Foliage of Eucalyptus-Punctata and the Maintenance Nitrogen Requirements of Koalas, Phascolarctos-Cinereus

Abstract

Nitrogen metabolism and balance were investigated in captive koalas fed mature foliage of a staple food tree, Eucalyptus punctata, in six feeding trials at various times of year. Winter foliage contained less nitrogen than summer foliage but koalas compensated by eating more in winter. Thus, nitrogen balance did not differ significantly between seasons. Urinary nitrogen excretion similarly showed no seasonal trend but losses of non-dietary (i.e. endogenous and microbial) faecal nitrogen were greater in winter than in summer, presumably because of the higher food intake. Estimates of the requirements of dietary and truly digestible nitrogen for maintenance of nitrogen equilibrium (283 mg . kg-0.75 . d-¹ and 271 mg . kg-0.75 . d-¹ respectively) were comparable with several estimates for other marsupials. Although urinary nitrogen.excretion by koalas was low compared with that by other marsupials, consistent with the koala's standard metabolic rate, losses of non-dietary faecal nitrogen were relatively high. It was concluded that relatively high losses of endogenous andlor microbial nitrogen in faeces are a consequence of feeding on Eucalyptus foliage and that the koala's low standard metabolic rate is an important adaptation permitting it to utilize this diet by partly compensating for faecal nitrogen losses.