Mate choice experiments are essential to further our understanding of sexual selection, but can be challenging to design and conduct with most wild animals. 3D printing technology is creating opportunities to conduct mate choice experiments in the field by facilitating the production of biologically accurate decoys. We used pairs of 3D printed female decoys differing only in size to test whether free-ranging male northern map turtles, Graptemys geographica, prefer larger females. Males interacted and attempted to mate significantly more with the larger decoys. By selecting larger females, males should increase their fitness because of the correlation between female size and hatchling size. Our experiment demonstrated that 3D printing technology can be a valuable tool to study animal behaviour in the field.