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Dryad

Size of brain, heart, liver, alimentary tract, and kidneys along altitudinal gradients in Asiatic toad

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Dec 06, 2024 version files 32.98 KB

Abstract

Brain size variation is frequently linked to energetic trade-offs with other metabolically demanding tissues or organs, a concept central to the expensive brain hypothesis (EBH). In this study, we investigate Asiatic toads (Bufo gargarizans) along altitudinal gradients, focusing on the trade-offs in size between the brain and four key visceral organs: the heart, liver, alimentary tract, and kidney. We estimated brain volume by measurement. We measured the dry weight of four organs: the heart, liver, alimentary tract, and kidneys. Additionally, we measured the snout-vent length and body weight of individuals. Our results indicate that both body size and scaled mass index—an indicator of total energy intake—decrease at higher altitudes, suggesting increased energetic constraints in high altitudinal environments. As altitude rises, we observe a reduction in relative brain size, while the relative sizes of visceral organs tend to increase. Through structural equation modeling, we identify a significant negative correlation between brain size and a latent variable termed "budget," which reflects energy allocation to the four visceral organs in high-altitude toads. Notably, the heart shows the most pronounced and consistent response to variations in energy distribution. In contrast, no such relationships are observed in toads at mid- and low-altitudes, where higher energy intake may enable individuals to bypass these trade-offs. When considering EBH in poikilothermic species, it is essential to emphasize total energy intake alongside energy allocation strategies. Future research on EBH should focus on intra-specific comparisons and explore the fitness implications that extend beyond mere energy constraints.