Are brain weights estimated from scaling relationships suitable for comparative studies of animal cognition?

Little brainiacs and big dummies: Are we selecting for stupid, stout, or small dogs?

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.

Leveraging brain-body scaling relationships for comparative studies

In Horschler et al. (Anim Cognit 22(2):187-198, 2019), we found that two components of executive function (short-term memory and self-control) were strongly associated with estimated absolute brain weight across dog breeds, and argued that dogs present a powerful model for studying evolutionary links between cognition and neuroanatomy due to their extraordinary degree of intraspecific morphological variation. In a commentary on this work, Montgomery (Anim Cognit, 2019) raises concerns about the practice of estimating brain weights from brain-body scaling relationships. Montgomery explores the practical significance of this approach, ultimately concluding that such estimations should be avoided. In this response, we point out some limitations of the analyses presented by Montgomery and consider his conclusions in light of these issues. We then explore the extent to which body weight serves as a valid proxy for brain weight under varying conditions. Through simulations, we show that the consequences of using body weight as a proxy for brain weight depend on parameters including effect size, the correlation between brain and body weight, and the variance in brain and body weight within a sample. Under conditions approximating those in Horschler et al. (Anim Cognit 22(2):187-198, 2019), we find that body weight is a reliable proxy for brain weight, and that statistical results from models using either brain weight or body weight as predictor variables are highly convergent. Nonetheless, we wholeheartedly agree with Montgomery that empirical data on brain weight, structure, and cellular composition will be critical for creating new opportunities to investigate the relationships between neuroanatomy and cognition in dogs.