Contour lines show the estimated energy density of polar bears (stored energy relative to lean body mass53; units: MJ kg−1) as a function of straight-line body length and total body mass. All bears die at zero energy density (lower thick lines) and females have never been observed to give birth if their energy density is below 20 MJ kg−1 before entering a maternity den15 (middle thick lines, panels c and d). An approximate upper bound to total body mass (upper thick lines) is estimated as M=59.76L3, which is approximately four times a bear’s structural mass53). G(M0,L0)(WH89-96) is shown in each panel as black circles. Top row: all body masses decreased by 20% (orange; resulting in reproductive failure in at least half of all solitary adult females, panel c), and 40% (red; resulting in reproductive failure in all females); bottom row: all body masses increased by 20% (green), and by 40% (blue; resulting in unrealistically high body masses in several bears). Based on this, we conclude that the fast-initiating body masses of a viable polar bear population are likely within the −20% to +40% range of WH89-96 values, with values at the lower (upper) end only possible if bears are simultaneously also shorter (longer), which would somewhat reduce their energetic requirements (increase their maximum possible body mass).