Target for lipid to carbohydrate intake minimizes cost of growth

Many theoretical treatments of foraging use energy as currency, with
carbohydrates and lipids considered interchangeable as energy sources.
However, herbivores must often synthesize lipids from carbohydrates since
they are in short supply in plants, theoretically increasing the cost of
growth. We tested whether a generalist insect herbivore (Locusta
migratoria) can improve their growth efficiency by consuming lipids, and
whether these locusts have a preferred intake target caloric ratio of
carbohydrate to lipid (C:L). Locusts fed pairs of isocaloric, isoprotein
diets differing in C and L consistently selected a 2C:1L target. Locusts
reared on isocaloric, isoprotein 3C:0L diets attained similar final body
masses and lipid contents as locusts fed the 2C:1L diet but ate more and
had a ~12% higher metabolic rate—indicating an energetic cost for
lipogenesis. These results demonstrate that some animals can selectively
regulate carbohydrate to lipid intake and that consumption of dietary
lipid can improve growth efficiency.