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 competition between the sperm of different males, and via female influences on sperm storage and usage. The existence of, and potential for, these selection forces have led to the evolution of a wide range of behavioural, anatomical and physiological adaptations for successful reproduction. Males increase fertilization either by enhancing the success of their own sperm, or by negating or eliminating rival sperm. Ultimately, however, sperm are under the potential control of the female. In the red flour beetle, Tribolium castaneum, the last male to mate gains fertilization precedence over previous males that have mated. This phenomenon may be due to the morphology of the female sperm storage organ, which could encourage the temporal stratification of sperm through its narrow tubular structure. In addition, males themselves enhance fertilization precedence by removing rival sperm from the female 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 show 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 variance may result from mechanisms of selection and competition that are driven by the complex genitalic structure of T. castaneum males.