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, r_{max} was determined by trial and error
substitution (Hennemann, 1983).
Because r_{max} 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 r_{max}. 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 r_{max}. Intrinsic rate of increase scales
to body mass (Fenchel, 1974), and we removed this effect by comparing
each calculated r_{max} with the value expected (r_{maxe}) on the
basis of body mass (Hennemann, 1983).