Relaxed feeding constraints facilitate the evolution of mouthbrooding in Neotropical cichlids

Multifunctionality is often framed as a core constraint of phenotypic evolution. Mouthbrooding, a form of parental care where offspring develop inside a parent's mouth, increases multifunctionality by adding a major function (reproduction) to a structure already serving other vital functions (feeding and respiration). Despite increasing multifunctionality, mouthbrooding has evolved repeatedly from other forms of parental care in at least 7 fish families. We hypothesized that mouthbrooding is more likely to evolve in lineages with feeding adaptations that are already advantageous for mouthbrooding. We tested this hypothesis in Neotropical cichlids, where mouthbrooding has evolved 4–5 times, largely within winnowing clades, providing several pairwise comparisons between substrate brooding and mouthbrooding sister taxa. We found that the mouthbrooding transition rate was 15 times higher in winnowing than in non-winnowing clades, and that mouthbrooders and winnowers overlapped substantially in their buccal cavity morphologies, which is where offspring are incubated. Species that exhibit one or both of these behaviors had larger, more curved buccal cavities, while species that exhibit neither behavior had narrow, cylindrical buccal cavities. Given the results we present here, we propose a new model for the evolution of mouthbrooding, integrating the roles of multifunctional morphology and the environment.