Background The evolution of sexual signals is not only determined by immediate sexual selection but also by selection arising from the environment and the interaction with developmental effects. In this study we aimed to investigate how the badge size of male house sparrows (Passer domesticus) is correlated to avian malaria infections as well as to prenatal testosterone exposure, measured as the 2D:4D digit ratio. The rationale behind this study is that the immunosuppressive effect of maternal testosterone deposition combined with the fitness cost imposed by parasites may cause important trade-offs to the development of secondary sexual signals.

Methods Assuming that vector abundance is a key variable associated with infection risk by avian malaria, we caught adult male sparrows from eight different populations in the Camargue, France, four of which were located in a vector-controlled area, and the other four in an untreated area. For each bird we measured its badge size, digit ratio and took blood to determine its infections status. We used PCR to identify the malaria parasite species and strain that caused the infection.

Results Contrary to our expectation, prevalence of disease did not differ across the vector-treatment regions, with around 80 % of birds being infected in both areas, and those infections were caused largely by a single strain, Plasmodium relictum SGS1. Although infected birds had a badge size not significantly different from uninfected males, there was a condition-dependent association between badge size, infection status and maternal testosterone deposition.

Conclusions This study illustrates that the complexity of temporal and ecological dimensions makes the relationships between disease, testosterone-related traits and secondary sexual signals that have been previously reported less general than thought.