Multiple factors contribute to female dominance in migratory bioflows.

Migration is a widely observed phenomenon supported by morphological, physiological and behavioural traits that vary with season and sex in many species. Recently, the genetic components underpinning migration in the marmalade hoverfly (Diptera: Syrphidae) have been unpacked through detection of differentially expressed genes between migrant and non-migrant females. Males also migrate, but changing sex ratios during autumn migration, from around 50% female in northern Europe to around 90% in southern Europe, suggests males are poor long-distance fliers. To elucidate the mechanisms underpinning this sex difference, we performed morphological, physiological and transcriptomic characterization of actively migrating females and males. Both sexes show similar physiological adaptations including hyperphagia and starvation resistance, but females display higher tolerance to cold, have lower wing loading values and display a greater flight capacity. In addition, females modulate the expression of genes involved in immunity, hypoxia and longevity while suppressing hormonal pathways involved in maintaining reproductive diapause. These traits contribute to the success of female migrants and underlie the diminishing pool of males, influencing population dynamics across huge geographic areas and through the whole migratory and overwintering period.