Summary
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.
Methodology
Printing the Female Decoys
A female map turtle was created by digitally scanning of a dried carapace and appendages of a preserved specimen using a desktop 3D scanner (Ultra HD, NextEngine Inc., Santa Monica, CA, U.S.A.). Because of the poor condition of the soft parts of the preserved specimen, a head was digitally rendered using photographs via 3D Studio Max 8 (Autodesk Inc., San Rafael, CA). A complete digital decoy was assembled and scaled in preparation for 3D printing in sections (ABS plastic) using a Makerbot® Replicator 2x and accompanying software (MakerBot, New York, NY, U.S.A.). We printed four decoys, scaled to two sizes: 210 mm and 250 mm (plastron length). An artist was commissioned to paint each decoy to represent biologically accurate coloration to mimic live turtles as closely as possible (Fig. 1a).
Male Mate Choice Experiment
We mounted the decoys in pairs (one large female decoy and one small female decoy) 40 cm apart on a platform made of 3.81 cm diameter ABS pipes (Fig. 1a). Each of the two decoy stations was equipped with a GoPro Hero 3 ® camera (GoPro, San Mateo, CA) with an extended Wasabi® battery pack (Wasabi Power, Pomona, CA). The camera was installed 80 cm above the decoys facing down. Both decoy stations were deployed between 0700 and 0800 hours and recovered at 1900 hours on 9 days. The location of both mate choice stations was determined haphazardly in the hibernation areas each day. For each deployment, we recorded the number of males approaching the decoys and the number of males attempting to mate with the decoys. We defined an approach as an event in which a male approached a female decoy to less than its own body length (Fig. 1b; Supplementary Video S1). Mature males range in carapace length from 10 to 16 cm (Bulté & Blouin-Demers, 2009). We defined a mating attempt as when the male mounted the dorsoposterior end of the female decoy (Fig. 1c, Supplementary Video S1). If male map turtles do not exhibit female size preferences, then the expected number of approaches and mating attempts should not differ between large and small female decoys. We used chi-square analysis to test whether the frequency of behaviours (approaches and mating attempts) directed towards the large decoy differed significantly from the expected 50%.