Size-structured mating niche differentiation in the male-polyphenic mite Rhizoglyphus robini

Condition-dependent expression of alternative male morphs evolves when males of different sizes experience different mating niches, requiring different morphologies. Such mating niche differentiation can be due to competitive asymmetry between large and small males in contests over mating opportunities. Here, we tested the hypothesis that aggressive interactions among males cause size-structured mating niches in an acarid mite with condition-dependent male polyphenism: the bulb mite Rhizoglyphus robini. In this species, large males mature as armed fighters with enlarged legs, and small males as scramblers without modified legs. We staged experimental dyadic contests over a mating opportunity between either a pair of fighter males, or a fighter and a scrambler male. We predicted that the larger male would have a higher likelihood of mating first in contests among fighters, that the fighter male would have a higher likelihood of mating first in fighter-scrambler contests, that fighters would have a higher likelihood of interrupting ongoing copulations if they are larger than their mating rival, and that copulations in the presence of a fighter rival therefore last shorter than copulations with a scrambler rival present. We found that in contests among fighters, the larger male had a higher probability of mating first. In contests among a fighter and scrambler, the fighter male was more likely to mate first regardless of the body size difference between the contestants. Ongoing copulations were only rarely interrupted by the rival male (always by a fighter), and the probability of interruption did not depend on the body size difference between the mating male and its rival. Copulations lasted shorter in the presence of a rival fighter, but this effect was not attributable to interruption of copulations. We conclude that the fighting niche is particularly accessible for large males, as larger males have a higher probability of winning pre-copulatory contests. Such mating niche differentiation likely contributes to evolutionary maintenance of condition-dependent male polyphenism, where small males are forced to adopt an alternative mating tactic and hence develop a dedicated morphology.