2.3.1 Precopulatory trait measurements
To investigate the sexual size dimorphism in the four populations
of O. babirussa , maximum pronotum width (Figure 2) of males and
females were measured as a proxy for body size with the
eyepiece reticle on the Olympus SZX10 microscope. This is widely used
proxy for body size because the pronotum width does not change in
adulthood and has been found to be the most appropriate measure for body
size in dung beetles (Emlen, 1997; Knapp & Knappová, 2013).
Horn lengths of male O. babirussa (Figure 2) were measured to
document variation in this precopulatory trait. Images were taken of the
anterior habitus. Heads of the beetles were separated and suspended with
Durex KY Jelly, with horns aligned parallel to the lens of the camera.
Images were captured using the EOS 800D and 6D camera body with
the Canon MP-E 65mm f/2.8 1-5x lens at 5x optical zoom. The camera was
suspended on the Dun, Inc. P-51 and the Camlift controller V2.9.3.0
software was used to take multiple images at different heights for focus
stacking. EOS Utility Launcher software was used to access the images
and stack them using the Zerene Stacker V. 1.04. software. Stacked
images were imported to Adobe Photoshop CS5 V. 12.0 x64 and a 1mm scale
bar was added to each image. Next, processed images were imported to
ImageJ V. 1.51 and the horns were measured from the tip to the bottom of
the outer edge of each horn, following previous studies (Moczek &
Emlen, 1999).