Behavioural flexibility allows ectotherms to exploit the environment to govern their metabolic physiology, including in response to environmental stress. Hydrogen sulfide (H 2 S) is a widespread environmental toxin that can lethally inhibit metabolism. However, H 2 S can also alter behaviour and physiology, including a hypothesized induction of hibernation-like states characterized by downward shifts of the innate thermal set point (anapyrexia). Support for this hypothesis has proved controversial because it is difficult to isolate active and passive components of thermoregulation, especially in animals with high resting metabolic heat production. Here, we directly test this hypothesis by leveraging the natural behavioural thermoregulatory drive of fish to move between environments of different temperatures in accordance with their current physiological state and thermal preference. We observed a decrease in adult zebrafish ( Danio rerio ) preferred body temperature with exposure to 0.02% H 2 S, which we interpret as a shift in the thermal set point. Individuals exhibited consistent differences in shuttling behaviour and preferred temperatures, which were reduced by a constant temperature magnitude during H 2 S exposure. Seeking lower temperatures alleviated H 2 S-induced metabolic stress, as measured by reduced rates of aquatic surface respiration. Our findings highlight the interactions between individual variation and sublethal impacts of environmental toxins on behaviour.

4Behavioural flexibility allows ectotherms to exploit the environment to govern their 5 metabolic physiology, including in response to environmental stress. Hydrogen sulfide (H2S) is a 6 widespread environmental toxin that can lethally inhibit metabolism. However, H2S can also 7 alter behaviour and physiology, including a hypothesised induction of hibernation-like states 8 characterised by downward shifts of the innate thermal setpoint (anapyrexia). Support for this 9 hypothesis has proved controversial because it is difficult to isolate active and passive 10 components of thermoregulation, especially in animals with high resting metabolic heat 11 production. Here, we directly test this hypothesis by leveraging the natural behavioural 12 thermoregulatory drive of fish to move between environments of different temperatures in 13 accordance with their current physiological state and thermal preference. We observed a 14 decrease in adult zebrafish (Danio rerio) preferred body temperature with exposure to 0.02% 15 H2S, which we interpret as a shift in thermal setpoint. Individuals exhibited consistent 16 differences in shuttling behaviour and preferred temperatures, which were reduced by a constant 17 temperature magnitude during H2S exposure. Seeking lower temperatures alleviated H2S-induced 18 metabolic stress, as measured by reduced rates of aquatic surface respiration rate. Our findings 19 highlight the interactions between individual variation and sublethal impacts of environmental 20 toxins on behaviour. 21