ORIGINAL PAPER Sexual dimorphism in immune function changes during the annual cycle in house sparrows Péter László Pap & Gábor Árpád Czirják & Csongor István Vágási & Zoltán Barta & Dennis Hasselquist Received: 11 May 2010 / Revised: 27 July 2010 / Accepted: 27 July 2010 / Published online: 13 August 2010 # Springer-Verlag 2010 Abstract Difference between sexes in parasitism is a common phenomenon among birds, which may be related to differences between males and females in their investment into immune functions or as a consequence of differential exposure to parasites. Because life-history strategies change sex specifically during the annual cycle, immu- nological responses of the host aiming to reduce the impact of parasites may be sexually dimorphic. Despite the great complexity of the immune system, studies on immu- noecology generally characterise the immune status through a few variables, often overlooking potentially important sea- sonal and gender effects. However, because of the differences in physiological and defence mechanisms among different arms of the immune system, we expect divergent responses of immune components to environmental seasonality. In male and female house sparrows (Passer domesticus), we mea- sured the major components of the immune system (innate, acquired, cellular and humoral) during four important life-history stages across the year: (1) mating, (2) breeding, (3) moulting and (4) during the winter capture and also following introduction to captivity in aviary. Different individuals were sampled from the same population during the four life cycle stages. We found that three out of eight immune variables showed a significant life cycle stage × sex interaction. The difference in immune response between the sexes was significant in five immune variables during the mating stage, when females had consistently stronger immune function than males, while variables varied generally non-significantly with sex during the remaining three life cycle stages. Our results show that the immune system is highly variable between life cycle stages and sexes, highlighting the potential fine tuning of the immune system to specific physiological states and environmental conditions. Keywords Acquired immunity . Annual cycle of immune responses . Cellular immunity . Humoral and innate immunity . Passer domesticus . Sex-differential immunity P. L. Pap : C. I. Vágási : Z. Barta Behavioural Ecology Research Group, Department of Evolutionary Zoology, University of Debrecen, 4032 Debrecen, Egyetem tér 1, Hungary P. L. Pap (*) : C. I. Vágási Department of Taxonomy and Ecology, Babeş-Bolyai University, 400006 Cluj-Napoca, Clinicilor Street 5–7, Romania e-mail: peterlpap@gmail.com G. Á. Czirják Department of Infectious Diseases, Faculty of Veterinary Medicine, University of Agricultural Sciences and Veterinary Medicine, 400372 Cluj-Napoca, Mănăştur Street 3–5, Romania G. Á. Czirják Laboratoire Évolution et Diversité Biologique (EDB), UMR 5174 Centre National de la Recherche Scientifique (CNRS), Université de Toulouse III Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse, France D. Hasselquist Department of Animal Ecology, Lund University, Ecology Building, 223 62 Lund, Sweden Naturwissenschaften (2010) 97:891–901 DOI 10.1007/s00114-010-0706-7