The relationship between parasites and spleen and bursa mass in the Icelandic Rock Ptarmigan Lagopus muta

The spleen and bursa of Fabricius in birds are organs that play an important role in fighting parasite infections. The size of these organs can be used by ecologists as a measure of immune investment, with larger size implying greater investment. The bursa only occurs in juvenile birds during the development of the B cell repertoire, whereas the spleen, which is the main site of lymphocyte differentiation and proliferation, is present in both juveniles and adults. We investigated spleen and bursa mass in relation to parasite measures for 541 Rock Ptarmigan Lagopus muta collected in northeast Iceland during October from 2007 to 2012. Of these 541birds, 540 carried at least one parasite species. Juveniles had heavier spleens than adults, and adult females had heavier spleens than adult males, but there were no sex differences in juveniles. Spleen mass increased from 2007 to 2009, then decreased up to 2011, before slightly increasing again in 2012. Spleen and bursa mass in juveniles increased with improved body condition, but decreased in adults, and this effect differed significantly among years. Spleen mass in juveniles was positively associated with parasite species richness and abundance, in particular endoparasite abundance, with coccidian parasites being the main predictors. Bursa mass was negatively associated with elevated ectoparasite abundance, with two chewing lice being the main predictors. These two immune defense organs appeared to relate to different stimuli. Mean annual spleen mass of juveniles changed in synchrony with Ptarmigan body condition and population density over the years of this study. The only parasite measure that showed any relation to density was coccidian prevalence in juvenile birds, with an approximately 2-year time-lag, suggesting that factors other than parasites are probably more important in triggering changes in spleen mass.