Predicting seed choice by Lumbricus terrestris through macronutrient regulation in a two–choice feeding assay using synthetic diets

Numerous animal species tend to actively regulate nutrient intake to meet physiological demands, yet the mechanisms underlying such regulation in soil fauna have been less extensively investigated. Earthworms can influence plant communities through seed consumption, but whether these seed feeding habits are governed by macronutrient regulation mechanisms is difficult to test using actual seeds, as nutritional effects are often confounded with physical seed traits. Here, we investigated macronutrient-driven feeding in the anecic earthworm Lumbricus terrestris Linnaeus (1758) using synthetic pelleted diets designed to simulate seed macronutrients while controlling the physical properties. Individual earthworms were offered two pellet types simultaneously in two-choice assays across three different protein-to-lipid (P:L) pairings and three levels of protein quality (high, moderate, and low quality). Across all treatments, L. terrestris tended to regulate lipid intake more tightly than protein intake. When lipids were scarce, earthworms over-consumed protein to reach their lipid intake targets. Reduced protein quality constrained protein over-ingestion, but did not significantly limit the acquisition of lipids. These findings show a nutrient-specific foraging behavior in L. terrestris and provide a mechanistic basis for predicting how the nutritional content of seed species, specifically lipid-rich seeds, may influence seed choice by earthworms in the field.