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

Abstract

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°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, 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.

Oxidative costs of reproduction in mouse strains selected for different levels of food intake and which differ in reproductive performance

“…As in previous studies, there were no differences in BM between mice from the L and H strain prior to mating, but reproductive success was increased in mice of the H line24. That is mice from the H line produced larger litters and weaned larger pups.…”

“…We measured a variety of biomarkers of oxidative stress, and compared these in lactating (R) and non-reproducing mice from two strains that had been selected for different levels of FI, that we have previously shown differ significantly in their levels of reproductive performance24.…”

“…We have recently shown that a correlated trait to the selection on FI is a large difference in reproductive performance24, with the high intake line having much greater reproductive investment than the low intake line.…”

“…We used a variety of biomarkers of oxidative protection (dROMS and antioxidant enzyme activities of SOD; superoxide dismutase, GPx; glutathione peroxidas and CAT; catalase) and damage (protein carbonyls and 8OHdG), and compared these in lactating and non-reproducing mice from two strains selected for different levels of food intake (high, H and low, L). We have previously shown that these lines differ significantly in their levels of reproductive performance with H mice having a higher performance than L mice, i.e., milk production was greater and weaned pups that were ~30% larger24. In line with the oxidative stress theory, we hypothesised that mice from the H line, with their higher reproductive effort, would reduce investment in protection and repair during lactation, leading to an increase in oxidative damage compared to L mice.…”

Sustained energy intake in lactating Swiss mice: a dual modulation process

“…A previous comparison of two mouse strains selected for different levels of thermal conductance found that the strain with lower thermal conductance assimilated less energy, produced less milk and had lighter litters at weaning than the strain with higher thermal conductance (Al Jothery et al, 2014). The results of our temperature experiment support the HDL hypothesis, which suggests that animals subject to warmer ambient temperatures have to restrict the production of body heat associated with food input and energy expenditure Król, 2005, 2011).…”