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Proceedings of the First Symposium on Marsupials in New Zealand

Conclusions

Conclusions

I have attempted here to outline what is known about sperm maturation and epididymal function in the common brushtail possum with a view to comparing it with the pattern for eutherian mammals. There are striking similarities in the structure of the epididymides, although relatively more of the possum epididymis appears devoted to progressive sperm concentration and fluid resorption than is normally seen in eutherians. In terms of morphological changes in the spermatozoa, again the picture for the possum appears basically similar to the eutherian pattern, but the changes are far more dramatic, and -unlike Eutheria - involve shedding of membranes by the sperm as well as elaboration of at least one major structural feature: the midpiece fibrous sheath.

I have also speculated as to how these contrasts may illuminate the selection pressures behind the evolution of the scrotum and epididymis in present day mammals. Clearly these pressures have produced some remarkable parallel developments in the marsupials and placentals. Among the more important pressures would appear to be the need for males to produce large excesses of fertile sperm in competition for breeding success; the increasing endocrine role of the testis; the increasing biochemical demands on sperm imposed by the adoption of internal fertilization and viviparity; and the problems of storing mature spermatozoa in a fertile state for prolonged periods. The storage problem would appear to have been eased in both groups of mammals by the evolution of the thermoregulatory scrotum for cooling the sperm accumulated in the distal portion of the epididymis. I suggest that the proximal regions of the epididymis have progressively assumed greater control over the final stages of sperm maturation in order to ease the demands on the page 35 testis, and if this is so, then the epididymis of the possum would appear to have specialized further along this path than is the case for eutherian mammals.