‘Dust you shall eat’: The complex nutritional and functional considerations underlying a simple diet

Animals assimilate macronutrients and mineral nutrients in specific quantities and ratios to maximise fitness. To achieve this, animals must ingest different foods that contain the needed nutrients or facilitate the digestion of those nutrients. We explored how these multidimensional considerations affect the desert isopods (Hemilepistus reaumuri) curious food selection, using field and laboratory experiments. Wild isopods consumed three‐fold more macronutrient‐poor biological soil crust (BSC) than plant litter. Isopods tightly regulated macronutrient and calcium intake, but not phosphorus when eating the two natural foods and when artificial calcium and phosphorus sources substituted the BSC. Despite the equivalent calcium ingestion, isopods performed better when eating BSC compared to artificial foods. Isopods that consumed BSC sterilised by gamma‐radiation ate more but grew slower than isopods that ate live BSC, implying that ingested microorganisms facilitate litter digestion. Our work highlights the need to reveal the multifaceted considerations that affect food‐selection when exploring trophic‐interactions. Foods are complex mixtures of organic and inorganic components that differ in digestibility. Animals often need to mix various foods to fulfill their specific nutritional needs. Our goal was to reveal the complex nutritional and functional considerations underlying a simple diet. Using field and laboratory experiments we found that desert isopods consumed 3‐fold more macronutrient‐poor biological soil crust than plant litter. This seemingly peculiar food choice allowed them to tightly regulate their macronutrient and calcium needs. Biological soil crusts served as the main calcium source but also as a source of microorganisms that enabled isopods to facilitate the digestion of the macronutrient‐rich but refractory plant litter. We highlight the need to reveal the multifaceted nutritional and functional considerations that affect food selection when exploring trophic interactions.