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General Discussion

CATT. Have you noticed a change in the appearance of the corpus luteum in the tammar during seasonal quiescence?

TYNDALE-BISCOE. There is no apparent change during quiescence but there does seem to be a change in the power of inhibition by the pituitary. We are studying this at present and we think the level of inhibition by prolactin may vary, so it is possible to trigger off reactivation more easily at certain times of the year.

B.D. BELL. You mentioned lower productivity, a shorter breeding season and a later maturity in the mountain brushtail Trichosurus caninus when compared with the common brushtail T. vulpecula. In New Zealand results from the Orongorongo Valley study, at least, suggest T. vulpecula tends towards the situation you describe for T. caninus - we have recorded low productivity, reduced breeding and delayed maturity there. I wonder to what extent the environment is influencing the pattern in T. vulpecula?

TYNDALE-BISCOE. That is a good question. In How's study there was a clear difference between the two species. One could carry out a parallel study in Tasmania where caninus does not occur and vulpecula occupies forest which is more similar to New Zealand forest. It is interesting that vulpecula might show this kind of strategy in the Orongorongo Valley since there the peak population density is probably past. We might be getting a situation where the more K-selected pattern is developing. The other aspect of caninus is that the association of the young with the mother is much longer, up to 250 days, as compared with 175 days for vulpecula. Do you have similar observations for possums in the Orongorongos?

B.D. BELL. Your figure of 175 days strikes me as low for vulpecula. I calculated a period of 170 days to the end of pouch-life and there is association with the mother for a period after that, in cases for at least a further six months.

TYNDALE-BISCOE. So your population of vulpecula is getting close to the pattern that How described for caninus.

B.D. BELL. Yes in some individuals, while others fall between the period 175 to 250 days.^*

TYNDALE-BISCOE. Is the Orongorongo possum population now lower than 1966-1968 when Dr Crawley made his study?

B.D. BELL. The population has fluctuated over the years rather than showing a conspicuous downward trend.

GREEN. I think we are all impressed by the examples you gave of how our current thinking can influence the interpretation of results. I quite often hear the possum referred to as a dumb animal, so perhaps you might have some comments on the intelligence of marsupials in general?

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TYNDALE-BISCOE. That's a very difficult question. I believe various marsupials have been the subject of standard psychological tests, with for instance Skinner boxes, but I would be rather sceptical about interpreting the results. When marsupials are presented with such situations they tend to perform less well than, say, laboratory rats. But they are not in a normal environment, not in the situation which is of significance in their survival so I'm not sure how one would go about testing their innate intelligence.

CUMMINS. In New Zealand we are concerned with disease in possums, especially leptospirosis and bovine tuberculosis. Have you any observations on diseases of forest dwelling marsupials in Australia?

TYNDALE-BISCOE. After a request from Dr Brockie two years ago I did make enquiries but came up with little recent information. However the earlier literature cites reports of epidemic-type diseases going through koala Phascolarctus cinereus populations and other marsupials. None of these were properly authenticated and most people tended to dismiss them. In defence, there were attempts to explain the sudden disappearance of many of the smaller marsupials. I think the hypothesis I mentioned in my address has more substance - an abnormal increase in their numbers initially, and then a subsequent crash when the habitat was no longer suitable. Most of the disease reports have little evidence and I'm not aware of anything in Australia resembling your outbreak of bovine tuberculosis.

WODZICKI. Many of us working on the possum in New Zealand are concerned with the problem of their control, and they have continued to spread since you left the country. Could you comment on how we might attack this problem over the longer term?

TYNDALE-BISCOE. I think it is the toughest problem that New Zealand biologists working on marsupials have got to face, and we have nothing like it in Australia. Given the idea that populations will ultimately stabilise at a lower level, then it follows there will be a natural adjustment. When deciding when to interfere with the situation, one has got to look at the life-cycle of the animal to see at what point control measures can be most effectively applied. In other words, if one were to attack the animal during the breeding season or just afterwards, one might have more effective control of the population than if it were applied later in the year when juveniles are already becoming independent.

KEBER. Is there any possiblity of biological control, such as with myxomatosis in the rabbit?

TYNDALE-BISCOE. Given that myxomatosis caused little problems with the endemic cottontail Sylvilagus braziliensis in South America, but was virulent in our rabbit Oryctolagus cuniculus, then I suppose New Zealand biologists could collect a variety of South American marsupial species and test any pathogens against brushtail possums. I believe some South American marsupials are considered reservoirs of viruses which affect humans, horses and cattle. It is a possible line of attack.

CUMMINS. It has been suggested the possum may be rather deficient in cell mediative immunity, and one idea is that instead of trying to eradicate tuberculosis in possums one should be spreading it!

TYNDALE-BISCOE. I thought that in general their immune response was pretty good and that it developed at quite an early age in pouch-life. This is work which as far as I know has not been published. There was some work done in New England on I think the development of the immune response in the possum.

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SPURR. I don't know if you're going to bite at this one. We want an opinion on the nomenclature 'possum' versus 'opossum'.

TYNDALE-BISCOE. I thought that might come up! I've prepared a photocopy from a book by Grzimek called 'Mammals and Man' and there is reference to the marvellous American opossum Didelphis which has spread through California and recently to New Zealand where it now occurs in many millions and is causing much damage to the forests! This indicates the desirability of trying to keep one common name for the American animals and distinguishing the Australian animal by a different vernacular name. 'Opossum' has become well-established in New Zealand which I think is unfortunate because it is generally used outside our countries by people to represent the American animal.

CUMMINS. I think it is very courageous of you as a New Zealander to point out this conservatism. On behalf of the meeting I would like to thank you very much for your stimulating address.