Dust you shall eat: Complex considerations underlying a simple diet

Animals must assimilate many different macronutrients, minerals, and vitamins in specific quantities and ratios to maximize their fitness. To achieve this multi-dimensional nutritional goal, most animals must ingest different food items that contain the needed nutrients or biotic and abiotic components that facilitate the digestion and assimilation of those nutrients. Since foods are complex mixtures of organic and inorganic components, revealing what drives food selection and dietary composition is challenging.

Desert isopods (Hemilepistus reaumuri) feed predominantly on both plant-litter (PL) and biological soil crust (BSC). BSC has 60-fold less carbohydrates and 27-fold less proteins than PL but is richer in calcium and phosphorus. We conducted laboratory and field experiments using natural and artificial foods to simultaneously explore the different considerations determining the isopods' food selection.

Isopods consumed more BSC than PL in the field and preferred Calcium-rich over macronutrient-rich artificial food in the lab. Simultaneous consumption of PL and BSC increased isopods' assimilation efficiency and fitness synergistically. When BSC was substituted with artificial Ca and P sources, isopods tightly regulated their macronutrient and Ca intake. Consumption of micro-organism deficient deep soil or sterilized BSC dramatically decreased isopod's fitness.

Overall, we show that along with regulating their macronutrient ingestion, Ca acquisition plays a major role in desert isopods' food selection. Their need for a thick calcified exoskeleton to cope with the arid environment drives this high demand for this micronutrient. Moreover, the BSC micro-organisms seem to benefit the isopods by lignocellulose breakdown and facilitation of PL digestion.