Optimal foraging for specific nutrients in predatory beetles

Abstract

Evolutionary theory predicts that animals should forage to maximize their fitness, which in predators is traditionally assumed equivalent to maximizing energy intake rather than balancing the intake of specific nutrients. We restricted female predatory ground beetles (Anchomenus dorsalis) to one of a range of diets varying in lipid and protein content, and showed that total egg production peaked at a target intake of both nutrients. Other beetles given a choice to feed from two diets differing only in protein and lipid composition selectively ingested nutrient combinations at this target intake. When restricted to nutritionally imbalanced diets, beetles balanced the over- and under-ingestion of lipid and protein around a nutrient composition that maximized egg production under those constrained circumstances. Selective foraging for specific nutrients in this predator thus maximizes its reproductive performance. Our findings have implications for predator foraging behaviour and in the structuring of ecological communities.

Figure 1.

(a) Nutrient rails for the 10 semi-artificial diets and individual nutrient intake points for 505 female Anchomenus dorsalis over 28 days of feeding. Each beetle was provided a single diet either in ad libitum amounts or at various levels of limitation (see below). The locust : lard : casein ratio for each diet is indicated by the rails within the figure and the diet lipid : protein (L : P) ratios are provided outside of the figure frame. The L : P ratios are equivalent to the slopes of the rails. All ratios are mass-based. The two L : P ratios marked in bold (L : P = 0.91 and 0.14) represent the two diets provided to beetles in the choice trial. The five diets containing two-thirds pulverized locusts (L : P = 0.91, 0.64, 0.43, 0.27 and 0.14) were provided ad libitum or in quantities representing approximately one-third, one-half or two-thirds of ad libitum intake. The five diets containing one-third pulverized locusts (L : P = 2.92, 1.35, 0.68, 0.30 and 0.06) were provided ad libitum only. (b) Effects of protein and lipid intake on the total number of eggs produced (no. of eggs/milligram beetle) by the 505 female A. dorsalis after 28 days of feeding on one of 10 semi-artificial diets. The landscape also includes 26 females that were killed at the start of the experiment prior to ingesting any food. The mean (±s.e.) protein and lipid intake of the 23 beetles from the choice test (i.e. the selected intake point) is overlaid (white) on the landscape. The mean (±s.e.) protein and lipid intake for 214 beetles that were provided ad libitum amounts of one of the 10 diets are overlaid (black) and the means are connected by a line. This is referred to as the intake array and represents the regulated intake when feeding on single, mainly imbalanced diets. As ingestion of only one nutrient ratio is possible on single diets, the error bars of the intake array follow the rails of the diets. (c) Nutrient intake trajectory (red) of the 23 beetles allowed to select their diet nutrient composition from a food with L : P = 0.91 and a food with L : P = 0.14 (solid black lines). Each intake point (mean ± s.e.) shows the cumulative lipid and protein intake for the seven feeding periods. The broken line (L : P = 0.49) shows the average nutrient composition of the two foods and is the nutrient ratio the beetles would ingest if they fed indiscriminately. The slopes of the intake trajectory show the selected nutrient ratio within each of the seven feeding periods. Asterisks indicate feeding periods where consumption from the two foods differed significantly (paired t-tests, p < 0.01). Overall, the beetles selected an average L : P ratio of 0.36 over the 28 days of the experiment.