Dorsal Crypsis and Chemical Defense Jointly Shape Brain Size in Toads.

The cognitive benefits of having a large brain must outweigh its substantial energetic demands for such an organ to evolve. The cognitive buffer hypothesis proposes that large brains facilitate survival in complex environments via flexible behaviors. Because active predator evasion is likely more cognitively demanding than a cryptic lifestyle, we reasoned that the evolution of crypsis could relax selection on brain size. To test this hypothesis, we examined the effect of dorsal crypsis, ventral crypsis, and granular glands on relative total brain evolution in eight toad species. As predicted, total relative brain size was negatively associated with dorsal crypsis across species, but was not related to either the volume or the count of granular glands. Additionally, because certain brain regions may be specifically involved in predator evasion, we assessed relationships between five major brain region sizes and both dorsal crypsis and granular gland traits. Only the olfactory nerves and telencephalon showed negative associations with dorsal crypsis, whereas optic tectum volume correlated positively with granular gland volume. These regions support complex evasion strategies. Our findings indicate that increased dorsal crypsis relaxes selection on cognitively demanding evasion, leading to smaller brains. Thus, crypsis evolution is a previously underexplored factor in vertebrate brain anatomy evolution.