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Copulatory wounding (CW) is widespread in the animal kingdom, but likely underreported because of its cryptic nature. We use four case studies (Drosophila flies, Siphopteron slugs, Cimex bugs, and Callosobruchus beetles) to show that CW entails physiological and life-history costs, but can evolve into a routine mating strategy that, in some species, involves insemination through the wound. Although interspecific variation in CW is documented, few data exist on intraspecific and none on individual differences. Although defensive mechanisms evolve in the wound recipient, our review also indicates that mating costs in species with CW are slightly higher than in other species. Whether such costs are dose- or frequency-dependent, and whether defense occurs as resistance or tolerance, decisively affects the evolutionary outcome. In addition to sexual conflict, CW may also become a model system for reproductive isolation. In this context, we put forward a number of predictions, including (1) occasional CW is more costly than routine CW, (2) CW is more costly in between- than within-population matings, and (3) in the presence of CW, selection may favor the transmission of sexually transmitted diseases if they induce resource allocation. Finally, we outline, and briefly discuss, several medical implications of CW in humans., (Copyright © 2015 Cold Spring Harbor Laboratory Press; all rights reserved.)

Copulation can involve the wounding of the mating partner by specialised devices. This type of mating, which we term traumatic mating, has been regarded as exceptional. Its prevalence, however, has not been compared across taxa, nor have its functions and putative evolutionary pathways. A categorisation has been lacking to date. We here show that traumatic mating is a widespread and diverse phenomenon that likely evolved via several pathways. Its putative functions include: (i) anchorage during mating; (ii) stimulation of short-term female reproductive investment; (iii) male paternity advantages; and (iv) enhanced fertilisation efficiency in transitions to internal fertilisation. Both natural and sexual selection have likely contributed to the parallel evolution of traumatic intromittent organs in phylogenetically distant taxa. These organs are sometimes remarkably similar in shape and often, but not always, inject sperm. The target sites of trauma infliction and the nature of secretions delivered alongside sperm are thus far poorly studied, but data on both are needed to elucidate the function of traumatic mating. The few existing studies that explicitly quantify fitness impacts of traumatic mating indicate that this strategy may often be costly to the party being wounded. However, a comprehensive approach to assess overall investments and returns for both sexes is a major target for future work. Finally, for the first time, we corroborate quantitatively the hypothesis that traumatic mating evolved relatively more often among hermaphroditic than among gonochoric taxa., (© 2013 The Authors. Biological Reviews © 2013 Cambridge Philosophical Society.)

Reciprocity constitutes the prevalent mating mechanism among simultaneous hermaphrodites. Yet, when copulations in the female role confer fitness costs through male manipulation, it becomes advantageous sometimes to mate unilaterally in the male role only. In the sea slug Siphopteron quadrispinosum, acting males stab their partner with a bipartite penis, which not only hypodermically injects prostate fluids, but also apparently mechanically enforces unilateral male matings. Despite a pronounced male mating drive in both partners, unilaterality ensued when one slug stabbed more rapidly than its partner. The acting male may thus avoid the costs inflicted by traumatic injections and penial spines. While future studies need to elucidate the net fitness consequences of stabbing, our behavioural evidence is in line with the hypothesis that mating in S. quadrispinosum represents conflicting rather than complementary mating interests between mates.