Surface-Contacts During Mating in Beetles: Stiffness Gradient of the Beetle Penis Facilitates Propulsion in the Spiraled Female Spermathecal Duct
Substantial diversity of genitalia found in animals is regarded as a result of sexual selection. This means that surface interactions of the male and female genitalia during copulation play a key role in its evolution. However morpho/physical diversity and biomechanics of the structures in sexual intercourse are not well linked. Here we estimated relative stiffness of hyper-elongated male and female genitalia in cassidine beetles based on material composition gradient analyses and numerically modeled this system to gain insights about how genital features affect penile propulsion. The material composition analyses suggested that only the hyper-elongated penis has stiffness gradient, the tip of the penis could be softer than the rest of it, and the highly spiraled female spermathecal duct is constantly stiffer than the penis. The numerical stimulation with different conditions of the penile stiffness showed effects of stiffness gradients of the penises on its propulsion into the female duct. This simulation demonstrated that the realistic type of the stiffness gradients aids in faster propulsion than other types and a constantly rigid penis causes largest local deviation of the female duct. It seems that the soft end of the penis is flexible enough to quickly adjust small curvature in the spermathecal duct, and at the same time, it may be strongly pushed by the rigid basal part. This study indicates that previously ignored physical properties of genital materials are of crucial importance in understanding physical interactions of sexes.
This book chapter is adapted from the publication Filippov et al. Stiffness gradient of the beetle penis facilitates propulsion in the spiraled female spermathecal duct, Sci. Rep. 7, 27608; doi: https://doi.org/10.1038/srep27608 (2016). This study was supported by the Yamada Science foundation and the Japanese Society of the promotion of Science (postdoctoral fellowship, grant no. 15 J03484) to YM. We thank J. Michels and E. Appel (Kiel University, Germany) for CLSM training and comments on the draft, H. Pohl (Jena University, Germany) for assistance with material collection in the field, N. Jacky (Kiel University, Germany) for the help with the rearing of beetles in the lab, and R. Niwayama (EMBL Heidelberg, Germany) for discussion on computer simulations.
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