Sex-specific hormone signatures in four fish species with divergent reproductive strategies.

Introduction: Steroid hormones regulate diverse biological processes, including development, immunity and reproduction. Because biological sex, pregnancy, parental care, and mate competition are closely associated in most established model organisms, it remains unclear whether endocrine differences between males and females are primarily linked to gametic sex or to the reproductive roles performed by each sex. Syngnathid fish, the pipefishes, seadragons, and seahorses, provide a powerful system for disentangling these effects, because males, rather than females, carry the developing embryos. Species further differ in sexual selection and in the extent of paternal investment, ranging from externally attached eggs to incubation with an enclosed brood pouch. Methods: This study provides baseline hormonal profiles of key steroid and peptide hormones in three syngnathid species across the unique evolutionary parental care gradient of male pregnancy ( Results: Sex-specific endocrine profiles broadly followed sexual selection differentials: species with stronger sexual heteromorphism, i.e., more extreme reproductive roles ( Discussion: By incorporating a cross-species comparative framework with both sex-role reversed and a conventional viviparous species, we can begin to understand which hormonal functions are conserved versus derived, and whether endocrine profiles are primarily aligned with biological sex or reproductive role, such as mate competition and parental care, thereby revealing how endocrine constraints and plasticity contribute to reproductive evolution.