Has a cushy home-life shrunk the brains of pet dogs?

Dogs are prone to small brain behaviour – it’s a phenomenon documented extensively, and hilariously, on social media.

Domestic dogs’ (Canis familiaris) brains are physically more than 24% smaller than their ancestor the grey wolf (Canis lupis).

It’s long been thought that the comforts of a cushy life of domestication led to exceptionally dramatic reductions in brain size because “relaxed selection pressures” – like reduced need for foraging, mating competition, and avoiding predators – decrease the need for energy-costly brain tissue.

But a new study in the journal Biology Letters challenges the idea that domestication shrank dogs’ brains to an unparalleled degree, compared to other wild canid species.

It found the reduction in domesticated dogs’ brain size falls within the expected range for most canids included in the study, suggesting domestication hasn’t had as important of an effect on dogs’ brain sizes as previously thought.

“There is a lot of controversy about the evolutionary effects of domestication,” lead researcher László Zsolt Garamszegi from the Institute of Ecology and Botany, HUN-REN Centre for Ecological Research in Hungary, told Cosmos.

“Given our human perspective, we tend to think that domestication has a ‘special’ evolutionary role, but if we look at this phenomenon from a broader evolutionary context, domestication can be considered as an ecological constraint to which species respond in some way — just as other evolutionary forces affecting wild animals.”

Zsolt Garamszegi and co-author Niclas Kolm, from the Department of Zoology at Stockholm University, Sweden, compared dogs’ brain sizes relative to their body size with 25 closely related wild canid species.

“The reduction in brain size due to domestication is typically considered as a component of a domestication syndrome (an entire suite of morphological/physiological/behavioural responses that are elicited by domestication),” says Zsolt Garamszegi.

“However, supporting evidence were generally lacking simply because we did not have data on the brain size of the ancestral wolf from which the dog was domesticated, and on the ancient dog breed which was domesticated.”

Their new analysis uses data from 11 ancient dog breeds (which originate more than 500 years ago) such as the Akita Inu, Siberian Husky, Afghan Hound, and Samoyed, to approximate the ancestral domesticated dog.

The findings indicate that, while domestication does contribute to brain size reduction in dogs, other ecological and evolutionary pressures, can drive comparatively small brains in wild species too.

In particular, the common racoon dog (Nyctereutes procyonoides) – the only known canid to hibernate during the winter – was the only clear outlier in terms of relative brain size.

Scientists think that hibernation, which is associated with long periods of low metabolic activity and food scarcity, limits brain size evolution due to the high energy demands of large brains. 

“Our main point is that these effects are comparable in terms of magnitude. This is interesting, because we typically focus on domestication due to its relevance to humans, but from an evolutionary perspective it is not that exceptional given that other ecological constraints lead to similar reduction in brain size,” says Zsolt Garamszegi.

“We were not able to derive an unambiguous support for the significant reduction in brain size in dog as compared to the wolf. Therefore, the effect of domestication on dog’s brain size should be less important than previously thought.”