The effects of testosterone on a viral infection in greenfinches (Carduelis chloris): an experimental test of the immunocompetence-handicap hypothesis.

The immunocompetence-handicap hypothesis suggests that the honesty of quality signals could be guaranteed if testosterone (T) suppresses immune function while enhancing male ornaments. In addition, it has been proposed that the cost of enhancing ornaments should be highest for males with small ornaments. Recently, the assertion that T causes obligate immunosuppression has been questioned. In this study, we tested whether elevated T levels would increase susceptibility to a viral infection, and whether this hypothesized effect would be most pronounced in males with small ornaments. We surgically inserted T implants into 15 male greenfinches (Carduelis chloris) and control implants into a further 15 males. All birds were then infected with a naturally occurring virus (Sindbis virus, Alphavirus genus), and each bird's daily viraemia (blood virus concentration) was measured for seven days. The specific antibody response was measured for eight weeks. T-implanted males did not exhibit increased viraemia or decreased antibody response, and males with small and large ornaments did not respond differently to T implantation. We did, however, find that T implantation decreased viraemia early in the course of the infection and increased viraemia late in the infection. Thus, our results demonstrate that T may act both to increase and to decrease viraemia.

Prevalence of sindbis virus neutralizing antibodies among Swedish passerines indicates that thrushes are the main amplifying hosts.

The Sindbis virus uses birds as vertebrate hosts in the summer amplification cycle, and the virus is transmitted by ornithophilic Culex species. Previous field and experimental studies have shown that mainly passerine birds are involved in the amplification. To delineate the pattern of Sindbis virus infections among passerines, we collected and sampled birds for blood at five study sites located in northern, central, and southern Sweden. All study sites were lowland forested wetlands and humid forests. The blood samples were assayed for Sindbis neutralizing antibodies, and we tested if the prevalence of Sindbis antibodies varied in relation to bird characteristics (i.e., species, body-mass, sex, and age), and environmental factors (i.e., year, month, and location). We found that Sindbis virus infections occurred in almost all passerine species sampled, but that the infection prevalence was unequally distributed among species. The fieldfare, the redwing, and the songthrush each had significantly higher prevalence than the average for all species. Large passerine species had higher infection prevalence than small species. The infection was less prevalent in hatching-year birds than in older birds during June and July, but not in August. Males and females had the same infection prevalence. The prevalence of Sindbis antibodies was higher in central than in southern Sweden, which coincided with a higher proportion of fieldfare-redwing-songthrush samples in the central region of the country. Thus, it is possible that regional and annual variations in the prevalence of Sindbis antibodies in Swedish passerine species depend on the number of fieldfares, redwings, and songthrushes available for feeding by vector mosquitoes.