The brain is directly responsible for governing an animal’s interactions with its environment. As such, the brain is often considered to flexibly respond to selection in changing environments (1–3). Brain size is, however, also commonly accepted to be restrained by energetic requirements that are considered universal across all vertebrates (4, 5). This apparent paradox highlights that brain size is one of the most salient traits for understanding the fundamental balance between adaptability and constraint in evolution. Despite this importance, crucial aspects related to the timing, pattern, and drivers that underlie modern phenotypic diversity in brain size remain undescribed.
It has long been recognized that brain size scales with body size following a standard linear allometric power law (6). The scaling coefficient (slope) of this allometry is assumed to be relatively stable across vertebrate classes and orders (most often estimated as between 2/3 and 3/4) (7) and is thought to reflect universal energetic growth constraints (4, 5). Largely because of methodological limitations in phylogenetic comparative statistics, this working hypothesis has received little scrutiny. Previous studies have therefore mostly been limited to comparing residual variation along a stable slope [i.e., mean relative brain size or encephalization quotient (EQ), quantified through differences in the intercept of the evolutionary allometry] (7, 8). There is, however, evidence to suggest