Nutritional missed opportunity costs: wild house mice (Mus musculus) consistently consume less preferred food, with implications for control.

BACKGROUND Foraging animals must navigate trade-offs among foods of varying quality and accessibility, with preferences generally predicted to be driven by energy maximisation. Limited access to preferred resources increases missed opportunity costs (MOCs), which should necessitate uptake of less-preferred alternatives. How animals trade off access to preferred versus less-preferred foods has important implications for wildlife management, especially where anthropogenic, natural, and management-related food sources co-occur. In cropping systems, mouse control relies on their uptake of unpalatable poisons coated on wheat grains, even when high-quality alternatives like sown and spilled grain are available. We tested whether increasing the cost of accessing preferred wheat seeds would shift mice toward consuming a less-preferred food, lentils. RESULTS We conducted two experiments manipulating the costs of searching for wheat, predicting mice would compensate by eating more lentils. Unexpectedly, mice ate similar quantities of lentils across treatments, regardless of wheat accessibility. We suggest this occurred because lentils contain substantially more protein than wheat, leading to a nutritional MOC associated with wheat - that is, a fitness cost incurred from an imbalanced, wheat-only diet. CONCLUSION MOCs are rarely defined and are typically framed as time or energy trade-offs, yet animals also incur fitness costs from nutritional imbalance. Bait substrates usually target pest preferences, but uptake may be limited when substrates match the macronutrient composition of background food (e.g., crops). We suggest that bait uptake could be improved by considering nutritional MOCs and selecting substrates that complement the macronutrient composition of background alternatives, thereby exploiting animals' nutritional requirements. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.