Limits to sustained energy intake. XXXIV. Can the heat dissipation limit (HDL) theory explain reproductive aging?

According to the heat dissipation limit (HDL) theory, reproductive performance is limited by the capacity to dissipate excess heat. We tested novel hypotheses that (1) the age-related decline in reproductive performance is due to age-related decrease of heat dissipation capacity and (2) that the limiting mechanism is more severe in animals with high metabolic rates (MR). We used bank voles (Myodes glareolus) from lines selected for high swim-induced aerobic MR, which have also increased basal MR, and unselected control lines. Adult females from three age classes - young (4 months), middle-aged (9 months) and old (16 months) - were maintained at room temperature (20 °C), and half of the lactating females were shaved to increase heat dissipation capacity. Old females from both selection directions had a decreased litter size, mass, and growth rate (GR). The peak-lactation average daily MR was higher in shaved than in unshaved mothers, and this difference was more profound among old than young and middle-aged voles (p=0.02). In females with large litters, the milk production tended to be higher in shaved (LSM±SE: 73.0±4.74 kJ day-1) than in unshaved voles (61.8±4.78 kJ day-1; p=0.05), but there was no significant effect of fur removal on the GR (4.47±2.29 g 4 days-1; p=0.45). The results provided a mixed support of the HDL theory and no support for the hypotheses linking the differences in reproductive aging with either thermoregulatory capability deterioration or genetically based differences in MR.