Abstract
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 ratio of carbohydrate (C) to lipid (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.