Evolution of warming tolerance alters physiology and life history traits in zebrafish

Evolution of warming tolerance may help species resist the impacts of climate change but can also lead to negative fitness outcomes. Identifying correlated responses to warming tolerance evolution could identify such negative consequences and help uncover the underlying mechanisms. By assessing the correlated responses of life history and physiological traits to seven generations of artificial selection to increase or decrease the acute upper thermal tolerance limit (CT_max) in zebrafish (Danio rerio), we show that warming-adapted lines have improved cooling tolerance. Furthermore, the absence of difference between selected lines in aerobic metabolic scope, brain heat shock protein levels, fecundity, growth or swimming speed contradicts several hypotheses concerning the mechanisms controlling acute warming tolerance. These results suggest that selection due to acute heating events does not target variation in metabolic rates but can benefit tolerance to cold, making individuals more resilient to extreme temperature events.