Macronutrient niches and field limitation in a woodland assemblage of harvestmen

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 its growth efficiency by consuming lipids, and whether these locusts have a preferred caloric intake 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 to locusts fed the 2C : 1L diet, but they 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 lipids can improve growth efficiency.

Different feeding preferences for macronutrients across seasons and sites indicate temporal and spatial nutrient limitation in the black bog ant

How the resource use by consumers vary in different environments and time scales is one of the fundamental ecological questions. Replicated field studies are rare, however; so the extent to which nutrient use varies and why is uncertain. We studied an endangered tyrphobiotic species, the black bog ant (Formica picea), and its feeding preferences in temperate peatlands. We conducted a baiting experiment at three different sites with high nest densities, repeated over three years and three periods of growing season. Preferences for three main macronutrients (carbohydrates, proteins and lipids) were assessed. We hypothesised that if nutrient limitation plays a role, ants will have an increased need for proteins and lipids in early seasons when brood is raised, while carbohydrates use will increase in late seasons. We also expected that site identity would influence nutrient preferences, but not year. Our results supported the nutrient limitation hypothesis for proteins that were consumed more in the early season. In contrast, preference for carbohydrates was rather high and did not increase consistently through season. Although the occupancy of lipid baits was low overall, it increased at colder temperatures, in contrast to carbohydrate and protein baits. Nutrient preferences varied more among sites than years, with the lowest nutrient use observed in a diverse fen-meadow, consistent with the nutrient limitation hypothesis. Year affected ant abundance, but not bait occupancy. Our results suggest that black bog ants flexibly adapt their diet to environmental conditions and that an interplay between nutrient limitation and climate determines their feeding behaviour.

Carbohydrates complement high-protein diets to maximize the growth of an actively hunting predator

In nature, food is often variable in composition and availability. As a consequence, predators may need to seek non-prey food sources. Some predators are known to feed on nectar when food is limited. Nectar and other carbohydrate resources could also be beneficial when prey are more abundant if it helps predators balance protein-biased diets. We tested if an actively hunting predator, the jumping spider, Phidippus audax, benefited from liquid carbohydrates when prey were not limited. We also tested if the benefit of carbohydrates varied with the nutrient content of prey (i.e., from protein to lipid biased). Spiders were reared on one of six live prey, Drosophila melanogaster, treatments that ranged from high protein to high lipid. Half of the spiders were given access to a 20% sucrose solution. After 2 months, we measured spider mass, cephalothorax width, instar duration, percent body fat, survival, and estimated number of prey eaten. Spiders reared on high-protein diets with carbohydrates were larger and heavier than spiders on other treatments. Access to carbohydrates also increased percent body fat and survival across prey treatments. Our results suggest that carbohydrates may be a valuable component of spider diets, especially when prey have high protein and low lipid content as is commonly observed in prey in the field. Our results highlight the importance of diet balancing for predators, and that liquid carbohydrates can be an important nutrient to supplement a diet of prey rather than just being an energy supplement during periods of starvation.