Prolonged sperm storage, rare among mammals, is widespread among bats and may promote sperm competition, assuming stored sperm are fertile. However, while sperm storage has been documented in many bat species, there have been few investigations of the fertility of stored sperm. Related to this, and a fundamental question in the study of competition at the gametic level, is the effect of mating order on success during sperm competition. For mammals there are no clear mating order effects, in part because females do not store sperm for prolonged periods, but there have been no investigations of mating order effects during sperm competition in sperm-storing bats. I report here on a study designed to investigate the fertility of stored sperm and the effects of mating order on success during sperm competition in a captive colony of Australian long-eared bats Nyctophilus geoffroyi (Chiroptera: Vespertilionidae). Bats were housed in outdoor flight-cages, under natural light and temperature regimes. Females were housed serially with two males and were separated from them at intervals throughout the year.

Blood plasma progesterone concentrations, measured using double antibody radioimmunoassay, were monitored to assess the duration of sperm storage. Paternity of offspring was assessed using genetic markers identified using gel electrophoresis of blood enzymes. Blood plasma progesterone concentrations were basal from early autumn till mid-late winter, increased markedly during pregnancy, and generally fell back to basal 5–6 weeks post-partum. Data indicated that females could store fertile sperm for more than 90 days, while sperm stored by males were fertile after 4–5 months' storage. Paternity was significantly biased toward one male which fathered all offspring. Results are discussed in the context of sperm competition.