Abstract
Background: Brain size has been associated with intelligence of various orders and families of animals, leading to the concept of encephalization. Brain size scales with body weight between species within mammals to approximately the 0.67 power. However, within species, this scaling exponent appears to be much smaller (approximately 0.27 power).
Aim: We examined whether this relationship has persisted in dogs over the 120 years since this was originally observed.
Methods: Comparative cross-sectional study of magnetic resonance imaging (MRI) data obtained from 127 dogs, compared to historical data from 157 dogs and 24 non-dog canid species.
Results: Brain size in dogs measured by MRI had a scaling exponent virtually identical to that observed previously (0.24 vs 0.26). However, the proportionality constant was smaller, suggesting that dogs in the study cohort had relatively smaller brains than the historical cohort. Absolute brain size appeared to have both a lower and upper limit in dogs. When compared to non-dogs canids, the most appropriate “representative” size for a “typical dog” when examining allometric scaling across Canidae appeared to be approximately 10-15 kg.
Conclusions: We interpreted the slight reduction in relative brain size to be a function of increased obesity in the study cohort compared to dogs examined 120 years ago. Further, we suggest that dog brains have a finite lower size limit. Finally, concepts of encephalization should not be applied to dogs.

Key words: encephalization, canine, brain size