A new study from Toho University reveals that female earwigs exhibit a similar pattern of exaggerated forceps growth as males, suggesting that both sexes may have evolved these traits through sexual selection.
Do larger male elk have proportionally larger antlers? The answer is no. In fact, larger individuals tend to have disproportionately larger antlers — a phenomenon known as positive allometry. This pattern, where certain body parts grow disproportionately large relative to body size, is observed not only in mammals but also in animals such as beetles and fiddler crabs. Evolutionary biologists interpret such traits as evidence of sexual selection — a process in which physical features evolve because they offer an advantage in competing for mates.
Male earwigs are known to show positive allometry in their forceps — pincer-like appendages at the tip of the abdomen — which are believed to have evolved as weapons in battles with rivals. But what about females? Female earwigs also have forceps — so what purpose do they serve?
Tomoki Matsuzawa (then an undergraduate) and Associate Professor Junji Konuma from Toho University’s Department of Biology conducted the first quantitative study of female earwig forceps. Using morphometric analysis on the maritime earwigs Anisolabis maritima, they found that female forceps also display positive allometry — suggesting that they, too, may have evolved through sexual selection.
The team measured the head, thorax, abdomen, and bilateral forceps dimensions and analyzed shape differences in both sexes. They found that males have thick, short, and curved forceps, while females have thin, long, and straight ones — indicating clear sexual dimorphism. When they plotted body size against forceps width and length on a log-log scale, the results revealed a pattern of positive allometry in males: forceps width increased disproportionately with body size. Surprisingly, positive allometry was also found in females — in the length of the forceps. These results suggest that while the sexes differ in forceps shape, both may have evolved them as weapons — albeit in different ways.
Associate Professor Konuma explains:”A previous behavioral study has shown that female earwigs compete for small, non-aggressive males. Our 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.”
These findings were published on June 12, 2025, in the Biological Journal of the Linnean Society.