Sperm storage occurs in bats inhabiting northern temperate regions such as the little brown bat (Myotis lucifugus), and also in many bats such as noctule (Nyctalus noctula). In the little brown bat, the testes become scrotal in the spring, and most sperm production is completed by September. The sperm are then stored until copulation commences months later. Females are inseminated in the fall and winter, while they are in hibernation. Sperm are then stored again, this time in the female reproductive tract, the uterus, where they remain motile for almost 200 days in the noctule. Females ovu-late much later, and active development of the embryos starts in the spring. For bats, delayed fertilization allows males to copulate when they are in best condition in the fall, and parturition to occur just prior to emergence of insects. Because of the energy required for copulation, mating in the spring would be at the time of worst male condition. Delayed fertilization also allows females to give birth immediately after spring arrives, thus allowing more time for offspring growth before the next hibernation period. Thus, delayed fertilization is especially advantageous for species with long periods of dormancy, and allows females to compare breeding males via sperm competition.
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