Male wasp genitalia as an anti-predator defense

Predators can cause selection that drives the evolution of various anti-predator defenses in prey^1,2,3. Some prey species have evolved specific defensive devices, while others simply use body parts that evolved for reasons other than defense to repel predators¹. For example, many animal species have strong canine teeth or mandibles to kill prey and to counterattack their enemies^1,2,3. Bees and wasps (Insecta: Hymenoptera) use their ovipositors as stings to repel attackers, including humans⁴. Stinging by injecting venom can cause intense pain and allergic reactions in humans⁴. Therefore, stinging hymenopterans that are recognized as dangerous have evolved warning signals, such as conspicuous body coloration⁴. Hymenopteran males, which lack ovipositors, are believed to be harmless, and the males avoid predators by mimicking the females' coloration and behavior⁴. Here, we report that males of the mason wasp Anterhynchium gibbifrons (Hymenoptera: Vespidae: Eumeninae) use sharp genital spines to counterattack predators. A potential predator, the tree frog Dryophytes japonica (= Hyla japonica) (Anura: Hylidae) or the pond frog Pelophylax nigromaculatus (Anura: Ranidae), was housed with male A. gibbifrons under laboratory conditions. All tree and pond frogs attacked the male wasps. Although all of the pond frogs ate the male wasps, 35.3% of the tree frogs ultimately rejected them. Male wasps were frequently observed to pierce the mouth or other parts of frogs with their genitalia while being attacked. The tree frogs were also housed with male wasps from which the genitalia were removed. All these frogs ate the genitalia-less males. Therefore, male wasps used their genitalia to prevent tree frogs from swallowing them. This study highlights the importance of male genitalia as an anti-predator defense and provides a new perspective for understanding the ecological roles of male genitalia in animals.