Earwig Evolution: Female Forceps Progress Mirrors Male Patterns
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Research on maritime earwigs (Anisolabis maritima) reveals that female earwigs exhibit positive allometry in their forceps, similar to males, suggesting a shared evolutionary path driven by sexual selection.
A new study from Toho University indicates that female earwigs show a similar pattern of exaggerated forceps growth as males. This suggests that both sexes may have evolved these traits through sexual selection, where physical features evolve to offer an advantage in competing for mates.
Positive allometry, where certain body parts grow disproportionately large relative to body size, is observed in various animals. Male earwigs are known to display this phenomenon in their forceps, pincer-like appendages used as weapons. But what about females, who also possess forceps? What purpose do these structures serve for them?
Tomoki Matsuzawa and Associate Professor Junji Konuma from Toho University’s Department of Biology, conducted a quantitative study of female earwig forceps. Using morphometric analysis on maritime earwigs (Anisolabis maritima), they discovered that female forceps also display positive allometry, suggesting a potential role in sexual selection.
The research team measured the head, thorax, abdomen, and bilateral forceps dimensions, analyzing shape differences in both sexes. males have thick, short, and curved forceps, while females have thin, long, and straight ones, indicating sexual dimorphism.When body size was plotted against forceps width and length, positive allometry was evident in males, with forceps width increasing disproportionately with body size. Surprisingly,females also exhibited positive allometry in the length of their forceps. These results suggest that while the sexes differ in forceps shape, both may have evolved them as weapons.
Female Earwigs Compete for Mates
According to the study, female earwigs may use their forceps as weapons when competing for mates.
“Female earwigs compete for small, non-aggressive males. Our findings suggest that female forceps may have evolved as effective weapons in such competition.”
Associate Professor Konuma explains that a previous behavioral study revealed that female earwigs compete for small, non-aggressive males. The current findings suggest that female forceps may have evolved as effective weapons in such competition. “While most earlier research focused only on males, our study highlights the importance of considering female traits as well when studying the evolution of insect morphologies,” Konuma stated.
These findings were published on June 12, 2025, in the Biological Journal of the Linnean Society.
Frequently Asked Questions About Earwig Forceps
- What are earwig forceps?
- Earwig forceps are pincer-like appendages located at the tip of the abdomen in earwigs. They are used for various purposes, including defense, mating, and capturing prey.
- Do both male and female earwigs have forceps?
- Yes, both male and female earwigs have forceps, although the shape and size may differ between the sexes. Males typically have more curved and robust forceps, while females have straighter and more slender ones.
- What is the purpose of forceps in female earwigs?
- While the exact function of forceps in female earwigs is still being studied, research suggests they might potentially be used for defense, competition for mates, or manipulating objects in their environment.
Sources
- Biological Journal of the Linnean Society, 2025
- National Park Service
- Rocky Mountain Elk Foundation
- Britannica
- Nature Scitable
- The Darwin Project
- Natural History Museum
- royal Society
- Annual Reviews
- National Institutes of Health
- Science Magazine
- Nature
- ScienceDirect
- The University of Chicago Press Journals
- Oxford Academic
