Sickness behaviour in the cricket Gryllus texensis: Comparison with animals across phyla

Immune activation alters behaviour (i.e. sickness behaviour) in animals across phyla and is thought to aid recovery from infection. Hypotheses regarding the adaptive function of different sickness behaviours (e.g. decreased movement and appetite) include the energy conservation and predator avoidance hypotheses. These hypotheses were originally developed for mammals (e.g. Hart, 1988), however similar sickness behaviours are also observed in insects (e.g., crickets). We predicted that immune-challenged crickets (Gryllus texensis) would reduce feeding, grooming, and locomotion as well as increase shelter use, consistent with the energy conservation and predator avoidance hypotheses. We found evidence of illness-induced anorexia in adult and juvenile crickets, consistent with previous research (Adamo et al., 2010), but contrary to expectations, we found an increase in grooming, and no evidence that crickets decreased locomotion or increased shelter use in response to immune challenge. Therefore, our results do not support the energy conservation or predator avoidance hypotheses. The difference in sickness behaviour between insects and mammals is probably due, in part, to the lack of physiological fever in insects. We hypothesize that the lack of physiological fever reduces the need for energy conservation, decreasing the benefits of some sickness behaviours such as increased shelter use. These results, taken together with others in the literature, suggest that ectotherms and endotherms may differ significantly in the selective forces leading to the evolution of most sickness behaviours.