A photo of a frog trying to eat a male potter wasp, and a close up of the male's pseudo-sting

Male potter wasps use their genital spines (close up image to the right) to fend off frogs trying to consume them. Image © Shinji Sugiura via Current Biology.

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Male wasps can use their genitals to fight off predatory frogs

Some wasps use their genitals to fight for their life when predators try to swallow them.

Male potter wasps have been observed using sharp spines on their genitals to jab attacking tree frogs until they let go.

One of the most unusual self-defence techniques in the animal kingdom has been uncovered.

While female wasps can rely on their venomous sting to defend against predators,  the males were believed to be defenceless. However, Japanese researchers discovered that male potter wasps can use sharp spines on their genitals, known as pseudo-stings, to fight off frogs.

Lead author Professor Shinji Sugiura says, 'We believed that male wasps were harmless, so we were surprised to get stung by male wasps.'

'As male wasps frequently visit flowers, these pseudo-stings may have developed to allow these insects to defend themselves against predators they could encounter there.'

Though the defence was only successful in around a third of cases, it gave male wasps a much better chance at survival, as those without the spines were always eaten. The chance of survival also didn't differ depending on size, but just how the genitals were used.

Dr Gavin Broad, the Principal Curator in Charge of Insects at the Museum, says, 'There are many wasp species where males have these spines, so I wouldn't be at all surprised if this behaviour is more widespread than just this one species.'

'The male spines would provide a sharp, jabbing pain, like having a thick needle shoved into your skin, and hurts quite a bit. There's no lasting effect unlike a female sting, which leaves a burning sensation behind because of the venom.'

Details of the behaviour were published in the journal Current Biology.

A female potter wasp with her nest made out of mud

Female potter wasps are renowned for their ability to make nests out of mud. Image © OUESTUSA/Shutterstock

The importance of insect genitals

Insect genitals have long been a source of interest for entomologists, but not just because of their role in reproduction. That's because looking at the genitals can sometimes be an important way to identify an insect.

'In groups of insects like the flies and butterflies, male genitalia are very important to tell species apart,' Gavin explains. 'In wasps, however, these genitalia are often overlooked as other characteristics tend to be more revealing.'

'In wasps, and the Hymenoptera as a whole, females also tend to be more interesting to study as they have a variety of adaptations such as their stings and ovipositors. Male wasps, on the other hand, tend to be smaller, fewer in number and less distinctive.'

The stings of females evolved as a result of modifications to a wasp's egg-laying organ, known as an ovipositor. Initially, it allowed the insects to drill into plants and lay eggs, but was later adapted to include a role as a weapon.

'The characteristics that made the ovipositor good for drilling into plants also made it useful as a weapon,' Gavin says. 'Female wasps use their venom to paralyse prey so that they can't move away, providing them with a fresh meal.'

'These venoms have then been co-opted for defensive stings as well.'

As males don't lay eggs, they don't have ovipositors. Instead, they were thought to rely solely on their visual similarity to females to ward off predators. The new research suggests that this may not be entirely true, at least in some species.

A wasp sting just above the skin of a human.

Wasp stings normally deliver venom, but the pseudo-stings are just sharp. Image © Biehler Michael/Shutterstock. 

How are wasp genitals used for defence?

As is often the case with science, the discovery of the pseudo-stings was a bit of an accident. While researching potter wasps, also known as mason wasps and which are known for building nests out of mud, co-author Misaki Tsujii had her finger pierced by a male wasp.

These spines are located on either side of the aedeagus, an insect organ broadly equivalent to a mammal's penis. Misaki observed that the male potter wasps did not use the spines during mating, which suggested it could have an anti-predatory role.

Misaki and Shinji investigated this theory by feeding 17 male potter wasps to both a Japanese tree frog and a black-spotted frog. In one group the males had their genital spines removed, whereas in the other group the males did not.

They found that 35% of the wasps with genital spines were spat out by the tree frogs after a struggle, during which the males attacked with their genital weapons. On the other hand, all the wasps whose genital spines had been removed were consumed.

As the Japanese tree frogs inhabit a similar environment to the male wasps, and prey on other nectar-feeding insects, the wasp pseudo-stings may have evolved as a defensive response. The researchers hope to investigate the role of male genitalia for defence in other wasps to build evidence for this.

Gavin adds that this research could provide a reason why as-yet-unexplained structures in other male wasps exist.

'There are quite a few instances of spine-like projections in male parasitoid wasps whose purpose is unclear,' Gavin says. 'They don't seem to be particularly sharp, so I can't imagine they're doing much with them, but defence could be a possible explanation.'

A short horned lizard sits on mud with blood draining from its eye

Short horned lizards are capable of emitting blood from around their eyes. Image © USFWS Mountain-Prairie, licensed under CC BY 2.0 via Flickr

Unusual animal defence methods

It's not just male wasps which can perform unusual feats with their genitals. Some hawkmoths, for instance, can rub their genitals against their abdomens to produce ultrasound that can jam the sonar of bats.

It's one of a variety of unusual defensive strategies that can be found in nature as evolution repurposes existing structures to aid an animal's survival.

For example, sea cucumbers such as Eupentacta quinquesemita are known to eject part of their gut at potential threats through their anus. They can then regrow the gut later if they survive the encounter.

Other sea cucumbers known as Holothuriids, meanwhile, can eject sticky structures known as Cuvierian tubules from inside the body cavity which expand and trap predators. Another marine creature, the hagfish, has a similar strategy whereby it rapidly produces slime to entangle threats.

Back on land, a wide variety of lizards can distract predators by intentionally throwing away part of their body in a process known as caudal autotomy. These lizards can break a weak section of their tail so that it drops onto the floor and writhes around, hopefully providing enough time for the individual to make its escape.

The defence of Trichobatrachus frogs, meanwhile, has been compared to the comic book character Wolverine. They have claws that are normally contained within their body, but when threatened tear out through the skin.

Perhaps the most unusual defence, however, is the Texas horned lizard's technique of firing blood out of sinuses around their eyes. This blood contains noxious compounds that are unappetising to predators and contributes to them learning to avoid the lizards.