Large Brains, Small Guts: The Expensive Tissue Hypothesis Supported within Anurans.

Brain size differs substantially among species, and several hypotheses have been proposed to explain the evolution of brain size. Because the brain is among the most energetically expensive organs in the vertebrate body, trade-offs have been hypothesized to exert constraints on brain size evolution. Prominently, the expensive tissue hypothesis (ETH) proposes that reducing the size of another expensive organ, such as the gut, should compensate for the cost of a large brain. But energetic constraints may also drive covariation between the brain and other costly traits-such as body maintenance, locomotion, or reproduction-as formulated in the energy trade-off hypothesis. To date, these hypotheses have mainly been tested in homeothermic animals and within the ectothermic animals, primarily in fishes. Here, we undertake a comparative test of the interplay between energetic limitations and brain size evolution within amphibians. After controlling for phylogenetic relationships and body size, we find a negative correlation between brain mass and the length of the digestive tract within 30 species of anurans. We further find that the evolution of large brain size is accompanied by an increase in female reproductive investment into egg size. Our results suggest that the evolution of brain size follows general patterns across vertebrate clades.