Quantifying variation in female internal genitalia: no evidence for plasticity response to sexual conflict risk in a seed beetle

Sexually antagonistic coevolution can drive the evolution of male traits that harm females, and female resistance to those traits. While males have been found to vary their harmfulness to females in response to social cues, whether female resistance traits vary in response to social cues remains to be examined. Among seed beetles, male genital spines harm females during copulation and females might resist male harm via thickening of the reproductive tract walls. Here we develop a novel Micro-CT imaging technique to quantify female reproductive tract thickness in 3-dimensional space. We compared the reproductive tracts of female Callosobruchus maculatus from populations that had evolved under high and low levels of sexual conflict, and for females reared under a social environment that predicted either high or low levels of sexual conflict. We show that neither social environment nor evolutionary history significantly affected reproductive tract thickness. Our novel methodology allows for the measurement of fine-scale changes in the internal female reproductive tract, and will allow future investigations into the internal organs of insects and other animals.