Morphology of Tribolium castaneum male genitalia and its possible role in sperm competition and cryptic female choice

It is now well recognised that sexual selection proceeds after copula via c ompetition between the sperm of different males, and via female influences on sperm storage and usage. The existence of, and potential for, these sele ction forces have led to the evolution of a wide range of behavioural, anat omical and physiological adaptations for successful reproduction. Males inc rease fertilization either by enhancing the success of their own sperm, or by negating or eliminating rival sperm. Ultimately, however, sperm are unde r the potential control of the female. In the red flour beetle, Tribolium c astaneum, the last male to mate gains fertilization precedence over previou s males that have mated. This phenomenon may be due to the morphology of th e female sperm storage organ, which could encourage the temporal stratifica tion of sperm through its narrow tubular structure. In addition, males them selves enhance fertilization precedence by removing rival sperm from the fe male tract. This study examines the detailed external and internal genital morphology of T. castaneum using scanning electron microscopy, and relates form to potential function in intrasexual competition and selection. We sho w that the aedeagus may articulate in a scoop-like manner to remove sperm, and we observe a retractable brush-like structure and discuss its function. In this beetle, a large and unexplained variation in reproductive success is observed between competing males and we suggest that some of this varian ce may result from mechanisms of selection and competition that are driven by the complex genitalic structure of T. castaneum males.