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

A male (left) and female (right) Nala lividipes earwig
Masaru Nishikawa

The insects known as earwigs are known for their pinchy pair of curved forceps at the end of their abdomens, but they do a lot more than induce panic when you find them under a rock. They snack on garden pests like aphids, care for their young (rare among insects, which usually lay-and-dash) and according to two decades of research by Yoshitaka Kamimura, an associate professor at Keio University in Japan, have elaborate sex rituals that include two penises.

Kamimura’s latest discovery, published in November in the Biological Journal of the Linnean Society, is that certain earwig species generally use only one of their penises when mating even though both are fully functional. Through a series of lab experiments, Kamimura found that individual earwigs are the penile equivalent of “right-handed” or “left-handed,” demonstrating laterality—a preference for one penis over another—and that the population of earwigs overall was split 50-50 between righties and southpaws. Humans, by contrast, are about 85 percent right-handed for reasons that are still unclear.

Kamimura and his team started their study by collecting a single male and six females of the species Nala lividipes from Ishigaki Island in Japan and brought them into the lab, where they reared them on a diet of water and cat food. Once the insects reproduced, the experiments began with the offspring: A virgin female was released into a small container and allowed to acclimate for 30 minutes, and then a male entered. After feeling each other with their antennae, the male rotated his abdomen almost 180 degrees and walked backwards, the two earwigs now facing away from each other. To initiate sex, the male pointed one of his penises towards the female and the organ swelled with insect blood for insertion into her vagina and spermatheca, the structure used to store sperm. The pair remained locked together for an average of 40 minutes while Kamimura video-taped them. By examining which penis was pointed towards the female to initiate sex (while the other remained flaccid), Kamimura could determine whether the species used its right or left penis. It turned out that 43.5 percent of the males favored their right penis, which was found to be statistically no different than 50 percent. In a second species examined, Nala nepalensis from Taiwan, 49.2 percent of the males used the right organ.

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

Earwigs mating

Yoshitaka Kamimura

When asked about this result, study co-author and Virginia Tech assistant professor Chin-Cheng Scotty Yang triumphantly raised both arms in the air.

“This was not what I expected,” says Yang, who had previously worked with Kamimura on a closely related species, Labidura riparia, which uses its right penis. “I thought we were going to see some obvious laterality [at the population level], but we didn’t. For these species, it’s half-half.”

After establishing which penis was used, Kamimura wanted to go further and figure out if males always used the same one and if the other penis was functional. After documenting which penis was primed for sex, he mated males with multiple females, watching them have sex with one virgin female, letting them rest for ten days, then having sex with a different virgin. In most cases, the male used the same penis the second time, indicating 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. To make sure both penises were functional, Kamimura used ice water to anaesthetize the earwig and then cut one penis off at random with fine forceps. After given time to heal, the male was mated with a virgin female. Most of these males successfully inseminated females with the remaining penis, with their success rate statistically no different than a control group, indicating that both penises were fully functional.

Kamimura wondered, if half of males are righties and half are lefties, perhaps there would be some structural differences in genitalia that favored one side or the other? In that past work with Yang, Kamimura had shown that 88.6 percent of L. riparia males consistently use their right penis—and the spermatheca of females is coiled to make insertion of the right penis easier than the left. But when Kamimura shot laser beams at the female genitalia of N. lividipes using autofluorescent microscopy to visualize the genitalic structures, he found no indication that females had evolved anatomically to better accept the left or right penis. When he did the same analysis of the male genitalia, there were no differences, either between left and right penises or between the penis that was readied for mating versus the one that was in repose.

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

A close up of a male earwig’s genitalia

Yoshitaka Kamimura

“This indicates penis use might be predominantly determined by neural control mechanisms,” Kamimura wrote in an email.

In other words, it’s all in the brain.

Much like human handedness is linked to centers of our brain that control motor use, whether N. lividipes uses its left or right sex organ may be controlled by their brain. Having a spare penis makes sense as a backup in case the primary penis is damaged during sex, but why have a preference for one over the other? Taking cues from other animals, Kamimura thinks that specialization and repeated use, essentially learning, might improve performance. If using the same penis over and over again increases the efficiency of penetration and mating, then it stands to reason that an individual will continue using the same penis during sex. Kamimura is looking into ways to test this hypothesis where he would compare lateralized males with those in the minority who alternate penises.

Another possible explanation for lateralization is that there is some energetic cost to penis flipping. If it takes a lot of work to switch penises, it might not be worth the effort, with lateralization providing higher evolutionary fitness.

“We are now trying some imaging techniques, including micro-computer tomography, for revealing muscular mechanisms that control penis flipping,” he writes. By looking at the muscles controlling earwig penises, Kamimura hopes to learn more about why penis flipping doesn’t occur more often.

Better understanding lateralization in earwigs will help scientists grasp how and why it has evolved across the animal kingdom, including in our own species. Sharks, for example, alternate penis use based on what side of the male the female is located on. But for Kamimura, his research is also motivated by his love of earwigs themselves. He finds these animals fascinating—he traces his passion back to high school, when he looked under a rock outside his house and saw a mother earwig next to her eggs. The mother ran away, startled by the intrusion, but when he looked the next day she was right back with her brood. This behavior was startlingly complex for an animal that is often dismissed as a creepy pest.

“They are some of the coolest insects,” his colleague Yang adds.

But cool doesn’t always translate into research funding. Despite there being 1,942 described species (and another thousand or so that remain undiscovered, according to earwig expert Fabian Haas, who was not involved with this study), most research has only been done on a handful of them. When I ask Haas how many biologists are left in the world who do earwig taxonomy, he says “less than a dozen.” Without major medical or agricultural importance, it’s hard to get much attention.

But to Kamimura and Yang, earwigs’ bizarre sex life and evolution of laterality is more than enough reason to keep putting them under a microscope.

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