In eutherian mammals, the size of the testes and number of spermatozoa produced and stored in the excurrent ducts vary widely between species, with the hydromyine rodents of Australia exhibiting a greater range of interspecific variation than any other closely related group of species. This study compared the efficiency of germ cell production and sperm storage capacity in the extra-testicular ducts of two arid zone species, the plains rat Pseudomys australis and the spinifex hopping mouse, Notomys alexis, that have vast differences in testes size and number of stored spermatozoa.

In the plains rat, the seminiferous tubules occupied 94% of the total testicular volume. Eleven germ cell associations were identified with a single association being present in a tubule cross section. By contrast, in the hopping mouse the seminiferous tubules comprised only 85% of testicular volume. Eight cellular associations were identified and were confined to small regions of the seminiferous epithelium, such that a multi-stage tubule cross-section was sometimes evident. In the plains rat, the duration of the spermatogenic cycle was calculated to be 11.2 days, spermatogenesis 44.8 days, and mean sperm production 26 x 10⁶ sperm/g testis/day or 40 x 10⁶ sperm/animal/day. By contrast, in the hopping mouse the duration of the spermatogenic cycle was 14 days, spermatogenesis around 56 days, and average sperm production only 9.8 x 10⁶ sperm/g testis/day or 0.5 x 10⁶ sperm/animal/day.

In the plains rat the epithelium of the caput and corpus epididymides displayed features (long stereocilia, numerous vesicles and vacuoles) indicative of absorption and secretion, whereas the cauda had a low epithelium surrounding a wide lumen filled with spermatozoa, suggesting storage. The vas deferens had a thick muscular coat, particularly towards its urethral end. The duration of sperm transit through epididymis and vas deferens averaged 10.1 days, with 2.3 days spent in the caput and corpus, 6.9 days in the cauda, and 0.9 days in the vas. The number of sperm present ranged from 360 to 1,890 x 10⁶ per side of the tract, with >94% of these, representing around 40 day’s production by the testes, located in the paired caudae epididymides, and <6% in the vasa deferentia.

In the hopping mouse, fewer structural specialisations were observed along the length of the epididymis. From the caput to the cauda region, there was a gradual decrease in the height of the principal cells and their stereocilia and a gradual reduction in the number of vesicles and vacuoles, although the Golgi became more prominent. Luminal diameter was relatively constant, apart from an increase in the cauda region. The urethral segment of the vas deferens showed extensive mucosal infoldings and had a very thin muscle coat. Sperm transit time averaged 5.6 days, with less than one day required by sperm to pass through the caput and corpus epididymides, 3.1 days in the cauda epididymidis and 1.6 days in the vas deferens. The total number of spermatozoa stored ranged from 0.8 to 3.1 x 10⁶ per side of the tract, with as many as 57% of these located in the vasa deferentia in some animals. Furthermore, total sperm stores per animal in the excurrent ducts represented only 4-5 day’s production by the testes.

In conclusion, the structural organisation of the testes, epididymides and vasa deferentia of the plains rat, the kinetics of sperm production, epididymal sperm transit and storage are similar to the pattern previously described in common laboratory rodents. By contrast, in the hopping mouse, the organisation of the testicular seminiferous epithelium, kinetics of sperm production, and storage were very different and, in fact, more reminiscent of that of humans. The low number of epididymal spermatozoa in the latter species is in part a consequence of small testes containing relatively less sperm producing tissue, coupled with a lower efficiency of sperm production and a longer duration of spermatogenesis. Furthermore, transit of spermatozoa through the extratesticular ducts is more rapid and cauda epididymal storage capacity is low, although some additional sperm are stored in the vasa deferentia. While the significance of such a reproductive strategy remains to be elucidated, it is evident that selective forces for maximising testis size and sperm numbers appear to have become relaxed in the hopping mouse, and that vast differences in the dynamics of germ cell production and storage have evolved in these two conilurine desert rodents.