work revealed these dogs had significantly smaller brains than ancient wolves even once body size was taken into account – an important consideration given they were smaller overall..

However, the team found no sign that the brains of two canines that lived alongside humans 35,000 and 15,000 years ago – sometimes called “protodogs” – were smaller than those ancient wolves. Indeed, one brain was relatively larger, with the authors suggesting that raises the possibility brain size may actually have increased in the early stages of the domestication.

Cucchi said it is unclear why domestication resulted in dogs with smaller bodies and brains than their wolf-like ancestors.

However, he noted research has suggested that when the size of the brain is reduced, it reorganises, meaning smaller dogs are less trainable and more wary of changes in their environment, making them potentially useful as “alarm systems”.

But Cucchi said it could also have been that limited food resources in the Neolithic village environment favoured smaller dogs with smaller brains, as these require less energy.

Dr Juliane Kaminski, an expert in canine cognition at the University of Portsmouth who was not involved in the study, said a particularly important revelation was that “protodogs” did not have smaller brains than wolves.

“They didn’t yet show this sign of domestication that we thought is [a] standard part of this domestication syndrome,” she said.

Kaminski said the study suggested the relationship between humans and dogs may have begun rather loosely before developing into a very strong bond.

“What [the authors] are simply saying is: OK, the timeline of the full domestication syndrome being in place is maybe a little bit later than the genetic data suggests,” she said.