Voluntary wheel use across animal taxa: motivation, enrichment, stereotypy and ethological interpretation

<jats:p>Exercise wheels are simple devices for recording animal movement, but they elicit surprisingly complex behaviours. In laboratory rodents, running wheels are key for studying voluntary exercise, circadian rhythms, motivation, energy balance, stress physiology and brain plasticity. Outside rodents, wheel-like devices have also been provided to rabbits, cats, squirrels, reptiles and invertebrates. This wide use prompts a key ethological question: what does wheel running mean? This review investigates wheel use as a behavioural phenomenon, not just as a method for measuring activity. Historically, running wheels seem to have developed from captive-animal husbandry and display practices, especially squirrel cages. They were later adopted as scientific instruments in early twentieth-century rat research. Since then, wheel running has been interpreted in overlapping ways: as locomotor exercise, reward-seeking behaviour, exploration, play-like activity, coping behaviour, environmental enrichment or abnormal repetitive behaviour. These interpretations are not mutually exclusive. Their relevance depends on species, individual phenotype, housing, motivation, wheel design and available behavioural alternatives. The review argues that wheel running should not be treated as a unitary behaviour. In some animals, wheel access may provide behavioural agency and support motivated activity. In others, high or rigid use may signal constraint, stress, or limited environmental complexity. For non-traditional wheel-using species, video confirmation is important. Wheel revolutions may reflect climbing, pushing, manipulation, or other non-locomotive interactions. Further work should report animal characteristics, housing conditions, wheel dimensions, access schedule, sensor methods, behavioural context and welfare outcomes. Interpreting wheel running through an ethological framework helps distinguish voluntary locomotion, enrichment, play, coping and stereotypy. This can improve the use of wheels in behavioural research and animal care.</jats:p>