Macroevolutionary brain scaling is a microevolutionary metaphenomenon.

From bees to blue whales, it has long been assumed that brain size scales with body size according to a simple log-linear relationship - with differences in the slope and intercept observed amongst different groups of animals. However, recent analyses in mammals contradict this view, revealing size dependency in the form of curvature in the brain and body mass relationship. Here, we use data from 4679 species across seven animal classes and spanning nearly 12 orders of magnitude to uncover near universal curvilinearity. We demonstrate that this body size dependence is a metaphenomenon emerging from a pattern of diminishing allometry within species with increasing body mass. This has fundamental implications for how we interpret macroevolutionary patterns - which can arise as a consequence of within-lineage dynamics. Our integration of inter- and intra-specific allometries reshapes perspectives on morphological evolution by providing a broader framework for understanding how microevolutionary within-species dynamics shape macroevolutionary phenomena.