Bioenergetics and inter-individual variation in physiological capacities in a relict mammal - the Monito del Monte (Dromiciops gliroides).

In evolutionary physiology, studies of inter-individual variation (i.e. repeatability) in functional capacities are valuable as they indicate - within populations - what attributes could respond to natural selection. Although repeatability and quantitative genetics of physiological traits in energy metabolism of eutherian mammals have been well characterized, few or no studies have been performed on marsupials. We studied the repeatability (i.e. intraclass correlation coefficient, tau) of bioenergetics for Monito del Monte (Dromiciops gliroides), the sole living representative of an otherwise extinct marsupial order (Microbiotheria). We measured resting metabolic rate as CO(2) production (V(CO(2))) and O(2) consumption (V(O(2))) simultaneously, together with minimum thermal conductance (C), evaporative water loss (EWL) and respiratory quotient (RQ), in a sample of ca. 20 individuals. Our results suggest that D. gliroides exhibits poor control of body temperature (T(b)), with a thermal amplitude of ca. 10 degrees C in normothermia. As a consequence, repeatability of T(b) and metabolic rate (either as V(CO(2)) or V(O(2))) were relatively low (tau(T)(b)=0.25+/-0.04, tau(VCO2)=0.14+/-0.03, tau(V)(O2)=0.24+/-0.02, jackknife estimations of standard errors). Thermal conductance exhibited near-zero or negative repeatability and was lower than expected for marsupials. However, we found significant repeatability for RQ and EWL (tau=0.32+/-0.03 and 0.49+/-0.09, respectively). In general, these results suggest that Monito del Monte exhibits some ;reptilian' physiological characteristics. The relatively low repeatability of physiological variables, which otherwise exhibit large inter-individual and genetic variance in eutherian mammals, suggests that these capacities do not exhibit evolutionary potential in the ancient order Microbiotheria.