Natural variation in temperature drives thermal preference in a freshwater fish, independent of infection status.

Aquatic environments are often thermally variable, and ectotherms may select thermal habitats based on their physiological state. However, it remains unclear whether immune reactions and sickness behaviours, including behavioural fever or chill, drive temperature preference and what role natural temperature shifts play in thermal choice. We used a shuttle box system to determine temperature preference in wild-caught pumpkinseed sunfish, Lepomis gibbosus, that were: (i) naturally co-infected by parasitic helminths; (ii) experimentally infected with trematodes, or (iii) immune challenged through lipopolysaccharide (LPS) endotoxin injection. We found no significant differences in preferred temperatures or activity levels based on parasite infection intensity, experimental trematode exposure or LPS injection, despite significant among-individual variation in temperature preference. However, LPS-injected fish caught in 2021 preferred higher temperatures compared with those caught in 2022, reflecting warmer water temperatures experienced in June and July of 2021. Additionally, fish from the natural infection experiment captured over three weeks in July 2021 showed increased preferred temperatures that paralleled rising water temperatures in their natural habitat. Our results suggest that variability in temperature preference reflects natural temperature shifts rather than infection status. Ultimately, this finding enhances our understanding of individual microhabitat choice while challenging the concept of behavioural fever/chill as a thermoregulatory strategy in sunfish. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.