Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

Proceedings of the First Symposium on Marsupials in New Zealand

Introduction

Introduction

Tyndale-Biscoe (1955) first investigated the age structure of common brushtail possum Trichosurus vulpecula populations. Using epiphysial fusion of the tibiae as a criterion he divided 120 Banks Peninsula animals into three age classes, the youngest of which were sexually immature and probably under a year old. Between May and August the proportion of 0–1 year olds fell from 48% to 23%. In one area with a history of severe trapping, young animals comprised one third of the sample whereas in another undisturbed area they comprised only 8% and those over 4 years comprised 55%. This implies that adult animals lived on average for more than six years (Tyndale-Biscoe 1975).

In 1953, 1449 possums were taken from the Orongorongo Valley, Wellington and, using tooth wear and skull sutures as an aging technique Kean (1975) estimated that 52.9% were over 4½ years old.

page 64

Pekelharing (1970) developed a reliable method for allotting possums to annual age classes based on the deposition of cementum layers of the molars. Earlier techniques could be misleading (B.D. Bell, A.J. White, unpubl.).

Throughout this paper, animals without an inner cementum layer on the lower first molar are classed as 0–1 year olds. More precisely their age lies between about 6 months, when they first leave the pouch (and are first liable to be poisoned, trapped or shot) until they lay down their first layer of cementum. This is customarily thought to occur at 12 months, but the first layer may be deposited from about 10 months onward (See Discussion). Summarising:

No cementum layer =0–1 year old

1 cementum layer = 1–2 years old

2 cementum layers =2–3 years old

etc.

This definition is necessary because some authors group results on another basis, e.g. Bamford (1972) divided his samples into "pivotal" age classes - a pivotal age of 1 including animals between six and eighteen months, a pivotal age of 2 including animals between eighteen and thirty months old, etc. Because of this, Bamford's results cannot always be incorporated into our analyses.

Since Pekelharing (1970) first developed his aging technique, several workers have sampled possum populations and analysed them according to this method. The samples are very uneven, having been gathered for various purposes. Some were gathered within a day or two, other samples accumulated over several years. Some came from a small area, others over large areas. Most were poisoned, trapped or shot; one sample was found dead or dying. Fragments of history or recent reproductive performance are known for some samples. Little of the information has been published, most of it appearing in internal government reports or student theses. Enough analyses have now been made to look for useful generalisations and to try to explain differences and similarities between populations.

Bamford (1972) aged 1356 possums from an area stretching 19 km along the north bank of the Taramakau River, Westland. 686 were taken in 1970 from 3 areas where he thought the possums were on the increase, static or in decline. He may have been right about the status of these populations but the age structure of the three populations is statistically homogeneous (chi square page 65 test) and affords no evidence of differences (Fig. 4a). An additional sample of 237 animals was gathered over the same ground a year later after it had been poisoned with aerially distributed chopped carrot impregnated with 1080 poison and another sample of 196 from an unpoisoned area. Bamford showed that one-year-old animals were significantly under represented in the poisoned sample (X2 1 = 4.8, p < 0.02). Bamford suggested that, as the poison was laid when the young were still in the pouch (August), the sucklings may have succumbed to doses which left their mothers unaffected but passed to them in their mother's milk; or at weaning, the young may be possibly more susceptible to traces of 1080 than adults. Another possibility is that sublethal doses of 1080 stopped the flow of milk and the young starved to death.

Boersma (1974) poisoned, trapped or shot 1001 female and 1176 male possums over a large area of the Hokitika River catchment. Only the 213 males without cementum layers were aged. The females were divided into annual age classes. He found mortality was high in the first year of life and again increasingly so after the fifth year. Between 2 and 4 years his females enjoyed a low death rate. Earlier control programmes appeared to have left no mark on the age structure of the Hokitika animals.

Cook (1975) investigating the incidence of bovine tuberculosis among possums of the Hohonu Forest, Westland, estimated the ages of 1203 animals by dental cementum layers. He interpreted an excessive number of 7–8 year old animals as reflecting the introduction of tuberculosis to the area 7 or 8 years earlier.

Clout (1977) studied possums in young and old pine forest near Tokoroa and estimated the age of 244 animals excluding pouch-young. He interpreted the large proportion of 3–4 year-old males in one sample as reflecting the clearing and burning of the site 4 years earlier.

Crawley (1970) reported that the oldest of 218 possums, tagged and recaptured in the Orongorongo Valley between 1956 and 1970, was in her twelfth year.

Fraser (1979) working in the Copland Valley, South Westland, found that males dominated the younger age classes, females the older age classes.