Population density and male polymorphism in the feather mite Falculifer rostratus (Acari: Falculiferidae)

Abstract

Male polymorphism, in which two or more male morphologies occur within a species, is a widespread phenomenon in the Astigmata. Factors determining which morph a male will become have been studied for several free-living species. In Sancassania berlesei (Michael) (Acaridae), male nymphs develop into relatively unmodified homeomorphs at high population densities, and into highly modified, aggressive heteromorphs at low densities. We tested whether population density correlated with male morph ratio in the dimorphic feather mite Falculifer rostratus (Buchholz) (Falculiferidae). Twenty-one pigeons (Columba livia Gmelin) from Illinois, USA, were thoroughly washed and all F rostratus extracted were identified to stage, sex, and male morph. Excluding four birds that had no F rostratus, total densities per host ranged from 1-1155. Numbers of heteromorph males correlated positively with population density, but homeomorph numbers had no obvious relationship with density. Ratios of homeomorph:heteromorph were frequently higher at low population densities than at high densities the opposite of the pattern observed for S. berlesei. An alternative hypothesis that quality of diet may determine morph ratio is tested and receives little support. It is possible that male morph is determined by density in F rostratus, but that it is controlled at a finer physical scale (e.g., per feather) than could be measured in this study. Other possibilities are that morphs are determined genetically or by host variables we did not account for (e.g., moulting status).