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Ectoparasitism may be recognized as one of the main costs of coloniality, but little is known about how it affects the fitness and social structure of bats, the most gregarious of mammals. We studied these issues using the colonial bat Miniopterus schreibersii and its haematophagous parasitic mite Spinturnix psi as a model. Body condition is an important indicator of individual fitness that is potentially affected by ectoparasitism. Thus, we measured host body condition and mite loads in a total of 969 bats throughout the annual cycle. Mites were rare while hosts hibernated, increased in abundance in spring and peaked during nursing season of bats, when they were particularly abundant on lactating females and young bats. This strong seasonal variation in mite loads is related to the reproductive cycle of mites, which in turn appears to be synchronized with the reproductive cycle of their hosts. Mite loads and the …