Brain structure, but not size, changes across an altitudinal gradient in the Plateau brown frog (Rana kukunoris)

High altitude imposes directional selection pressure from energetic constraints and other environmental challenges, driving population-level trait diversity along altitudinal gradients. Given energetic constraints are a major driver of intraspecific variation in vertebrate brain size, we tested this in a high-altitude system that is cognitively demanding and resource-scarce. Accordingly, we quantified brain size and structure of the Plateau Brown Frog (Rana kukunoris) across an altitudinal gradient to test the predictions of the cognitive buffer (CBH), expensive brain (EBH), developmental constraint (DCH), and functional constraint (FCH) hypotheses. We failed to find evidence for a general trend of reduced relative brain size at higher altitudes, or in relation to latitude. Notably, however, female cerebellum size decreased while optic tectum size increased with altitude, a pattern consistent with the FCH positing trade-offs between brain regions under environmental pressure. Additionally, an increased female optic tectum size supports the CBH, as the optic tectum's relative size has also been shown to correlate positively with discrimination learning performance. However, no significant correlation was found between the relative size of any male brain structure and altitude or latitude. We therefore propose a sex-specific evolutionary strategy driven by altitudinal selection. For larger females, high energetic costs necessitate a trade-off that prioritizes investment in the optic tectum to maximize fitness components such as survival and fecundity, likely at the expense of investment in the cerebellum. Further investigation along altitudinal gradients is required to unravel the complex mechanisms driving adaptive brain evolution.