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General Discussion

MORIARTY. Could you give some indication of the time sequence required for this dramatic change in sperm morphology to take place?

CUMMINS. We have some indication that the time period from spermatogenesis to release of ejaculate is about two weeks. This is comparable to most eutherian mammals of the same weight and size. So it takes some two weeks for sperm to pass through the epididymis and I would estimate that they probably remain viable in the tail of the epididymis for up to 2 or 3 weeks.

BROCKIE. How long do you think the sperm could remain viable in the vagina of the female?

CUMMINS. I really do not know. We have kept them in culture for up to 48 hours. My guess would be 24 hours maximum.

ANONYMOUS. Do you find sperm is released into the urine?

CUMMINS. The sperm is normally released into the urine in the possum. At one time it was thought that in eutherian mammals the sperm is absorbed when not ejaculated. I think the idea has largely gone by the board. For example rams that are not allowed to mate will pass into the urine daily amounts of sperm equivalent to the daily testicular sperm production. So it happens in eutherians as well as in marsupials.

GREEN. Does your work suggest that chemo-sterilant control is a viable possibility sometime in the future, or would you look rather to the female from this point of view.

CUMMINS. If I were attempting to devise a strategy I think I would look at the female, but I think the male must not be ignored. If we look at chemosterilants in the male we must look at the prostatic secretions - that is something that will affect the sperm once they have been ejaculated. While in the epididymis they are pretty untouchable.

GREEN. Are you going on to look at this aspect?

CUMMINS. Yes, I hope we will.

BROCKIE. Would I be right in thinking the sperm is produced regularly throughout the year without seasonal variation?

CUMMINS. Yes. The evidence for the possum is that there is no seasonal variation in sperm output or testis weight, though there is seasonal variation in prostate weight. Certainly we have found no seasonal variation in sperm quality in the animals we have examined.

BROCKIE. So seasonality in the reproductive cycle will depend on the female rather than the male?

CUMMINS. Yes, I think so.

WODZICKI. The work of Dr Cummins has described demonstrates how easy it is in New Zealand to carry out not only applied but fundamental work on marsupials. I remember Dr Tyndale-Biscoe had to travel hundreds of miles to obtain a few tammar wallabies, while here if we wish to study wallabies we can get them in hundreds. The same is of course true of the possum. There is a great prospect for university zoology departments to contribute to fundamental studies of page 39 these marsupials. Also Dr. Cummins has shown that the reproductive system is not at all antiquated or primitive, but rather that the marsupials have successfully evolved during the long time they have been in Australia.

MORIARTY. I am sure that the message will come from this symposium that we are sitting on a gold-mine of experimental material.

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