Masculinized dominant females in a cooperatively breeding species.

The molecular mechanisms underlying complex social behaviours such as dominance are largely unknown. Studying the cooperatively breeding African cichlid Neolamprologus pulcher, we show that dominant females were similar to dominant males in dominance behaviour, high testosterone levels and brain arginine vasotocin expression (a neuropeptide involved in vertebrate territorial, reproductive and social behaviours) compared to subordinate helpers, but had lower levels of 11-ketotestosterone than males. Furthermore, brain gene expression profiles of dominant females were most similar to those of the males (independent of social rank). Dominant breeder females are masculinized at the molecular and hormonal level while being at the same time reproductively competent, suggesting a modular organization of molecular and endocrine functions, allowing for sex-specific regulation.