Values of oxygen consumption, evaporative water loss, and body temperature were plotted as a function of chamber air temperature. Linear regressions of oxygen consumption at temperatures below the thermoneutral zone (Tn), and evaporative water loss at temperatures above freezing, were determined with the SAS (1982) GLM procedure. Lower critical temperature (Tlc) was determined graphically from intersection of the line representing Ḣb and the regression line representing oxygen consumption below Tn. Slopes and intercepts of regression lines, as well as other mean values, were compared with t-tests (Statistical Analysis System, 1982; Ott, 1984:138-175). Unless indicated otherwise, data are expressed as mean ± standard deviation (s.d.).
Estimating Intrinsic Rate of Natural Increase
We employed the method first described by Cole (1954) to calculate rmax:
| 1 = e-rmax + b·e-rmax(a) - b·e-rmax(n+1) | Eq. 2 |
where a is potential age of females first producing young, b is potential annual birth rate of female young, and n is potential age of females producing their final young. After life-history data were substituted into [Eq. 2], rmax was determined by trial and error substitution (Hennemann, 1983).
Because rmax represents the genetically fixed, physiologically determined maximum possible rate of increase, data on earliest possible age of female reproduction, highest possible birth rate of female young, and longest possible female reproductive life span were used for a, b, and n, respectively. Calculated values, therefore, represent physiologically possible, not ecologically possible, intrinsic rates of increase (Hennemann, 1983, 1984; Hayssen, 1984; McNab, 1984b). Values of n were derived from longevity records for captive animals, and as these were all large values of similar duration (14-16 years), they had very little effect on rmax. All species considered have one litter per year, and because their sex ratios at birth are about 50:50, variation in b was due to differences in litter size. Therefore, age of first reproduction and litter size had the greatest effect on rmax. Intrinsic rate of increase scales to body mass (Fenchel, 1974), and we removed this effect by comparing each calculated rmax with the value expected (rmaxe) on the basis of body mass (Hennemann, 1983).