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

In several species of the mite family Acaridae, fighter males, possessing a sharply terminated third pair of legs which they use to mortally stab othe r males, coexist with scrambler males with unmodified legs. We studied the effect of population size on the relative mortality of fighters versus scra mblers 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 s mall (10 individuals) colonies. Colony size did not significantly affect th e relative survivorship of the two morphs, and we also found no differences between large and small colonies in the proportion of fighters in the prog eny. Thus, the possibility that variation in population size maintains gene tic variation for male morph in R. robini was not supported by our result. Nor was another possible mechanism, the negative frequency-dependence of mo rph 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 ex plain the maintenance of morph heritabilitv in R. robini in spite of the hi gher mortality of scramblers. We discuss potential mechanisms.