Phenotypic Matching Without Genetic Correlation in Dimorphic Legs of Bulb Mites

The evolution of exaggerated structures used as weapons in male-male contests often drives correlated changes in traits that compensate for their costs or enhance their benefits. Given their interdependence, weapons and compensatory traits are expected to evolve with some degree of genetic integration. The bulb mite Rhizoglyphus echinopus, a male-dimorphic species, offers an opportunity to examine this question. In this species, the third legs are dimorphic in width, functioning as a weapon, while the fourth legs are dimorphic in length, potentially serving as a compensatory trait. Fighter males, identified by their thicker third legs, also tend to have longer fourth legs than scramblers. Here, we first tested whether body size and the width of the third legs predict contest success, and whether the length of the fourth legs also contributes to fighting ability. We then evaluate phenotypic and genetic integration between these traits. Behavioral trials showed that variation in body size, third leg width, and fourth leg length did not explain contest outcomes. Moreover, although males with thicker third legs tended to have longer fourth legs, quantitative genetic analyses revealed that the traits are phenotypically, but not genetically, correlated. This suggests that their phenotypic match likely results from shared environmental thresholds rather than genetic integration. Overall, our findings emphasize the need to move beyond phenotypic correlations when evaluating the evolutionary role of compensatory traits, as such correlations alone may mislead interpretations of their adaptive significance.