Testis mass of the spintex hopping mouse and its impact on fertility potential

Captive-bred Australian spinifex hopping mice Notomys alexis have very small testes regardless of their age. Compared with other rodents of similar body mass, these animals also produce and store comparatively low numbers of spermatozoa. In the present study, we thus ask the following questions: (1) what is the testis weight of sexually mature spinifex hopping mice in the natural environment and does this change at different times of reproductive activity of the population and (2) what is the fertility potential of adult sexually mature males? The results show that wild-caught individuals, like the captive-bred animals, invariably have very small testes, regardless of whether females in the population are, or are not, reproducing. Spermatogenesis continues at times when females are reproductively inactive, thus suggesting males may remain potentially able to inseminate females as soon as they enter oestrus. In spite of their very small testes and low epididymal sperm stores, an adult male can fertilize the ovulated oocytes of at least two females within a period of a few days. Thus, although sperm reserves are likely to be heavily depleted after the first ejaculation, males rapidly return to maximum fertility, which is no doubt due, at least in part, to the short sperm epididymal transit time that occurs in this species.