Ecological energetics and reproduction in the common ringtail possum, Pseudocheirus peregrinus (Marsupialia: Phalangeroidea)

This study examines the annual cycle of energy and time expenditure in a small folivorous marsupial, the common ringtail possum, Pseudocheirus peregrinus. Particular attention was given to the energy expended in lactation by the females. Field metabolic rate (FMR) and water flux were measured using an isotopic technique (doubly labeled water). Feeding rates were estimated from measurements of FMR in conjunction with information on the composition of the diet and a digestibility study. FMR's and subsequent feeding rates estimated by the doubly-labeled water technique do not include the proportion of food consumed which is diverted to milk solids and is not metabolised by the mother. The amount of energy transferred directly to the young ringtail possum via the milk was estimated from measurements of milk composition and production. Reproduction in the common ringtail possum in Tasmania was seasonal, with the majority of births (mean litter size 1.8) in late autumn and early winter. In general, the young leave the pouch during early spring and are fully weaned by the early summer months. There was no significant seasonal variation in the energy expenditure or water influx of males. The mean FMR of males and non-lactating females was approximately 2.5 times basal metabolic rate which is consistent with the hypothesis that a low total energy cost of free existence (or field metabolic rate) is a characteristic shared by arboreal folivores. Females showed significant changes in water influx and energy expenditure according to their lactational status. The greatest metabolisable energy expenditure was that of females during Phase 3 of lactation (30% above nonreproductive metabolism). Water influx was correspondingly high in these females (36% above non-lactating females). In general, ringtail possums in both the field and captivity lactated for approximately seven months. However, the length of lactation was shorter in females which bred twice in a year. The composition of the milk varied throughout lactation. A peak in milk solids and energy content coincided with emergence of the young from the pouch. Milk solids represented around 18% (w/w) with milk fat representing only 10% of milk solids. Milk production peaked during Phase 3 of lactation. The dilute milk with a relatively low fat content combined with a long period of lactation result in slow growth of the young. Peak milk energy output was 154.5 kJ.kg-0·75 .d-1 and peak metabolisable energy allocation during lactation was 763.2 kJ.kg-0.75 .d-1. These were lower than values available for other herbivores. However, the total output of milk energy by ringtail possums ( 11 MJ/kg) and total metabolisable energy allocation during reproduction (23.4 MJ/kg) were similar to estimates available for other herbivores. The lactational strategy of the ringtail possum has been selected, most likely, in order to spread the energy demands of reproduction over time due to constraints on the rate of energy intake imposed by a leaf diet. The total energy requirement for reproduction (34.4 Ml/year, or 14% of total annual energy budget) suggests that the ringtail also has a relatively low overall energy investment in reproduction. Estimates of total body water made from isotopic dilution and measurements of body mass suggest that females utilise body fat stored during the early stages of lactation to cope with the additional energy required for late lactation. However, reproduction is apparently timed such that late lactation coincides with the increased production of young foliage. Therefore females may also increase their food and water intake during late lactation by consuming young foliage. Differences were found in the composition of milk collected from wild and captive animals. Chemical analyses of the leaves eaten suggested that these differences were due to variations in diet composition. It was, therefore, proposed that the intraand inter-population variation in reproductive traits shown by the common ringtail possum may be related to variations in milk composition and/or production caused by variations in diet quality.