Beneficial effects of a heat wave: higher growth and immune components driven by a higher food intake.

While heat waves will become more frequent and intense under global warming, the ability of species to deal with extreme weather events is poorly understood. We investigated how a heat wave influenced growth rate and investment in two immune components (phenoloxidase activity and melanin content) in larvae of two damselfly species, Ischnura elegans and Enallagma cyathigerum Late instar larvae were kept at 18°C (i.e. their average natural water temperature) or under a simulated long heat wave at 30°C. To explain the heat wave effects, we quantified traits related to energy uptake (food intake and growth efficiency), energy expenditure (metabolic rate measured as activity of the electron transport system, ETS) and investment in energy storage (fat content). The two species differed in life strategy, with I. elegans having a higher growth rate, growth efficiency, ETS activity and fat content. In line with its preference for cooler water bodies, the heat wave was only lethal for E. cyathigerum However, both species benefited from the heat wave by increasing growth rate, which can be explained by the higher increase in food intake than metabolic rate. This may also have contributed to the increased investment in energy storage and immune components under the heat wave. This mediatory role of food intake indicates the critical role of food availability and behaviour in shaping the impact of heat waves. Our results highlight the importance of including behavioural and physiological variables to unravel and predict the impact of extreme climate events on organisms.