Proceedings of the First Symposium on Marsupials in New Zealand
Keynote Address: — Advances in Marsupial Biology – A Review of 25 Years' Research
Advances in Marsupial Biology – A Review of 25 Years' Research
I must preface this paper by saying how much I appreciated the invitation to attend this first symposium on marsupials in New Zealand and to deliver this address. My own interest in marsupials began in New Zealand and I was looking forward keenly to meeting all of you today. It is a very great disappointment that I cannot be at the symposium and can only participate as a disembodied voice.
When I was asked to give this address I thought I should review the most salient features of marsupial research over the last 25 years. It soon became obvious, however, that it would be quite impossible to do this and I have chosen, instead, to trace the way in which our attitudes to marsupials have changed and how these attitudes colour the research we do. Science like religion satisfies a deep need to find order in a chaotic world, and scientific theories that provide a simple, clear arrangement are often held tenaceously long after their usefulness has passed. This idea has been very well developed recently by Professor Hayes (1977) in relation to the development of molecular biology, and I think it may be instructive to apply it to marsupial biology.
Twenty-five years ago the prevailing view about marsupials was essentially still T.H. Huxley's view that they are physiologically intermediate between reptiles and higher mammals and that their present distribution in only two of the southern continents is because they were driven to the remote corners of the world by later evolved, superior mammals and saved from extinction by isolation there. If there was any merit in studying them it was for the light that might be shed on the evolution of higher mammals. As a consequence, virtually nothing was known about the physiology, ecology or reproduction of any Australian marsupial 25 years ago, despite the fact that some species, such as the red kangaroo Macropus rufus in Australia and common brushtail possum Trichosurus vulpecula in New Zealand, were becoming serious economic pests.page 10
The person who was largely responsible for changing this attitude in Australia was Harry Waring who came to the Chair of Zoology in Western Australia in 1948, bringing the outlook of a comparative physiologist and the conviction that the study of marsupials was intrinsically worthwhile (Waring 1956). The group he attracted to Perth from other parts of Australia and New Zealand concentrated their attention on the quokka Setonix brachyurus, a small wallaby common on Rottnest Island, and in February 1954 preliminary notices of the first two important discoveries were published in Nature. These were the phenomenon of delayed pregnancy or embryonic diapause (Sharman 1954) and the ruminant-like digestion and metabolism (Moir et al. 1954) of the quokka. Both phenomena were subsequently found to be shared by most or all the Macropodidae and profoundly influenced the subsequent direction of research in reproduction and metabolism respectively. Other work by this group on temperature regulation and water metabolism of the quokka and other desert-living macropods indicated regulatory capacities quite the equal of desert-adapted eutherian mammals.
About the same time in the mid-1950's research programmes were begun by several state and commonwealth bodies on the ecology of the large kangaroos in response to the growing public concern at the adverse effects these species were having on the pastoral industry. For instance, in north western Australia the marginal lands were carrying increasing numbers of kangaroos while sheep numbers steadily declined. It soon became evident that this was due to the better physiological adaptation of the kangaroos to the country and to their opportunistic breeding strategies. So almost from the beginning basic and applied research on macropods proceeded simultaneously and knowledge is now much more advanced about this than any other family of marsupials. Also as a result of these interrelated studies across the country a new appreciation of marsupials developed so that in the preface of our review (Waring et al. 1966) published in 1966 we felt constrained to say of 'the old and recurrent notion that marsupials are inferior mammals. That among other things we think this review kills that superstition.' We hoped to establish a new view of marsupials as the equals of other mammals, not from misplaced chauvanism but because the older view was arbitrary and restrictive. While the new view certainly influenced thinking in Australia the rest of the scientific world took little heed and even as recently as January this year Nature carried a commentary by Cox (1977) entitled 'Why marsupials can't win'.page 11
Fig. 1. Relationship between body size and minimum heart rate in marsupials compared to eutherians (from Kinnear and Brown 1967)
Fig. 2. Relationship between body size and standard metabolic rate in marsupials compared to eutherians (from Dawson and Hulbert 1969)
These results found ready acceptance, as they accounted for other aspects of physiology, such as basal temperature which tends to be a few degrees lower than in Eutheria and water turnover and circulating hormone levels, particularly adrenal and thyroid hormones. Terry Dawson (1972) has developed the hypothesis most fully and has contrasted marsupials with monotremes and some of the more primitive Eutheria such as insectivores. He concludes that low metabolic rate is a primitive characteristic that all these mammals have retained, but overlaid on this the marsupials have evolved advanced mechanisms of temperature regulation, kidney function and digestion. Indeed the low metabolic rate may have pre-adapted the macropods to desert conditions as they require less water and less protein nitrogen. However, Victor Macfarlane (1971) has challenged this whole concept; he considers the double log relationship of body weight to metabolic rate obscures substantial differences page 13 between the species of one taxon and that the marked difference reported between Eutheria and marsupials can be partly accounted for by the fact that Kleiber used largely temperate zone species while the Australian studies used almost exclusively desert or semi-desert species. When the water turnover of rodents, antelope or dasyurid marsupials from different climates are compared the desert-evolved species generally have lower metabolic rates than those species from cool temperate climates. The generalization that marsupials have a fundamentally different rate setting, which affects all their physiological functions, has been a very useful and stimulating concept for almost a decade, but like its predecessors it may have outlived its use and we should now be ready to modify it.
Let us turn now to another field of study and another example where prevailing attitudes influenced thought. Sharman's discovery of embryonic diapause in the quokka provided a powerful impetus to research in reproduction because it afforded such a beautiful experimental model with which to examine the endocrine and other control mechanisms operating in marsupials. Nevertheless, with hindsight I can see that we were heavily conditioned in our thinking by the attitude that the marsupial pattern would be a simplified eutherian plan with the placenta playing little or no part. Since pregnancy is accommodated within a single oestrous cycle of one month's duration and the corpus luteum of pregnancy does not have an extended life, we assumed that its role in the control of pregnancy was paramount. When in 1953 I found that pregnancy would continue quite adequately after ablation of the corpus luteum at the end of the first week we entertained ideas about long-acting hormones in circulation being responsible but not placental involvement. Ten years later when circulating levels of progesterone were measured in the tammar wallaby Macropus eugenii (Lemon 1972), which indicated differences between pregnant and non-pregnant animals, none of us were yet convinced that the placenta was responsible. Eventually Marilyn Renfree's (1972) work showing differences in the endometrial tissue and secretions of the two uteri, convinced some of us that the yolk sac placenta was having a local stimulatory influence on the pregnant uterus but this idea is only now gaining general acceptance. Contemporaneously other studies indicated differences in response of the myometrium of the pregnant and non-pregnant uterus to oxytocin and now finally the placenta of the tammar and quokka have been shown to have the capacity to synthesise progesterone. If our minds had been less prejudiced the evidence was at hand 15 years ago.page 14
The discovery of the role of the pituitary in diapause is also interesting in the same way. During diapause the corpus luteum is small and follicle growth is suppressed. By analogy with the rabbit and rat it was assumed that the suckling stimulus caused reduced secretion of gonadotrophin and raised the levels of prolactin. Prolactin in rats is luteotrophic, but clearly it was not having a luteotrophic effect in the quokka; so we supposed that this might be because insufficient was available to stimulate both the mammary glands and the corpus luteum. When John Hearn (1973) hypophysectomised tammars however, the results were quite unexpected for the corpus luteum immediately reactivated and pregnancy proceeded, which clearly showed that the pituitary exercises a tonic inhibition of the corpus luteum, which is otherwise autonomous. Subsequent experiments have shown that gonadotrophin levels are unchanged in diapause and pregnancy and that the corpus luteum inhibitor is prolactin, a corpus luteum stimulator in rats. It now appears likely that prolactin is the hormone that suppresses the return to oestrus in lactating cows and causes lactational amenorrhoea in women.
Ecological thinking about marsupials was dominated in Australia until quite recently by kangaroos of the arid and semi-arid zones and some of the most stimulating ideas are currently being generated by this work. In his recent review, Alan Newsome (1975) marshalled the results of 20 year's research on red kangaroo and common wallaroo Macropus robustus to develop the thesis that the huge increases in populations of these two species early this century were the result of the removal of the primeval constraints of fodder and water shortage when stock were introduced. A succession of droughts through the 1940's took heavy toll of stock and red kangaroos but the wallaroos were able to exploit the ravaged country because of their less demanding dietary requirements (Fig. 3). He predicted that red kangaroos may never again reach the level of the earlier years wherever there is competition with any two introduced herbivores, cattle, sheep or rabbits. He also suggested that similar sequences of sudden abundance and subsequent collapse also befell the small marsupials of the grasslands but much earlier when the first introduction of stock converted tall grasslands to close cropped pasture. The relevance of this to our present meeting is that when their habitat was profundly altered these species behaved like invaders in a new territory. I understand that there is evidence in New Zealand that some possum populations may be undergoing this sort of sequence. If so the population densities of more than 10 adults per hectare recorded in New Zealand forests (Crawley 1973), which are 20 times the densities recorded in Australian forests, may be a transient phenomenon.page 15
Research on forest-dwelling marsupials has a short history in Australia and only began in earnest after the growth of public interest in forest conservation in the 1970's. At present the only completed studies were all initiated in universities as graduate student projects. Nevertheless the ideas that have arisen from these may have more relevance to our deliberations, since almost all the marsupial species now in New Zealand originated from this habitat.
The forests of the eastern seaboard harbour a greater diversity of mammals than any other part of Australia and were, until recently, the least affected by European settlement. The radiation of marsupials across Australia and New Guinea probably began in forests like these and, from the few studies done so far, it is apparent that the present day inhabitants show adaptations for the regulation of their populations at low densities, which do not conform to the conventional theories derived from fecund species of the northern hemisphere. Males of several species of Antechinus page 16 die immediately after the breeding season, while the females give birth and support a litter of young for several months, and these young then become the breeding population of the next year. The cause of the male die-off is complex (Lee et al. 1977); an androgen-driven aggressive behaviour pattern with prolonged bouts of copulation is associated with high plasma concentrations of free cortisol, negative nitrogen balance and increasing susceptibility to diverse pathogens. Selection may have favoured aggressive behaviour but it has also removed one half of the population immediately after its biological function is completed, which must favour the survival of the females and their progeny.
|Trichosurus vulpecula||T. caninus|
|Season of birth||April - October||March|
|Mother-young association (days)||175||250|
|Dispersal age (years)||0.5 - 1.5||1.5 - 2.5|
|Sexual maturity (years)||1||2-3|
|Male-female association||polygamous||probably monogamous|
page 17 particular male and female pairs, which suggests that the species may be monogamous and that the adults have a high attachment to their territory which is held exclusively. Where the two species are sympatric, the common brushtail is excluded from rain forest or wet sclerophyll forest by the mountain brushtail, but is more abundant in open or drier habitat. In Tasmania, where the mountain brushtail is absent, the common brushtail occupies the wet sclerophyll forest. So far as I can gather from Pracy's (1962) account, the mountain brushtail was never introduced to New Zealand but it is interesting and not entirely academic to speculate from current knowledge on what might have happened if it had been. It is likely that it would have excluded the common brushtail from the high rainfall forests and would have occupied them at low density as it does in eastern Australian rain forests. However, had we been able to advise the Acclimatisation Societies of 100 years ago to liberate the mountain brushtail it would not have commended itself to people concerned to populate the forests of New Zealand with a good fur bearer as quickly as possible. It is interesting to read the opinions of Cockayne and Kirk, both eminent botanists, on the common brushtail. Both were in favour of its spread and believed that its effects on the native vegetation would be negligible. Now opinion in New Zealand has wholly changed and the serious and seemingly intractable problems raised by this highly adaptable species will engage most of our attention at this meeting.
It would be a pity if the pressing urgency of the problems caused by the common brushtail possum should lead to the view that all liberations of marsupials in New Zealand were an unmitigated disaster, for in the small but established populations of tammar wallabies near Rotorua, in the wallabies on Kawau, Rangitoto and Motutapu Islands and in the red-necked wallaby Macropus rufogriseus in South Canterbury (Wodzicki and Flux 1967), New Zealanders have the opportunity to make important contributions to marsupial biology and shape new ideas. Ideas which may also contribute to a better understanding of the common brushtail. In applied research no less than in basic research we must beware of the seductive charm of prevailing attitudes and hypothesis, and be ready to entertain heterodox ideas from wherever they come. Of course it is axiomatic that for new ideas to emerge the results of all research must be open to critical appraisal by the whole scientific community. Nowadays the major problems in any field of research can only be adequately tackled by government-funded organisations able to deploy substantial resources over a long time. A disadvantage of this is that it imposes the constraint of definite objectives which may be political or economic as well as scientific.page 18
University research groups provide an important dialectic; they are not thus constrained and graduate students seem to be more ready to entertain heterodox ideas than their seniors and the best of these ideas get tested out in their projects. Some are fizzlers but a few transform the subject. As I prepared this talk I was impressed by how many of the new directions in thought about marsupials during these last 25 years have begun with a graduate student's research.
It is most appropriate that the organisers of this symposium are providing time in the programme for discussion on the universities' role in research on marsupials. I hope that our deliberations, or the published proceedings, may stimulate more students to seize the real opportunities New Zealand offers for research in this field of biology.
The organising committee deserve our thanks for conceiving the idea to hold this symposium and the response to it is evidence that they were right; it is a splendid opportunity for all of us engaged in research on marsupials to share experiences and problems in the certain expectation that this way leads to new ideas and better research.
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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?page 21
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.page 22
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.
* Refer also to abstract of paper by J.N. Jolly, later in this issue - Editor.