Diet Balancing in the Cockroach Rhyparobia madera: Does Serotonin Regulate this Behavior?

Animals, including insects, have the ability to self-select an optimal diet from a choice of two or more incomplete diets that lack an essential nutrient. This paper demonstrates that nymphs of the cockroach Rhyparobia madera also have this ability. The nymphs chose a protein:carbohydrate (P:C) ratio of approximately 25:75 when faced with a choice between one cube of protein (casein) and another of carbohydrate (sucrose). This self-selected ratio was shown to promote growth as well or better than other diets tested. When given a wide range of P:C choices, the R. madera nymphs consistently selected a P:C ratio of approximately 25:75, suggesting that they have the ability to diet-balance. Finally, injections of various serotonergic drugs into self-selecting nymphs influenced their choice of diets. Serotonin promoted a decrease in carbohydrate feeding, while injection of the antagonist ?-methyltryptophan caused the nymphs to overfeed on carbohydrate. The results suggest that serotonin may help alter the carbohydrate feeding response in cockroaches.