Abstract
Hamilton and Zuk's hypothesis suggests that male showiness allows females to assess male's ability to resist parasites. In this hypothesis the full expression of secondary sexual traits and parasite load are respectively assumed to mainly depend on individual vigour and genetic resistance. Here, we propose that a negative correlation between brightness and parasite load can incorrectly suggest a causal relationship between parasitism and mating success. Parasitism can negatively covary with host age in certain circumstances of host mortality induced by parasite accumulation, while both male brightness and female preferences positively covary with age. Thus some properties of host-parasites associations, coupled with host reproductive behaviour can provide fortuitous support for the Hamilton-Zuk hypothesis.