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.

Methods
To study whether locusts regulate their C:L energy intake during the 5th instare of migratory locust, we designed iso-caloric artificial diets. We supplied a pair of pre-weighed artificial diet dishes to each locust. Half the locusts were given a bowl of 3P:5C:1L (protein:carbohydrate:lipid ratio) diet and a bowl of 3P:1C:5L diet, and another half were provided bowls of 3P:5C:1L and 3P:2C:4L diets. For these diets, ratios refer to the relative proportion of all energy in that diet provided by that nutrient. In addition to artificial diets, we provided ad libitum water in a tube with a cotton plug. After three days, we replaced the diet dishes with new pre-weighed dishes. The consumption of each diet was measured as the difference between initial and final dry mass in each diet dish (following drying for 24h at 60°C). To test whether the specific lipid source affects how locusts regulate C:L consumption, we carried out this experiment three times using three different oil sources (canola, sunflower, grapeseed).

To study how locusts perform on diets with and without lipids we measured developmental time, survival, energy consumption rates, and body lipid growth when locusts were confined to a single diet during 5th instar nymph development. We designed a new artificial diet, 3P:4C:2L, based on the self-selected C:L intake target ratio in the choice experiment, and compared it to developmental performance of nymphs that were reared on diets equivalent in protein and energy content, but with only carbohydrates as their non-protein energy source (3P:6C:0L). Pre-weighed diet dishes were inserted into each individual cage, replaced after three days with new fresh dishes, and removed at the end of the experiment when the locusts molted to adults. In addition, we measured CO2 production and oxygen consumption and calculated the RER (respiratory exchange ratio) for each of the diet treatment groups.