Extra-pair paternity and antiparasitic defence

Background Extra-pair paternity (EPP) in birds provides benefits in terms of more offspring, and characteristics for maintenance of this behaviour have been the subject of investigation. Microorganisms are known to be transmitted during mating, especially when mating with multiple partners, and factors reducing this cost of multiple mating are expected. Further, plumage brightness and colour intensity have been shown to be important traits to benefits from multiple mating as predicted by sexual selection. The aim of this study was to investigate the relationship between the rate of extra-pair paternity and the relative size of the uropygial gland at the interspecific level, as the uropygial gland is an exocrine gland hypothesized to produce antiparasitic substances and further identified to affect plumage brightness. Because of the expected benefits of large uropygial gland in scenarios of sexual selection, we predicted a positive correlation with EPP. Methods We collected information from the literature of uropygial gland size and frequency of extra-pair paternity of 60 avian species of different families and explored the predicted positive correlation between them. We did so with means of comparative analyses that considered phylogenetic relationship as random factor and included body mass as covariate. We used Markov chain Monte Carlo generalized linear mixed models that were weighted by number of nests used to estimate extra-pair paternity. Results We detected a positive relationship between level of extra-pair paternity and uropygial gland size at an interspecific level. This finding is consistent with the prediction. Conclusions We discuss the importance of this result in scenarios of sexual selection and argue that the detected relationship may have arisen by utilizing antiparasitic secretions through secondary sexual characters indicating parasite resistance.

Exploring the adjustment to parasite pressure hypothesis: differences in uropygial gland volume and haemosporidian infection in palearctic and neotropical birds

Parasites are globally widespread pathogenic organisms, which impose important selective forces upon their hosts. Thus, in accordance with the Adjustment to parasite pressure hypothesis, it is expected that defenses among hosts vary relative to the selective pressure imposed by parasites. According to the latitudinal gradient in diversity, species richness and abundance of parasites peak near the equator. The uropygial gland is an important defensive exocrine gland against pathogens in birds. Size of the uropygial gland has been proposed to vary among species of birds because of divergent selection by pathogens on their hosts. Therefore, we should expect that bird species from the tropics should have relatively larger uropygial glands for their body size than species from higher latitudes. However, this hypothesis has not yet been explored. Here, we analyze the size of the uropygial gland of 1719 individual birds belonging to 36 bird species from 3 Neotropical (Peru) and 3 temperate areas (Spain). Relative uropygial gland volume was 12.52% larger in bird species from the tropics than from temperate areas. This finding is consistent with the relative size of this defensive organ being driven by selective pressures imposed by parasites. We also explored the potential role of this gland as a means of avoiding haemosporidian infection, showing that species with large uropygial glands for their body size tend to have lower mean prevalence of haemosporidian infection, regardless of their geographical origin. This result provides additional support for the assumption that secretions from the uropygial gland reduce the likelihood of becoming infected with haemosporidians.