Abstract
Sick animals generally behave differently than healthy individuals by, for example, being less active and exploratory. How an individual responds to illness is also likely to be mediated by the individual's age because age will dictate the individual's ability to fight a challenge. To date, empirical research on sickness behaviour has focused on the population-level average effect of ill health on behaviour. No study has examined how sickness affects individual behavioural variation, which can affect not only survival and reproductive success but also disease transmission via interactions with conspecifics. In this study, we use a repeated measures design to experimentally test the hypothesis that an immune challenge will induce sickness behaviour in yellow mealworm beetles (Tenebrio molitor) and that the effect on behavioural expression will be dose- and age-dependent. We test the prediction that an immune challenge will reduce beetle activity and exploration at the population level as well as modify variation in behavioural expression among individuals with individuals receiving a stronger challenge expressing more sickness behaviour. Although we found little evidence that T. molitor experiences sickness behaviour, we did find that older beetles were more active than younger ones. There was very little evidence that age, sex, and immune status affect behavioural variation among and within individuals but the phenotypic correlation between activity and exploration is driven by a correlation within individuals. Observed effects within individuals are likely driven by a significant effect of test sequence; behavioural expression significantly decreased in the second of the repeated tests.