Metabolic physiology of the numbat (Myrmecobius fasciatus)

Abstract.

The numbat (Myrmecobius fasciatus) is unique amongst marsupials as it is exclusively diurnal, feeds only on termites and is semi-fossorial. This study examines the thermal and metabolic physiology of the numbat to determine if its physiology reflects its phylogeny, diet and semi-fossorial habit. Numbats (mean adult body mass 552 g) were able to regulate body temperature at ambient temperatures of 15–30 °C, with a body temperature at thermoneutrality (30 °C) of 34.1 °C. The thermoneutral body temperature was not significantly different from that predicted for an equivalent-sized marsupial. Basal metabolic rate, measured at 30 °C, was 0.389±0.025 ml O₂ g^–1 h^–1, and was slightly but not significantly lower at 82.5% of that predicted for a typical marsupial of equivalent body mass. Metabolic rate increased with decreasing ambient temperatures below 30 °C. Patterns of metabolic cycling observed for completely inactive numbats at ambient temperatures below 30 °C are likely to be related to sleep phase. Wet thermal conductance of 1.94 J g^–1 h^–1 °C^–1 (at 30 °C) was 131% of that predicted for a marsupial. Evaporative water loss of the numbat remained constant below the thermoneutral zone (<30 °C) at approximately 0.6 ml g^–1 h^–1, only 47.4% of that predicted for a marsupial. It increased to 1.01±0.16 ml g^–1 h^–1 at an ambient temperature of 32.5 °C. The thermal and metabolic physiology of the numbat is generally similar to that expected for other marsupials, and is also comparable to that of termitivorous placental mammals. Thus the reduction in body temperature and basal metabolic rate of placental termitivores is a "marsupial-like" low energy turnover physiology, and the numbat being a marsupial already has an appropriate physiology to survive exclusively on a low energy diet of termites.