Sexual dimorphism of weapon allometry in the European earwig

Static allometries describe how trait size covaries with body size and are often treated as stable descriptors of trait architecture. We quantified forceps allometry in the European earwig (Forficula auricularia) using a split-brood experiment manipulating diet and social density. We analysed females, brachylabic males, and macrolabic males as three separate phenotypic groups using a Bayesian mixed-effects model that allowed scaling relationships to vary among groups, environments, maternal families, and rearing dishes. Most posterior median slopes were below one. We found no clear evidence that female and brachylabic male slopes differed across environments, and macrolabic male slopes were also similar under most conditions. A steeper relationship was estimated for macrolabic males under poor diet at medium density, but the credible interval included isometry and the estimate was based on 18 individuals. Environmental differences in slopes were otherwise small or uncertain. A quadratic model received somewhat greater predictive support and suggested curvature near the extremes of the body-size distribution, particularly in macrolabic males, but did not alter the broad pattern of similarity. Variation among maternal families and rearing dishes was small. Overall, the results suggest that forceps allometry is broadly similar across phenotypes and environments, with limited and uncertain divergence associated with the exaggerated male morph.