A phantom decoy shifts wild forager decisions in a natural environment

Abstract Seemingly irrelevant options, such as inferior or unavailable “decoys”, can shift decisions people and captive animals make in simplified environments. But do decoys influence animals in messy, complex, natural environments? Here, we explored this question in a foraging context, a process involving countless daily decisions that ultimately underpins animal fitness. We tested whether a “phantom decoy” (inaccessible but preferred food) influenced food choice of free-ranging swamp wallabies across 3 scenarios; Binary—2 available, equally preferred foods differing nutritionally; Phantom—2 available foods plus an inaccessible phantom decoy nutritionally similar to one; Trinary—all 3 foods available. We analysed data using a new approach that overcame cross-level bias, when individual choices shift in opposite directions leading to no overall shifts. We found that the phantom decoy made wallabies more likely to switch their choice away from the nutritionally similar food—a net reactance effect. Unexpectedly, when all foods were available, wallaby choices appeared random and haphazard, suggesting cognitive overload. These findings demonstrate that decoys can shape ecologically relevant foraging decisions, so their influence is not simply an inadvertent byproduct of stylized simplistic settings. As such, traditional foraging models may better predict real-world foraging decisions by incorporating decoy effects.