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The energetics of an egg-laying mammal, the echidna (Tachyglossus aculeatus), were studied in the wild by means of isotope turnover techniques. Water and sodium influx rates were highest in summer (47.7±15.3 ml kg-1 day-1 and 1.20±0.52 mmol kg-1 day-1, respectively) and associated with high metabolic rates (0.509±0.048 ml CO2 g-1 h-1). Water and sodium influxes and metabolic rates were lowest in May and June (7.8±6.4 ml H2O kg-1 day-1, 0.21±0.12 mmol Na kg-1 day-1 and 0.205±0.129 ml CO2 g-1 h-1, respectively). These low rates in late autumn/early winter are associated with reduced activity, the animals spending substantial periods of time in torpor. The comparatively low isotope turnover rates of echidnas are a consequence of their diet; ants and termites which have low mass-specific energy contents.