Male dimorphism in the bulb mite, Rhizoglyphus robini: fighters survive better

Abstract

In several species of the mite family Acaridae, fighter males, possessing a sharply terminated third pair of legs which they use to mortally stab other males, coexist with scrambler males with unmodified legs. We studied the effect of population size on the relative mortality of fighters versus scramblers in the bulb mite, Rhizoglyphus robini, a species in which male morph is heritable. Fights were an important determinant of male mortality, and fighters of R. robini survived better in both large (120 individuals) and small (10 individuals) colonies. Colony size did not significantly affect the relative survivorship of the two morphs, and we also found no differences between large and small colonies in the proportion of fighters in the progeny. Thus, the possibility that variation in population size maintains genetic variation for male morph in R. robini was not supported by our result. Nor was another possible mechanism, the negative frequency-dependence of morph fitness. We estimated morph mortality and mating success in populations of 42 adults, with the fighter/scrambler ratio ranging from 1:19 to 19:1. Neither measure depended on the proportion of fighters in a population, but fighters survived better than scramblers. Further research is needed to explain the maintenance of morph heritability in R. robini in spite of the higher mortality of scramblers. We discuss potential mechanisms.