Temporal variation in shedding of coccidial oocysts: implications for sexual-selection studies

Studies of the effects of parasites on the sexual ornamentation and behavio r of animals are becoming increasingly common. A potential difficulty in ma ny studies of this type is the accurate assessment of parasite prevalence w ithin a population and of the parasite burden of individual hosts. We prese nt data that illustrate how a simple factor such as the time of day at whic h samples are collected can drastically alter the results of parasite asses sments. House finches (Carpodacus mexicanus) infected with coccidians of th e genus Isospora shed oocysts in their feces predominantly during the late afternoon. The importance of recognizing such circumstances (e.g., diurnal periodicity) is discussed in the light of possible negative effects on stud ies of host-parasite relationships.