Food intake and food choice are altered by the developmental transition at critical weight in Drosophila melanogaster

An animal's metabolism changes throughout development, obliging the animal to coordinate its feeding behaviour with its stage-specific nutritional requirements. Previous studies in the fruit fly Drosophila melanogaster showed that the developmental transition known as critical weight alters the response to nutrition in larvae; starvation reduces survival and dramatically delays development in precritical weight larvae, whereas it has more moderate effects on survival and accelerates development in postcritical weight larvae. We thus hypothesized that this change in sensitivity to nutrition might result in differences in feeding behaviour between the two stages. Using both no-choice and two-choice assays, we found that pre- and postcritical weight larvae had similar strategies for macronutrient balancing, both regulating protein intake at the cost of under- or overconsuming carbohydrates. Despite these similarities, precritical weight larvae regulated protein intake within more narrow limits than postcritical weight larvae. In addition, the larvae showed significant differences in the way they regulated macronutrient intake in the presence of bitter, potentially noxious compounds. Whereas precritical weight larvae avoided bitter food and showed only mild deficiencies in protein intake, postcritical weight larvae responded to these compounds by consuming less. When larvae were forced to choose between a higher quality diet tainted with quinine or caffeine and a lower quality diet containing less protein, larvae of both stages showed similar avoidance strategies but precritical weight larvae maintained a more constant protein intake than their postcritical weight siblings. Together, our results show that the developmental transition at critical weight modifies larval feeding behaviour, increasing our understanding of how developmental processes influence behaviour.