Sex-dependent species discrimination in auditory forebrain of naturally hybridizing birds.

Pairs of individuals breed together only if they recognize each other as the same species, but the process of recognizing conspecifics can depend on flexible criteria even when species-specific signals are innate and fixed. This study examines species recognition in naturally hybridizing sister species, California and Gambel's quail (Callipepla californica and Callipepla gambelii), that have vocalizations which are not learned. Specifically, this study tests whether being raised in a vocalizing mixed-species cohort affects neural activity in the adult auditory forebrain in response to heterospecific and conspecific calls. After hatching, quail chicks were raised either with their own kind or with both species. Once reaching reproductive condition, each adult was played a recording that was one of three types: Gambel's quail opposite-sex contact calls; California quail opposite-sex contact calls; or synthetic tones. Brains were collected following playback and assessed for neuronal activity by quantifying expression of the protein of the immediate early gene, ZENK, in two brain regions, the caudomedial nidopallium (NCM) and the caudomedial mesopallium (CMM). ZENK levels were greater in NCM of males than females, but female NCM cells responded differentially to conspecific compared to heterospecific calls. Namely, females had more immuno-positive NCM cells when they heard conspecific calls rather than heterospecific male calls. Early experience with heterospecific broodmates did not alter neural responses in the NCM or CMM to heterospecific vocalizations. This study suggests that the NCM plays a role in species discrimination but that rearing condition does not alter the response in these non-vocal-learning species.