Physiological and haematological consequences of a novel parasite on the red-rumped swallow Hirundo daurica

Parasite virulence has been hypothesised to increase with the degree of hos t sociality because highly social hosts have a greater probability of encou ntering horizontal transmission of parasites and experiencing infections wi th multiple strains of the same parasites than do solitary hosts. As compar ed with the defences of closely related social host species, we predicted t hat solitary hosts should have relatively weak defences against parasites, thus being relatively more affected when parasitised by a novel parasite. W e tested this prediction by either experimentally infesting 12 nests of the solitarily nesting red-rumped swallow Hirundo daurica with 50 individuals of the generalist martin bug Oeciacus hirundinis or by fumigation of nine n ests. Nestlings 13 days old from the parasite addition group experienced in creased mortality, attained lower body mass and tended to have shorter tars i compared to nestlings from fumigated nests. Surprisingly, nestlings from the parasite addition group had higher packed cell volume (cellular fractio n of blood) and lower levels of heat shock proteins (HSP60) than nestlings from the fumigation group. A measure of immunocompetence was not significan tly affected by treatment, but its magnitude was positively related to pack ed cell volume and negatively related to level of HSP60. Solitary hosts lik e the red-rumped swallow have weak immune responses and low levels of heat shock proteins when infested with ectoparasites while highly social hosts h ave strong immune responses and high levels of heat shock proteins when inf ested. These findings partially support the hypothesis that potential host species with weak defences are more susceptible to infection and the delete rious effects of evolving parasites than potential hosts with strong defenc es. (C) 2001 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.