Illness-induced anorexia may reduce trade-offs between digestion and immune function

Animals from across the animal kingdom decrease feeding during an infection. Superficially this response seems maladaptive because the decline in food intake occurs at the same time as immune activation increases energy expenditure. However, illness-induced anorexia could be beneficial by decreasing trade-offs between the immune system and digestion. For example, in insects (i.e. crickets) there is a trade-off between lipid transport and immune function. We predicted that increasing the need for lipid transport (e.g. when digesting a high fat meal) would reduce immune function. After consuming a high fat meal, crickets (Gryllus texensis) showed an increase in haemolymph lipid concentration. Crickets also showed a decrease in resistance to bacterial infection (Serratia marcescens). After an immune challenge, crickets not only ate less, they also preferred foods containing less fat. This occurred whether the target food was an ecologically valid food item (dead cricket), natural foods (e.g. lettuce and ground meat) or an artificial diet containing different amounts of lipid. Therefore, the change in feeding behaviour after an immune challenge is consistent with the need to reduce lipid transport in order to maximize immune function. Illness-induced anorexia may be one method by which animals can bias physiological pathways towards enhanced immune function. Some behaviours may be adaptive because they can bias the direction of physiological trade-offs.