What’s on Milo’s mind?: Why the dog’s brain is growing in modern times

Picture a fire-lit cave in the Stone Age, about 14,000 years ago. A wolf stands at the entrance; large, with a proportionately large head.

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It has been using its mental faculties to hunt and fight off predators; navigate its place in the pack, and its changing landscape. Now, it is welcomed into the cave; becomes a companion, guard and leader of the hunt. In return, it receives food, shelter, protection. The wolf, particularly the gray wolf (Canis lupus), becomes the dog (Canis familiaris), smaller, with a smaller brain-to-body ratio.

Fast-forward to today. Bustling cities; busy lives. Stresses and strains. And a dog with a larger brain-to-body ratio… except, it’s clearly using it for very different things.

Modern breeds of dog, those developed over the last 150 years, have a larger brain-to-body ratio than their early ancestors, found a study published in the journal Evolution in April, led by László Zsolt Garamszegi, an evolutionary biologist with the Centre for Ecological Research in Hungary.

“Perhaps the more complex social environment, urbanisation, and adaptation to more rules and expectations have caused this change,” says Enikő Kubinyi, a senior research fellow with the department of ethology (or the science of animal behaviour) at the Eötvös Loránd University in Hungary, and one of the co-authors of the study.

This growth in brain-to-body ratio can be traced to the dawn of the industrial revolution, a period that also coincided with the rise of intensified dog-breeding standards for specific tasks such as hunting, tracking, herding, guarding and companionship, about 160 years ago.

The Hungarian study examined 865 canine skulls and brains across 159 contemporary dog breeds and 48 wolf specimens. About half the skulls were from the canine collection of Tibor Csörg, a researcher with the department of anatomy, cell and developmental biology at Eötvös Loránd, and a co-author of the study, who maintains a vast collection acquired over 10 years. The study, which analysed the data over a year, also sourced tissue samples from the Canine Brain and Tissue Bank at Eötvös Loránd established in 2017.

CT scans were used to reconstruct the volumes of the brains that would have occupied each skull. The researchers found that the farther a dog breed was, genetically, from a wolf, the higher the brain-to-body ratio. This ratio in the wild wolf remains 24% higher than that of a similarly sized dog.

What’s interesting is that the brain remained shrunken through the phases of settled living and agriculture. “Pastoralism and the accumulation of wealth offered various tasks for dogs, including fowl-guarding, sheep-herding and the guarding of property,” says Kubinyi. Still, the brain did not grow.

So why now? What parts of modern living have caused the canine to make greater demands of its brain? Exactly what parts of the brain have grown, or remained shrunk? These are potential areas of further study.

It takes immense selection pressures on cognitive abilities to yield large enough changes in brain morphology for changes in relative brain size to be detectable, Kubinyi says. “Future studies in the field of brain evolution in dogs could focus on changes acting on particular brain regions to uncover the potential links between brain morphology and other traits.”

Interestingly, a 2022 study conducted by researchers at the University of Vienna and the National Museums Scotland, published in the journal Royal Society Open Science, indicates that the skulls and brains of domestic cats have grown significantly smaller over the last 10,000 years, compared to their African ancestors.

Data does not indicate a reversal of brain size such as has now been observed in dogs. It’s unlikely there is a significant change in this direction, says Kubinyi, because cats are not as domesticated as dogs and remain independent, even when they are fed.