Neuronal sex identity shapes social buffering of defensive freezing behaviour in fruit flies

Sex differences are pervasive across the animal kingdom1,2, shaped by reproductive strategies, ecological roles, and physiological demands. Neurobiological studies have largely focused on sex differences in reproduction and aggression3,4, leaving understudied how sex shapes other survival-relevant behaviours such as responses to predatory threats. Previous work in vertebrates uncovered sex differences in individual defence responses deployed by rats5 and in social modulation of freezing responses in zebrafish6. The origin of these differences and their potential underlying neuronal circuitry remain however unknown. Here we characterise defensive responses to a visual threat of female and male fruit flies tested alone and in groups. Individually, defensive responses are comparable across sex, with similar freezing levels and moderately increased escape responses in males. In groups, however, we uncovered strong sex differences in freezing as males show consistently shorter freezing bouts, reflecting a stronger social buffering effect than in females. Using a generalised linear mixed model, we demonstrate that the shorter freezes of male flies, when tested in groups, are not explained by well-known locomotor differences between sexes7,8. Although males move more and faster, generating stronger social motion cues that mediate the "safety in numbers" effect in female flies9,10, their response to the surrounding motion is not increased. We further identify the nervous system as the source of this sex-dimorphic behaviour by feminising males through pan-neuronal ectopic expression of transformer, a key regulator of the cell-autonomous sex-differentiation pathway. Finally, we ruled out the visual projection neurons, lobula columnar neurons 11 (LC11), detectors of social motion10, as the source of the neuronal-sex differences observed. This study identifies a neuronal origin of sex differences in socially modulated behaviour that extends beyond sex-specific behavioural modules or locomotor function, important to understand differential impact on survival upon threat across sexes.