Why do so many flour beetle copulations fail?

Copulations that fail to result in fertilisations are common across a broad range of species, and remain enigmatic given the inherent costs that are being paid by both partners. The determinants of failures are varied and can take effect before, during, or after copulating. In the red flour beetle, T ribolium castaneum ( H erbst) ( C oleoptera: T enebrionidae), population estimates of copulations failing to result in fertilisations as high as 55% have been reported, suggesting that the causes of these failures play an important role in the mating system, or that the costs experienced by those determining failure, in this system likely the females, is low. Here, we show that failure is determined peri‐copulation; successful spermatophore transfer (indicated by change in mass of mating individuals) typically results in offspring production, suggesting that mechanisms of post‐copulatory female choice do not cause outright failure. The proportion of copulations that failed to result in fertilisations was apparently not influenced by the relatedness of mating pairs, the age, mating status, or mass of the female, or the interval she experienced between matings. This suggests that alternative adaptive explanations are responsible for the prevalence of copulations that fail to result in fertilisations in this species, or that the costs involved are sufficiently small, so that selection against such failures is weak.