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Inactive trout come out at night: behavioral variation, circadian activity, and fitness in the wild.

Theory suggests that high activity levels in animals increase growth at the cost of increased mortality. This growth-mortality tradeoff has recently been incorporated into the wider framework of the pace-of-life syndrome (POLS) hypothesis. However, activity is often quantified only in the laboratory and on a diurnal basis, leaving open the possibility that animals manage predation risk and feeding efficiency in the wild by modulating their circadian activity rhythms. Here we investigate how laboratory activity in wild brown trout parr (Salmo trutta L.) associates with circadian activity, growth, and mortality in their natal stream. We found that individuals with high activity in the laboratory displayed high dispersal and cathemeral activity in their natal stream. In contrast, trout with low laboratory activity showed variation of activity in the wild, which was negatively related to the light intensity. Our results do not support the growth-mortality trade-off of the POLS hypothesis as highly active, fast-growing individuals showed higher survival than inactive conspecifics. These novel results show for the first time that active and inactive individuals, as scored in the lab, can show different circadian patterns of behavior in the wild driven by light intensity. This implies that studies conducted under a narrow range of light conditions can bias our understanding of individual behavioral variation and its fitness consequences in the wild.