Evidence for the expensive-tissue hypothesis in the Omei Wood Frog (Rana omeimontis)

Brain size variation across the animal kingdom can be interpreted as a trade-off between selective advantages of higher cognitive ability and the prohibitively high energy demands of a large brain. The Expensive-Tissue Hypothesis (ETH) predicts that brains are costly, and increases in brain size will decrease the size of other metabolically costly tissues. Here, we tested this prediction using the anuran Rana omeimontis. Brain size was negatively correlated with gut length, supporting the ETH. We did not find associations between brain size and the size of other organs (heart, liver, lungs, kidneys), but found positive correlations between brain mass and testes as well as limb muscle mass when correcting for the effects of body condition. The negative correlation between gut length and brain mass suggests that diet quality may play a role in the evolution of brain size in R. omeimontis.