A price to pay: long-term selection for divergent basal metabolic rate affects reproductive success in mice.

The adaptive nature of basal metabolic rate (BMR) variation and its link to sustained metabolic performance, for example during reproduction, is one of the most important problems in physiological ecology. Different hypotheses are considered: one claims that low BMR (L-BMR) is adaptive under conditions favouring energy conservation, and natural selection should reduce BMR so that energy can be allocated to other functions; the second links high energy-expenditure capacity to high BMR (H-BMR) through larger guts and the capacity to acquire energy. Here, we examine this relationship by analysing reproductive outcome across multiple generations in two lines of laboratory mice subjected to long-term artificial selection for high or L-BMR. Our results link high energy-expenditure capacity to H-BMR, reflecting greater energy-acquisition abilities. Mice with H-BMR wean larger litters than females with L-BMR. However, the between-line difference is driven mainly by a progressive decrease in litter size in the L-BMR line and is less pronounced than the between-line type divergence in BMR. Our results emphasize that life-history models based exclusively on energy allocation typically overlook potential physiological constraints associated with traits as complex as BMR and cannot provide a single definitive explanation for the link between BMR and sustained performance.