Polymorphism in male feather mites (Astigmata: Analgoidea, Pterolichoidea): patterns and potential explanations

Male polymorphism is common in mites from the cohort Astigmata. The best studied cases involve combat-related morphology in mites associated with stored products, such as Sancassania berlesei, in which some males have legs modified for attacking rivals (heteromorphs) while others have female-like walking legs (homeomorphs). In these free-living species, induction of the hetermorphic state can be environmentally or genetically determined. Polymorphism is even more commonly observed in astigmatan mites associated with birds. Male nest mites from the family Pyroglyphidae can show strong variation in the degree of hypertrophy of their forelegs. Feather-dwelling mites often display homeo- and heteromorphic states, which can be further complicated by right- vs left-handed asymmetries. To date, instraspecific variation in male form in these bird-associated mites has received attention mainly from a taxonomic point of view with little effort put into determining whether the variation is truly a distinct polymorphism (vs, e.g., continuous variation) and whether there are any environmental correlates associated with variation in male morphology. Here I report on three examples of variation in male morphology in bird associated mites: the nest mite Sturnophagoides bakeri, the pigeon feather mite Falculifer rostratus and the cormorant feather mite Michaelia neotropica. For the nest mite, there were no clearly distinct morphs, but rather a continuum of leg and body sizes. For F. rostratus, unlike in S. berlesei, there was no relationship between population density and propensity for expression of the heteromorphic state. Asymmetry of male M. neotropica was strongly correlated with which side of the host bird they were found on. Future research could address the relationship between male morphology at a finer scale (e.g., per feather rather than per bird), and how the modified male structures are actually used by the male mites.