Male-biased parasitism by common helminths is not explained by sex differences in body size or spleen mass of breeding cormorants Phalacrocorax auritus

In vertebrates, males are often more parasitised than conspecific females. This bias in parasitism might result from sex differences in parasite exposure and/or susceptibility to infection. Such information is important for testing hypotheses about allocation of resources to life histories of males and females and for testing hypotheses about factors thought to influence parasite fitness and parasite dynamics. We tested whether double-crested cormorants Phalacrocorax auritus exhibit male-biased parasitism by gut helminths. The prevalence of nematode Contracaecum spp. and trematode Drepanocaphalus spathans infections was similar to 90% and 39%, respectively. Cestode, primarily Paradilepis caballeroi and acanthocephalan Andracantha gravida infections were less common (< 10%). Male and female cormorants did not differ in prevalence of infection by any helminth species. However, males had twice the abundance and intensity of Contracaecum spp. infections and twice the intensity of D. spathans infections than found in females. For common parasites showing male-biased parasitism, degree of parasitism was also unrelated to body size or mass in either sex. Males and females did not differ in spleen mass and spleen mass was unrelated to abundance of common parasites. Furthermore, abundance of trematodes and nematodes was not correlated. At present, male biases in parasitism by nematodes and trematodes in cormorants are independent patterns that remain unexplained, but are most likely attributable to sex differences in exposure and/or immunological differences not yet assessed.