Limits to sustained energy intake. XXII. Reproductive performance of two selected mouse lines with different thermal conductance

Maximal sustained energy intake (SusEI) appears limited, but the factors imposing the limit are disputed. We studied reproductive performance in two lines of mice selected for high and low food intake (MH and ML, respectively), and known to have large differences in thermal conductance (29% higher in the MH line at 21 degrees C). When these mice raised their natural litters, their metabolisable energy intake significantly increased over the first 13 days of lactation and then reached a plateau. At peak lactation, MH mice assimilated on average 45.3% more energy than ML mice (222.9 +/- 7.1 and 153.4 +/- 12.5 kJ day(-1), N=49 and 24, respectively). Moreover, MH mice exported on average 62.3 kJ day(-1) more energy as milk than ML mice (118.9 +/- 5.3 and 56.6 +/- 5.4 kJ day(-1), N=subset of 32 and 21, respectively). The elevated milk production of MH mice enabled them to wean litters (65.2 +/- 2.1 g) that were on average 50.2% heavier than litters produced by ML mothers (43.4 +/- 3.0 g), and pups that were on average 27.2% heavier (9.9 +/- 0.2 and 7.8 +/- 0.2 g, respectively). Lactating mice in both lines had significantly longer and heavier guts compared with non-reproductive mice. However, inconsistent with the 'central limit hypothesis', the ML mice had significantly longer and heavier intestines than MH mice. An experiment where the mice raised litters of the opposing line demonstrated that lactation performance was not limited by the growth capacity of offspring. Our findings are consistent with the idea that the SusEI at peak lactation is constrained by the capacity of the mothers to dissipate body heat.