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Rethinking the Effects of Body Size on the Study of Brain Size Evolution

Authors
  • Font, Enrique
  • García-Roa, Roberto
  • Pincheira-Donoso, Daniel
  • Carazo, Pau
Type
Published Article
Journal
Brain, Behavior and Evolution
Publisher
S. Karger AG
Publication Date
Aug 22, 2019
Volume
93
Issue
4
Pages
182–195
Identifiers
DOI: 10.1159/000501161
PMID: 31437837
Source
Karger
Keywords
License
Green
External links

Abstract

Body size correlates with most structural and functional components of an organism’s phenotype – brain size being a prime example of allometric scaling with animal size. Therefore, comparative studies of brain evolution in vertebrates rely on controlling for the scaling effects of body size variation on brain size variation by calculating brain weight/body weight ratios. Differences in the brain size-body size relationship between taxa are usually interpreted as differences in selection acting on the brain or its components, while selection pressures acting on body size, which are among the most prevalent in nature, are rarely acknowledged, leading to conflicting and confusing conclusions. We address these problems by comparing brain-body relationships from across >1,000 species of birds and non-avian reptiles. Relative brain size in birds is often assumed to be 10 times larger than in reptiles of similar body size. We examine how differences in the specific gravity of body tissues and in body design (e.g., presence/absence of a tail or a dense shell) between these two groups can affect estimates of relative brain size. Using phylogenetic comparative analyses, we show that the gap in relative brain size between birds and reptiles has been grossly exaggerated. Our results highlight the need to take into account differences between taxa arising from selection pressures affecting body size and design, and call into question the widespread misconception that reptile brains are small and incapable of supporting sophisticated behavior and cognition.

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