According to scientists, the reason dogs buck this trend is because they were bred and evolved by humans to do what we want, not by nature.

“The results are actually counterintuitive and contrary to the patterns we have observed throughout the evolution of the mammalian brain,” Dr. Balcarcel said.

“Over time, most animals have increased the relative size of their brains. The hypothesis is that a bigger brain helps us survive, to outcompete other animals.

“In dogs, it’s exactly the opposite: it’s the only way to measure cognitive abilities.”

Small dogs more fearful and aggressive

The big-brained trait of small dogs has also been found to be linked to higher fearfulness and aggression, and could explain the combative nature of feisty terriers.

Dogs with large brains for their size were found to be more attention-seeking and more prone to separation anxiety, data showed, and more likely to be aggressive toward humans and other dogs.

Working dogs with smaller brains, however, were much easier to train, the study found.

“Working dogs are known to fulfill the greatest diversity of human assistance roles of any functional category,” the scientists write in their paper published in Biology Letters.

“These specializations include search and rescue, detection (i.e. medical), assistance (i.e. guidance), policing and protection (i.e. say military) and are undoubtedly more complex than those of other dogs.”

These dogs have better “executive function,” the scientists say, which previous studies have used as a definition of intelligence in its own right because it is linked to better behavioral control and short-term memory skills.

But Dr Balcarcel says owners of other dogs, including toy breeds, may now reject the claim that working dogs with high “executive functions” are the smartest.

“While your dog may not have ‘higher executive function’ like ‘working’ class service dogs, he probably has intelligence in his own way,” she said.

“It’s probably just a different set of skills and behaviors that they have, but it doesn’t correlate with larger brain size.” In short, intelligence can and should be measured in many different ways, not just by total brain size.