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RESEARCH PRODUCT

Inbreeding depression in intraspecific metabolic scaling

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Metabolic scaling (i.e., the relationship between the size and metabolic rate of organisms) has been suggested to explain a large variety of biological patterns from individual growth to species diversity. However, considerable disagreement remains regarding the underlying causes of metabolic scaling patterns, and what these patterns are. As in all biology, understanding metabolic scaling will require understanding its evolution by natural selection. We searched for evidence of natural selection on metabolic scaling indirectly by manipulating the genetic quality of male and female Drosophila montana flies with induced mutations and inbreeding, building on the notion that mutations and inbreeding will cause predictable changes in characters under directional selection. Irradiation-induced mutations had no effect on the examined traits, most likely because of purging at an early stage. However, inbreeding increased the energy use of larger females, suggesting that selection has favoured low metabolic scaling in females. Inbreeding did not affect metabolic scaling of males. Together, our results suggest that natural selection on metabolic scaling acts differently on the sexes, depending on the relative importance of body size and energetic efficiency to individual fitness. The results call attention to the important notion that size-specific energy use can be an evolutionarily malleable trait.