Dimensionless numbers for each of the four variables used in
calculating composite scores were derived as follows. Ratios of
measured to predicted values were used for basal metabolism (H_{br})
and minimum wet thermal conductance (C_{mwr}). Thermoregulatory
ability at low temperatures is closely related to the ratio
H_{br}/C_{mwr} (McNab, 1966). This ratio was used, therefore, to gauge
each species' cold tolerance. For D_{d} we used the ratio of food
categories actually used by a species to the total number of food
categories taken by all species tested (D_{dr}). The ratio of
calculated to expected intrinsic rates of natural increase was used to
derive r_{maxr}. Composite scores were calculated as