ORIGINAL PAPER Leucocyte proﬁles and H/L ratios in chicks of Red-tailed Tropicbirds reﬂect the ontogeny of the immune system Nina Dehnhard • Petra Quillfeldt • Janos C. Hennicke Received: 16 April 2010 / Revised: 23 November 2010 / Accepted: 9 December 2010 Ó Springer-Verlag 2010 Abstract Immune defence is fundamentally important for the survival prospects of young animals. While innate immunity offers initial protection from a variety of pathogens, acquired immunity responds more speciﬁcally to pathogens, but is considered to be more costly and to respond slower. Moreover, the acquired immunity is not yet fully developed in neonatal chicks. Little is known about the ontogeny of the immune system of wild birds. Long-lived seabirds, with their slow chick development, are good models to investigate how young birds invest in both arms of their immune system. We determined leuco- cyte proﬁles and heterophil to lymphocyte (H/L) ratios of Red-tailed Tropicbirds (Phaeton rubricauda westralis) on Christmas Island, Indian Ocean. Young chicks (N = 10) had signiﬁcantly higher H/L ratios than older chicks (N = 19), while adults (N = 47) showed intermediate values and did not differ from either chick age class. High H/L ratios in young chicks were caused by high initial numbers of heterophils per 10,000 erythrocytes that declined with age. In contrast, the number of lymphocytes per 10,000 erythrocytes was similar for young and older chicks. These data suggest that young chicks invested heavily in innate immunity to protect themselves from pathogens, while investment into acquired immunity became more important in older chicks with a functional acquired immune response. Body condition did not have a signiﬁcant inﬂuence on any leucocyte parameter. Keywords Tropical seabirds  H/L ratio  Ontogeny of immune system  Leucocyte proﬁles Introduction Animals must cope with hostile environmental inﬂuences including food scarceness, pathogens and parasites that can affect their health, survival and reproduction (Ho ˜rak et al. 1998). Internal control mechanisms, like the immune sys- tem, have evolved as a defence against some of these factors and an intact immune system is essential for sur- vival (e.g., Sheldon and Verhulst 1996; Lee 2006). How- ever, investment in the immune system and therefore own survival, is in trade-off with other life history components, like growth and development, as building and maintaining an intact immune system is energetically costly (Sheldon and Verhulst 1996; Norris and Evans 2000; Lee 2006). Thus, developing offspring speciﬁcally have to balance their energetic investment between growth and immune system. The immune system in vertebrates is largely based on different types of leucocytes, which are variable in number. Infectious diseases usually lead to leucocytosis in adult birds, which is an elevation in the number of leucocytes (Fudge 1989). Leucocytes-related immune response is traditionally divided into innate and the acquired immu- nity, even though they are highly integrated (Juul-Madsen Communicated by G. Heldmaier. N. Dehnhard (&)  P. Quillfeldt Max Planck Institute for Ornithology, Vogelwarte Radolfzell, Schlossallee 2, 78315 Radolfzell, Germany e-mail: dehnhard@orn.mpg.de J. C. Hennicke Abt. Animal Ecology and Conservation, Biozentrum Grindel, Universita ¨t Hamburg, Martin-Luther-King Platz 3, 20146 Hamburg, Germany J. C. Hennicke Centre d’Etudes Biologiques de Chize ´, CNRS UPR 1934, 79360 Villiers-en-Bois, France 123 J Comp Physiol B DOI 10.1007/s00360-010-0548-9