Half of These Earwigs Use Their Right Penis. The Other Half Use Their Left Penis. Why?

They are best known for their curved forceps at their abdomens. However, they can cause panic if you find them under rocks. They eat garden pests such as aphids and care for their young, which is rare among insects that usually lay-and-dash. Yoshitaka Kamimura (an associate professor at Keio University, Japan) has found that they have elaborate sex rituals that include 2 penises.
Kamimura's most recent discovery, published in November by the Biological Journal of the Linnean Society in the Biological Journal of the Linnean Society is that certain earwig species use one of their penises for mating, even though they have both fully functional penises. Kamimura discovered that individual earwigs can be either "right-handed" (or "left-handed") in penile terms. This demonstrates laterality, which is a preference for one penis or the other. The population of earwigs was 50-50 split between righties and southpaws. For reasons that remain unclear, humans are approximately 85 percent right-handed.

Kamimura and his colleagues began their research by collecting six females and one male of the species Nala vidipes from Ishigaki Island, Japan. They brought them to the laboratory and fed them water and cat food. After the insects had reproduced, experiments were started with their offspring. A virgin female was placed in a container and allowed to adjust for 30 minutes. Then a male was brought in. After feeling their antennaes together, the male turned his abdomen nearly 180 degrees and began walking backwards with the two earwigs facing each other. The male pointed one penises at the female to initiate sex. Kamimura videotaped the pair for 40 minutes on average. Kamimura was able to determine whether the males used their right or left penis by examining the penis that was pointed towards the female in order to initiate sex. Statistics showed that 43.5 percent preferred their right penis to the females. This was statistically not different from 50 percent. A second species, Nala Nepalensis, was also examined. It was found that 49.2 percent used the right organ.

Professor Chin-Cheng Scotty Yang, a Virginia Tech assistant and study coauthor, triumphantly raised his arms in celebration when asked about the result.

Yang says, "This wasn't what I expected," Yang who previously worked with Kamimura to study a closely related species, Labidura Riparia, which uses its right penis. "I assumed we would see some obvious laterality [at population level], but it didn't. It's only half-half for these species.

Kamimura decided to investigate if males used the same penis or if there was another. After determining which penis was ready for sex, Kamimura mated males with multiple women, and then let them rest for ten more days before allowing them to have sex again with another virgin. The male used the same penis twice in most cases. This indicates individual-level laterality. When a male "flipped" and used the other penis, there was no pattern in the directionality--left-to-right was equally likely as right-to-left. Kamimura anesthetized the earwig with ice water and then used fine forceps to cut off one penis at random. This was done in order to ensure that both penises functioned. After allowing the wound to heal, the male was mated to a virgin woman. These males inseminated the females with the remaining penis with great success. Their statistical success rate was no different from a control group which indicates that both penises were functional.

Kamimura wondered if there were structural differences in the genitalia favoring one side or another if half the males are righties and half the lefties. Kamimura's previous work with Yang had shown that 88% of L.riparia males use their right penis, and that the spermatheca (females) is coil to facilitate the insertion of the left penis. Kamimura used autofluorescent microscopy and laser beams to examine the female genitalia in N. lividipes. He found no evidence that the females have evolved anatomically to accept either the right or left penis. He also did the same analysis on the male genitalia. There were no differences between the left and right penises, or the penis in repose versus the penis ready for mating.

Kamimura sent an email saying that this indicated penis use may be primarily determined by neural control mechanisms.

It's all about the brain.

Similar to how human handedness can be linked to the centers of our brains that control motor use and whether N. vipers uses its right or left sex organ, their brain may also influence which one they choose. It is a good idea to have a spare penis in case your primary penis gets damaged. But why choose one over another? Kamimura believes that repetition and specialization, which is basically learning, can improve performance. It is possible that a penis will be used repeatedly if it increases efficiency in mating and penetration. Kamimura is currently looking at ways to test this hypothesis. He would like to compare lateralized males and those who have alternate penises.

One possible explanation for penis flipping's lateralization is the energy cost. It might be difficult to switch penises. However, lateralization can provide greater evolutionary fitness.

He writes that "We are currently trying some imaging techniques, such as micro-computertomography for revealing muscular mechanisms which control penis flipping." Kamimura is interested in learning more about penis flipping by looking at the muscles that control earwig penises.

Scientists can better understand lateralization in earwigs to help them understand how and why it evolved in other animal species, such as our own. For example, sharks have a penis that alternates depending on which side the male is on. Kamimura's love for earwigs drives his research. These animals are fascinating to him. He traces his fascination back to high school when he saw a mother earwig near her eggs under a rock outside of his home. She fled, startled at the intrusion. But when he returned to her nest the next day, she was back. This was a remarkable behavior for an animal often regarded as a creepy pest.

Yang, his colleague adds that they are "some of the most coolest insects."

Cool doesn't always translate to research funding. Although there are 1,942 species that have been described (and a further thousand undiscovered according to Fabian Haas who is an expert on earwig taxonomy), only a few studies have been conducted. Haas says that there are only a few biologists who specialize in earwig taxonomy. It's difficult to get attention if it isn't of major agricultural or medical importance.

Kamimura and Yang see earwigs as more than just a sexy specimen.