Aversion learning in response to an invasive venomous prey depends on stimulus strength

Learned avoidance can allow animals to survive the introduction of noxious prey. The effectiveness of aversion learning can depend on the intensity of the stimulus. The red imported fire ant, Solenopsis invicta, is a novel prey of the eastern fence lizard, Sceloporus undulatus, but can prove lethal especially to juvenile fence lizards. Thus, avoiding consumption of fire ants would increase survival for juveniles. We tested whether juvenile lizards would exhibit learned aversion of fire ants, and whether aversion would be longer lasting following exposure to greater numbers of fire ants. Lizards were exposed to one of three 8-ant treatments with varying relative proportions of fire ants for 5 days: (1) 100% fire ants, (2) 50% fire ants and 50% native ants, and (3) 0% fire ants (8 native ants). Juveniles in the 100% fire ant treatment showed clear aversion learning, consuming nearly two-thirds fewer fire ants after the first trial day. Juveniles exposed to the 50% fire ant treatment, consumed slightly fewer fire ants after day 1 of the trial, but recovered by day 5. Juveniles that received native ants only did not alter their consumption of ants over time. These results suggest that juveniles show species-specific aversion to fire ants, but this was dependent upon stimulus strength. When presented with both fire ants and native ants, lizards reduced their consumption of both species. Investigating whether exposure to noxious invasive prey alters the consumption of native prey would provide insight into long-term impacts of invasive species.