Elevational Variation in Organ Size and Brain–Viscera Energetic Trade-Offs in the Minshan Toad (Bufo minshanicus)

Energy trade-offs underlying brain size evolution represent a central topic in evolutionary biology. The Expensive Brain Hypothesis (EBH) posits that the high energetic costs of large brains must be compensated for by increased energy intake or reduced allocation to other costly tissues and functions. We examined the Minshan toad (Bufo minshanicus) along an elevational gradient (2594–3441 m) on the eastern Tibetan Plateau, measuring relative sizes of the brain, five visceral organs, and two reproductive organs, as well as body condition (SMI). Body size and condition significantly decreased with elevation. Relative brain mass showed a non-significant increasing trend with elevation, whereas relative masses of the liver, heart, and alimentary tract significantly increased, and lung mass significantly decreased, with pronounced sexual dimorphism. Relative brain mass was positively correlated with most viscera and male reproductive organs, while a significant brain–ovary trade-off occurred only in females at mid elevation. Overall, elevational environmental conditions shape divergent organ mass covariation patterns. High-altitude habitats may relax energetic constraints to allow coordinated organ development, while moderate mid-altitude stress induces brain–ovary resource trade-offs in females. Our findings do not universally contradict the EBH and instead demonstrate that energetic trade-offs are strongly context-dependent rather than ubiquitous across elevational gradients.