THE FOOD OF THE OPOSSUM (Trichosurus vulpecula Kerr) IN THE ORONGORONGO VALLEY, NEAR WELLINGTON

The diet of a population of opossums from an area of 2.25 ha in the Orongorongo Valley near WelJington was investigated, using faecal analyses to identify the cuticles of the leaves eaten. Because of the variation in the digestibility of the leaf cuticles of the different species [the cuticle of hangehange (Geniostoma ligustrifolium) is twelve times more digestible than that of climbing rata (Metrosideros fulgens)] it was necessary to calculate an index of digestion for each of the tree species known to be eaten in quantity. These indices were determined by feeding captive opossums with known amounts of the leaf species eaten in the wild, and then measuring the relative frequency of the specific cuticle in the faeces. This factor was then used to correct the observed frequencies of each species occurring in the faeces. Implicit in the calculations to determine the index is a computation to include the weight per unit area of the leaf, which allows the corrected results to be compared using proportional weights rather than areas. Faeces from 25-35 opossums routinely live-trapped in the study area each month were combined and prepared for microscopic examination. Five hundred and fifty identifications of abaxial cuticle were made from each monthly combined sample. The results from the four years' analysis. Mav 1969 to April 1973 showed that two species, kamahi (Weinmannia racemosa) and northern rata (Metrosideros robusta) comprised about 60% of the total leaf intake. Climbing rata (Metrosideros fulgens) and supplejack (Ripogonum scandens) contributed a further 15..160/" but occurred seasonally, most climbing rata being eaten during the spring and early summer, and suppIejack in winter. Of the further ten species contributing regularly to the diet, mahoe (Melicytus ramiflorus), hangehange (Geniostoma ligustrifolium), five finger (Pseudopanax arboreum) and tawa (Beilschmiedia tawa) .occurred most commonly. Some other species were present intermittently. Foods other than leaves are also eaten by opossums; from the frequency of occurrence of different types of fragments in the faeces, it was found that leaves contributed about 60-70% of the diet. The rest comprised other types of food, e.g. flowers, fruit, seeds, bark, and petiole. Bark fragments were present consistently though in, varying amounts throughout the period. but it i$ not known if bark biting fulfils a nutritional or social need. Though hinau (Elaeocarpus dentatus) leaves were not present often in the diet, the opossums browsed on the developing flower buds of hinau from March, when they first appeared, until they expanded into flowers, which were also eaten in N ovemberDecember. In addition opossums also ate unripe hinau fruit which the trees carried well into the winter months. Fruit of other species was also eaten; pigeonwoo-d (Hedycarya arborea) and supplejack with fruit ripening over a three to four month period. and horopito (Pseudowintera axil/aris) and kaikomako (Pennantia corymbosa) both with a short fruiting season. Flowers and fruit of other species were also eaten, as well as other types of food but more work is required to determine the contribution that these supplementary foods make to the diet.