Elevational variation in brain size of the Minshan toads ( <i>Bufo minshanicus)</i> on the Tibetan Plateau

Intraspecific brain size variation in vertebrates is primarily driven by selective pressures associated with environmental harshness and life-history strategies. We investigated these drivers in the Minshan Toads (Bufo minshanicus) by simultaneously examining the sizes of the brain and brain regions across three altitudinal gradients on the eastern Tibetan Plateau while accounting for ecological covariates (elevation, latitude, annual total precipitation and normalized difference vegetation index) and life-history covariates (age and sex). Our findings do not support the Expensive Brain Hypothesis (EBH), as relative brain size does not systematically decrease with elevation. Furthermore, individual age had no influence on relative brain size or structure. Rather, we found evidence of mosaic brain evolution, supporting the Functional Constraint Hypothesis (FCH). The significant association between elevation and the relative sizes of the telencephalon, optic tecta and cerebellum demonstrates that selective pressures drive the independent evolution of specific brain regions. Notably, low-elevation populations, despite experiencing the highest temperatures, exhibited the lowest annual precipitation and primary productivity. While low-altitude areas accumulate surface runoff from higher elevations, the warm and dry local environment restricts the persistence of small water pools, which cannot satisfy the toad's demand for large, permanent still-water spawning sites. This environmental context may explain the larger telencephalon in low-elevation females, supporting the Cognitive Buffer Hypothesis (CBH) and indicating stronger selection for cognitive abilities in more challenging low-elevation habitats. A comprehensive understanding of these adaptive mechanisms will necessitate future studies across more extensive environmental gradients.