What if, just like males, female earwigs are evolving powerful tools not for raising offspring—but to fight for a mate? The surprising measurements suggest women wield weapons, too!
Key Points at a Glance
- First evidence of positive allometry in female earwig forceps length.
- Males show positive allometry in forceps width, while females show it in length.
- Both sexes exhibit weapon asymmetry, but much less variation than overall shape.
- Findings suggest sexual selection shapes both male and female weapon traits.
The recent morphological study of Anisolabis maritima earwigs offers a fresh challenge to traditional sexual-selection theories. For decades, researchers believed exaggerated physical traits—like oversized forceps—were driven almost exclusively by males competing for mates. But new data demonstrates that females too are not just passive players: their forceps grow disproportionately long as body size increases, a classic signature of sexual selection. In other words, females have weapons—literally, and they evolve them strategically.
Scientific analysis of 68 male and 45 female earwigs from a Japanese population revealed striking sex differences in forceps design. Males generally have thick, short forceps that benefit from width—likely useful in gripping rivals. Their forceps width scales steeply with overall size, showing a strong positive allometry pattern: larger males gain disproportionately thicker forceps.
Females, in contrast, sport slender, elongated forceps. Crucially, the length of their forceps increases more steeply with body size than expected—another form of positive allometry. It seems that as females grow larger, their weapons don’t just get bigger proportionally: they become dramatically longer. These findings challenge the assumption that female morphology is driven only by survival or fertility factors. Instead, it points to a role in competition—perhaps among females vying for mates—a behavior already observed in some earwig species.
The study also investigated asymmetry—the subtle differences between left and right forceps. While asymmetry exists, it represents only about 3 % of total variation. That pales in comparison to the weapon shape itself, which constitutes a dominant 85 % of variation in body traits. This suggests asymmetry may be less relevant to selection pressures, or its variation too limited to influence competitiveness meaningfully.
What does this mean for evolutionary biology? First, it broadens the scope of sexual selection research: females, too, can develop exaggerated structures as competitive tools. Second, it highlights the importance of measuring all body traits, including female-specific ones, to fully understand selective forces. The common assumption that weapons evolve mainly in males might be too restrictive.
Methodologically, the study moves beyond univariate allometry based on single body metrics. By using a geometric mean of ten body traits, the researchers offered a robust measure of body size across sexes. They then applied Burnaby’s size–correction and principal component analysis to reveal the forceps shape axis (PC1), accounting for most variation across sexes. Regression slopes clearly indicated which forceps dimension grows disproportionately in each sex.
This underscores the dynamic interplay between morphological form and sexual behavior. If females use forceps in competition, they face selection pressures just as males do. It adds a new dimension to our understanding of sexual selection—not as a male-centric domain, but as an arena where both sexes may evolve weaponry for social struggle.
Future research should explore whether female forceps length correlates with reproductive success or dominance in female–female contests. Behavioral assays, ecological observations, and genetic analyses could clarify the adaptive value of weapon-bearing females. After all, a long weapon may win more than just fights—it may reshape our view of how species evolve complex traits.
Source: Biological Journal of the Linnean Society
Enjoying our articles?
We don’t show ads — so you can focus entirely on the story, without pop-ups or distractions. We don’t do sponsored content either, because we want to stay objective and only write about what truly fascinates us. If you’d like to help us keep going — buy us a coffee. It’s a small gesture that means a lot. Click here – Thank You!
