Phantom decoys cause irrational choice in lizards

Birgit Szabo, Rachel Behrmann, Frederick Verbruggen, Simon C. Griffith, Martin Whiting

Choice theory generally predicts that individuals act to maximize their benefits and behave according to simple rationality assumptions. However, the predictions of choice theory are often violated when an additional option is added to a choice set leading to a shift in choice between the original options. While this “decoy effect” has been documented across several animal taxa, it has not been investigated in non-avian reptiles. Here, we examined the phantom decoy effect in foraging decisions in the central bearded dragon (Pogona vitticeps). After establishing individual food preferences, we first determined choice indifference between the two most preferred foods by manipulating food quantity and then testing how the addition of an unavailable, asymmetrically dominant phantom decoy influenced subsequent choices. Our results show that the addition of a phantom decoy shifted lizards choice towards the less preferred food option. This study provides the first test of contextual decision-making and the phantom decoy effect in a non-avian reptile, offering insight into the evolutionary distribution of irrational choice behaviour.