Trapping

Possums were caught at night in baited cage traps spaced at regular intervals through the forest. In area A the population was monitored continuously over 1966-75; traps were usually set at least one night per month, with two periods of more intensive study: (i) 1966 to 1968 described by Crawley (1973); (ii) May 1970 to December 1973 when 64 traps were set at 30 m intervals on a 210 × 210 m grid (4.4 ha) for four successive nights each month, with supplementary trapping in adjacent forest for 2–4 nights every 3–4 months. In area B a parallel trapping regime to that used in area A was adopted, 64 traps being set on the ground on a 210 × 210 m grid during the same nights each month from June 1971 to December 1973, (then less intensively until September 1974).

Animal examinations

To facilitate examination and measurement, many possums were immobilised using succinylcholine chloride (Taylor and Magnussen 1965); others, mostly retrapped males, did not require detailed examination at each capture so were simply identified, weighed and released. Free-ranging possums were page 92 individually marked with a numbered tattoo and a fingerling tag in the ear; pouch-young up to about 30–40 days were too small to mark; from then until about 90 days they were marked by cutting small notches in up to three positions on the ear margin; from about 80 days they could be ear-tagged; they were not ear-tattooed until they were free-ranging (<170 days).

Animals were weighed on at least the initial capture each trapping-month and only this initial weight was used in the present analysis. Those immobilised were measured externally following Lyne and Verhagen (1957): total length was taken by running a flexible tape mid-dorsally along the body contours from tip of tail to tip of snout. Pouch-young measurements comprised head length, tail length and, in smallest young, crown-rump length (Lyne and Verhagen, 1957); they were only weighed after they had voluntarily released the teat; except for smaller young, all were sexed, and when evident, fur-colour was recorded. During the potential breeding season (March-November) females were immobilised and examined every month, but at other times of year pouches were usually checked without drugging the animals.