To understand what makes an invasive species successful, we must understand the behavioral mechanisms these invaders employ. In this study, I examined traits associated with the “boldness” behavioral syndrome (i.e., aggression, general activity levels, and behavioral flexibility), and the morphology of brain regions associated with those traits. I assessed boldness by conducting a series of four behavioral tests designed to measure aggression towards prey, aggression towards a conspecific, overall activity in an open field test, and flexibility in completing a novel task. I compared these measures in two species pairs: the native green anole (Anolis carolinensis; n = 12) and the invasive Cuban brown anole (Anolis sagrei; n = 15), and the native Texas banded gecko (Coleonyx brevis; n = 4) and the invasive Mediterranean house gecko (Hemidactylus turcicus; n = 8). I found that the brown anole was “bolder” than the green anole in two of the four behavioral tests conducted, but there was no difference between the two gecko species for any of the behavioral tests conducted. In contrast to my predictions, the native green anole had a larger relative brain mass (a general indicator of behavioral flexibility) and a relative total brain volume than the invasive brown anole. Green anoles also had larger neuron somas in the ventromedial nucleus of the amygdala and the medial cortex (regions associated with the boldness behaviors), contradicting my predictions. However, the Mediterranean house gecko had a larger relative brain mass than the native Texas banded gecko, consistent with the predicted pattern. I also found several within species relationships between behaviors expressed during the trials and the brain regions with which I predicted they would be associated. Together, these results provide one of the first studies of the relationships between brain and behavior in invasion biology
