Metabolic–behavioural performance for overwinter survival in a wild rodent

Modern evolutionary theory predicts that integrated phenotypes, including interacting fitness-related complex traits such as behaviour, physiology and morphology, jointly influence animal performance. However, studies have examined these aspects of biology independently in relation to fitness. We investigated how phenotypic characteristics collectively affect overwinter survival in a wild population of yellow-necked mice. We measured behavioural activity, basal metabolic rate and body mass in late autumn, released marked mice and recaptured survivors in spring to reassess their phenotypes. Mice grew, increased metabolic rate and decreased distance moved over the winter, but when those changes were accounted for, the phenotypes were intra-individually consistent. The simultaneous selective advantages of elevated metabolism and behavioural activity during autumn suggest that the interplay between physiology and behaviour is subject to selection. Despite the lack of significant phenotypic covariation between metabolic rate and behaviour, their additive survival benefits suggest that elevated activity facilitates the development of costly metabolism in free-ranging animals. As metabolism and body mass share 84% of the common variance, we hypothesized that selectively advantageous high metabolism supports overwinter growth. The results support postulates that the behaviour-physiological phenotype is the first line of selective responses and that plasticity is an important source of variation in individuals' performance.