The morphology of mating plugs and its formation in scorpions: Implications for intersexual participation

Mating plugs have been proposed as a mechanism that has evolved to avoid sperm competition. Their structure and composition vary across taxa and are related to the effectiveness of its function. This effectiveness could be related to different evolutionary interests of the sexes. Urophonius brachycentrus and Urophonius achalensis (Scorpiones, Bothriuridae) are highly suitable species to study mating plugs because both are monandrous species with specific morphological and physiological responses in the female's genitalia. Here, we analyze (a) the morphology and fine structure of the mating plugs of both species, (b) the site of production in males and the formation process of the mating plug, and (c) the changes that it undergoes over time in the female's reproductive tract. In both species, a complex mating plug obliterates the female's genital aperture and fills the genital atrium. We observed considerable interspecific variation in the mating plug morphology. A mating hemi‐plug was found surrounding the capsular lobes of the hemispermatophore, which could have a mixed composition (involving portions of the hemispermatophore and glandular products). The glandular portion was transferred in a semi‐solid state filling the female's genital atrium and then hardening. Changes that the plug undergoes in the female's genitalia (darkening and increase of the “distal” area of the plug) indicate a participation of the female to the formation of this type of plug. Our study provides new insights into the plugging phenomenon in scorpions, and we discussed the adaptive significance as a post‐copulatory mechanism to avoid sperm competition. Among the post‐copulatory mechanisms that males have evolved to avoid sperm competition are mating plugs. Here, we described the mating plugs of two scorpion species (Urophonius brachycentrus and Urophonius achalensis). The plugs are complex structures with different portions indicating an intersexual origin. A hemi‐genital plug present in the male hemispermatophore with glandular substances (mixed hemi‐genital plug) would be transferred to the female after the sperm transfer. The female triggers physiological processes that result in changes in the plug over time (anchoring and darkening). Our findings provide new insights into the plugging phenomenon in scorpions stimulating experimental studies on post‐copulatory sexual selection.